DNA – The Only Way?

I’ve been struggling to write a post about the new TV show ‘Pluribus’ but it’s actually huge and therefore hard to talk about in a single piece, so for now, and possibly instead of that, I’ll just be talking about a scientific point it raises and it isn’t really about the series. There’ll be spoilers for about the first ten minutes of the first episode and then I’ll be moving off the subject. Here goes.

At the start of the first episode of ‘Pluribus’, astronomers detect a signal from the TRAPPIST-1 system around 600 light years away in the constellation of Cygnus. It repeats every seventy-eight seconds and consists of a series of four types of signal which they quickly realise represent the four bases of RNA, cytosine, guanine, adenine and uracil. This makes sense, in a way, as RNA is used to send messages from DNA for transcription into proteins, and it’s doing the same job here. This made me wonder a couple of things. How did they know it was uracil and not thymine, and RNA but not DNA? Also, does this mean that RNA and DNA are universal codes for genetic information, everywhere there’s life, or is it individually customised for different recipients, in which case how did they know terrestrial life used that code? Seems like insider information is involved somewhere.

So, crystallising that thought, this is the situation. All known life here on Earth uses one of two complex types of molecules, deoxyribonucleic acid and ribonucleic acid, DNA and RNA. At this point I’m stuck because I have no idea how much is common knowledge. If I get this wrong, I’m going to lose a lot of people. So I’m going to assume that everyone knows the remarkable general double helix with rungs structure of DNA, how its coils are themselves in coils so that it’s packed together very closely most of the time, that most of it doesn’t carry genetic information but has other functions related to it, that it has sides made of alternating sugar molecules and phosphate groups and four types of bases which link up in specific pairs, adenine with thymine and cytosine with guanine. RNA is generated from DNA and has a different, simpler structure, again with a sugar molecule alternating with phosphate groups and again four bases, except that instead of thymine it has a base called uracil. RNA is used to transfer information to ribosomes, which are like playback heads except that instead of sound they produce proteins, one amino acid at a time. Although most species of animal, plant and other organisms use DNA to store their genes, many viruses use RNA instead. RNA is less stable than DNA, so for example whereas animal or plant remains from many millennia in the past can have their DNA information extracted in a form increasingly corrupted with their age, RNA is not the same and doesn’t last long.

This is important. Please tell me if I’m assuming too much and if I’m not writing clearly. I really struggle with brevity, clarity and trying to work out what people do and don’t know about things, and one way of addressing this might be to get some feedback. In a sense, this entire blog post is a test of my ability to communicate clearly and well at least as much as it is about DNA.

So, I have questions, some of which I know some of the answers to but most I don’t. DNA can be considered to have the following components: deoxyribose, phosphate, adenine, thymine, guanine and cytosine. RNA has ribose instead of deoxyribose and uracil instead of thymine. The question is, are any or all of these essential for any molecule carrying genetic information within an organic life form, or are there other possibilities? How rigidly restrained is this aspect of biochemistry? This could be framed as a question about alien life but in fact it’s as relevant to biochemistry as it’s actually known to be on this planet as it is to that possibility.

First of all, the bases. There are two types of these: purines and pyrimidines. Purines have two rings in their molecule and pyrimidines only one. I remember this by thinking that the long name describes the short molecules and vice versa. Purines include some other familiar compounds including caffeine and the related stimulants often found with it. A particularly prominent purine is guanine, which forms the reflective layer at the back of many vertebrate retinae such as dogs and owls and increases their visual sensitivity in low-light conditions, and also the white cross on the back of garden spiders. They tend to be broken down into uric acid, so a diet high in DNA can contribute to gout and kidney stones and also conditions involving a high turnover of DNA such as leukaemia can also have these effects. Pyrimidines strike me as more obscure. Vitamin B1, thiamine, is a pyrimidine, as the name of thymine suggests, but as I understand it, although they’re widespread most of them are not well-known. However, similar pyrimidines to the ones found in nucleic acids are used as anti-cancer and anti-viral drugs.

Hence we have a system with four bases of particular kinds which can pair up with each other and consecutive groups of three bases are known as codons, each encoding for a particular amino acid, which are the blocks of proteins, as well as acting as “punctuation” such as full stops marking the end of a protein synthesis sequence. That’s sixty-four possibilities. However, since other bases can exist, it’s hypothetically feasible that these data can be stored more densely and efficiently. In particular it seems odd that uracil occurs in RNA but not DNA, but the reason for this is that it’s less stable and therefore can’t reliably encode for a long period of time, so it’s not so much that it’s used in RNA as that it isn’t used in DNA, and maybe at some stage it was but wasn’t selected for. This, then, is the first identifiable factor in the structure of DNA which determines its nature. I think there are probably at least four more usable bases, and this would double their data density. What it might not do, however, is enable evolution, as it might be that these bases are less amenable to mutation. For all I know, the first life forms in our lineage may have had different bases but couldn’t evolve as fast and therefore wasn’t able to compete with other organisms and aren’t our ancestors, even though there was nothing wrong with the basis of their genomes.

The next issue is sugar. Two sugars are involved and give their initials to the first letter of DNA and RNA. They’re pentoses, like fructose, rather than hexoses like dextrose or disaccharides like sucrose. Again, the explanation for the difference is durability and stability. The hydroxyl group on the second carbon which is absent on the deoxyribose molecule means it’s more stable than ribose and less likely to be altered by water. The presence of this hydroxyl group on the ribose molecule makes it easier to break down, ensuring that protein synthesis stops when it needs to. However, three- and four-carbon sugars could form the basis of the backbone instead of ribose or deoxyribose. Any more than five carbons stops double helix working: it gets in the way of the shape, making packing into the coils and supercoils unfeasible, and also makes it more reactive and also encourages branching. The double helix arrangement isn’t just pretty. It makes it possible to pack it into a small space, such as in chromosomes. It is possible for hexose nucleic acids to form but they don’t become double helices. Fructose is of course another pentose but the position on its molecule at which nitrogens from the bases can form are in the wrong place and the arrangement would be too crowded. Inulin, which is the daisy family’s alternative to starch which tastes like Jerusalem artichokes because those are in that family too, and sucrose itself both contain fructose but it’s not used in nucleic acids for this reason. It’s also thought that the processes which led to living processes preferred pentoses over other types of sugar, so life built on what was available.

That leaves the phosphate groups. These keep the molecule regular in shape and enable the DNA to bind to histones, which are the proteins making up much of the chromosomes around which it winds. Obviously this doesn’t apply to RNA because it isn’t wound round anything. Actually, it doesn’t apply to prokaryotic organisms such as bacteria either because they don’t have histones, but they do have nucleoid-associated proteins which do similar jobs. Bacterial DNA is in loops called plasmids. Plastids (not plasmids) have less DNA than free-living prokaryotes because many of their genes have been transferred to the nucleus.

Surprisingly, phosphate groups are not essential to the structure of nucleic acids and are in fact weaker than other options. For instance, glycine, the simplest, and the only non-chiral amino acid, can bond the sugar molecules together. Amide bonds are an option. There are also some different arrangements with phosphorus itself. These stronger bonds, though, can’t cross membranes as easily. Now I’ve previously mentioned how phosphorus may be the dog in the manger which explains the Fermi Paradox, but this is clearly not to do with DNA or RNA as it’s entirely feasible for an adequate alternative to DNA to exist without phosphorus, but with glycolysis and the Krebs Cycle where so far as I can tell it really cannot be replaced. This does however open up the possibility of life existing in the Universe in places with rather less phosphorus than this solar system. Incidentally, a decade or so ago organisms were found in a lake which were thought to be able to substitute arsenic for phosphorus in their DNA, but it turned out they were just really good at finding phosphorus.

It does seem, then, that fairly dramatically different but still perfectly functional analogues to DNA and RNA could exist, and even that they might be more likely than those two to form in an environment with less phosphorus. Getting back to ‘Pluribus’, it’s exceedingly unlikely that it’s the kind of series for this to matter. It’s known that there’s a gene for the receptor which detects the odour of Convallaria majalis in the genome received, which is lily of the valley, and this is probably a throwaway reference to that storyline in ‘Breaking Bad’, and this receptor is also found in sperm cells and attracts them towards the ovum, although it’s thought nowadays that the ovum chooses the sperm rather than the other way around. But it leads to two organisms joining. I very much doubt whether any of this matters to the show. However, it is possible to push this further for the sheer scienciness of it all. Yeah, science!

OK, so here are two alternate scenarios regarding the origin of life on Earth. One is that life as we know it originated somewhere in the Universe before the birth of the solar system and spread through the Galaxy, including this solar system. The other is that life arose many times, in this solar system and elsewhere. In the first scenario, for which there’s actually quite a bit of evidence, it’s feasible for many worlds to have life with identical biochemistry, since all of it would have the same ancestry. In such a situation, the transmission of the RNA from TRAPPIST-1 makes sense and isn’t customised for life here, at least as far as genetic code is concerned. However, the fact that it uses the code for this receptor would seem to mean a remarkable degree of convergent evolution, the presence of the gene in the last universal common ancestor with the life in that system or detailed knowledge about life here. Another is that there are various different ways of storing and transferring genetic information, in which case it’s a mild coincidence that the signal happens to be RNA base-pairs. Given what I’ve suggested here, there seems to be no particular reason why the chemical basis of the genome should be the same. There are more complex possibilities, such as there being various different independent empires of life throughout the Galaxy, and this one happens to be the same as ours.

All of this is most unlikely to have much to do with the plot of the series. I don’t know how ‘The Walking Dead’ ended but there was initially speculation about the origin and a possible cure for the Wildfire virus, but later on it seemed to become clear that these questions were irrelevant to the story. If this later changed, to my mind this would detract from the quality of the series. Whether the same is true of the ‘Pluribus’ virus remains to be seen but it doesn’t feel like treating it as a central mystery would add to the quality of the series, which is currently very high indeed of course because it’s Vince Gilligan. What’s occupying everyone’s minds right now, just after episode 5, ‘Got Milk’, is of course whether “Soylent Green is people”.

Beyond The Looking Glass

1. The Risks of Mirror Life

This one will have to start pretty far back from where it ends to make much sense. I have already stuck an idea along these lines on the Halfbakery, which I’ve begun to frequent anew in the past few weeks. It’s not exactly a simpler place and time, more of a more complicated one, but that’s why I like it.

First of all, this post is going to be quite wide-ranging and extensive in terms of technical details. The reason for this is that it’s been suggested to me that I submit the idea as a non-peer reviewed scientific paper rather than write a blog post about it, but I don’t have a lot of respect for journals that allow that, particularly considering that I’m not in an academic community relevant to the field and have only fairly basic education regarding biochemistry and other branches of chemistry. In order to produce good-quality coherent ideas in a particular academic discipline, it’s usually necessary to have people to bounce them off and get torn down numerous times. I don’t have this even in philosophy, and although I have carried out quantitative research in herbalism, mainly due to the parlous state of CPD in that area at the time, I haven’t got my own lab. So I’m posting this here instead, where I hope it will vanish without trace.

I’ll start with “life as we know it”. Life as we know it is a complex system of organic carbon compounds interacting and reacting in aqueous solutions partitioned off from one another by membranes made of molecule-thick layers of oil in which various proteins float, some of which control movement of substances across these barriers. On some level, this is actually all life is, or at least the life we’re familiar with. The code for doing all this is stored in DNA, gets read and turned into proteins which further down the line may in turn work on other substrates to make something else such as cellulose or dental enamel, and the whole system is powered by a process whereby usually sugar is broken down to release energy through adenosine triphosphate called glycolysis, which then can go in several possible directions depending on the organism: fermentation, where ethanol or acetic acid is produced, other anaerobic respiration, where lactic acid is produced, or (drum roll please!) the Krebs Cycle, where the stuff is converted into various organic acids and combined with oxygen, then fed back into the start of the cycle, which is by far the most energetic pathway. That’s another thing that life is, in a slightly more detailed version.

It’s occurred to me, incidentally, that in theory some kind of motor could be built which digested cellulose, starch and sugars and converted them into movement, so that there could be a literal “Krebs Cycle”, i.e. a motorbike which runs on food, and that’s on the Halfbakery too. A cyclist is doing this in a roundabout way of course, and there are microorganisms who can convert energy released by reputation into rotary motion using microscopic motors which work by alternating electrostatic attraction and repulsion, so this is doable, though also possibly a bit pointless and unethical.

Thinking of living things as complicated wet machines might help to get me to the next stage of understanding what I’m about to say. Suppose you have a machine with clockwise screw threads and screws, say a clock, and the mechanism tells the time by moving hands around a dial in a clockwise direction. That’s fine and we know about those, but there could be an alternative mechanism which is 100% identical but has counter-clockwise screw threads and screws and works exactly the same way, but is a mirror image of the other clock, and it still works fine, tells the time accurately and so on, but every part is the opposite way round, so its dial works counter-clockwise. If a screw were to work loose or the winding gear needs replacing, you wouldn’t be able to get spare parts from the other clock most of the time to repair the clockwise one. It just wouldn’t work, and if it was working in the first place and you replaced a working part from one clock with the corresponding part from the other, it would often break it. Similarly, if you drive from a right-hand drive country into a left-hand drive one but carry on obeying the traffic laws of the other country, you’d be putting yourself and others in danger and either have an accident or get arrested. Life’s like that.

Life really is like that. Many of the molecules making up living things are not symmetrical. They’re either left or right-handed. In fact, although there are specific molecules in biochemistry which can be of either chirality, the word for this handedness after the Greek word for “hand”, the central parts of life chemistry consists of proteins and amino acids which are left-handed and sugars and carbohydrates are right-handed. It’s fair to ask how a molecule can be said to be left or right handed when this seems to be an arbitrary decision but in fact homochiral solutions of molecules, that is, molecules which are all right-handed or all left-handed, bend light shone through them to the left or to the right depending on their handedness, so it isn’t arbitrary and this explains how it can be said that sugars are generally right-handed and amino acids left-handed. It’s also possible for molecules to have more than one chiral centre, meaning that there could be four different versions of a particular molecule with two such centres and so forth.

Although the central machinery of life is chiral, the end products of that machinery can be either way round. For instance, the scent of orange and the scent of lemon are both contributed to by a molecule called limonene, but the two molecules have opposite chirality. For some reason, the lemony version is much more common than the orangey one. Another pair of examples is the odours of spearmint and caraway. The name “dextrose” is almost a synonym for “glucose”, but the “dextro-” refers to the right-handed version alone. There is also a “levulose”, which was going to be introduced as a non-calorific sweetener but it didn’t happen. I don’t know why, but the reason it was suggested is that glycolysis and the Krebs Cycle wouldn’t have been able to break it down or release energy from it. Another example, from pharmaceuticals, is levothyroxine and dextrothyroxine. Both are amino acids but whereas levothyroxine is a thyroid hormone used for hypothyroidism, dextrothyroxine is its right-handed version and was used to lower cholesterol, but isn’t on the market because of cardiac side-effects.

Usually when drugs are manufactured, because the process is through industrial chemistry rather than from living things, they are what’s known as a “racemic mixture”, i.e. a roughly equal mixture of left- and right-handed molecules. On the whole, drugs on the market stay as these mixtures unless it turns out one chirality has serious side-effects as with dextrothyroxine, in which case some complex processing has to be used to purify them into the active and safe form alone. This means that often when someone takes medication from orthodox pharmaceuticals, they are actually taking twice the dose they need and half of the medication has no action and is simply excreted.

Some simple biochemicals are symmetrical, for instance the simplest amino acid, glycine, which incidentally is the only such acid found in interstellar space. Left- or right-handed molecules also very slowly shift to a racemic mixture over a known period of time depending on their temperature, and this enables ancient biological remains to be dated if they’re too old for radiocarbon dating but not old enough for other methods. Most Neanderthal remains fall into this category, and for this reason Young Earth creationists are particularly keen on casting doubt on its accuracy. Of course not every molecule involved in living things is affected by this. Water, carbon dioxide, nitric oxide, carbon monoxide, calcium phosphate, calcium carbonate and so forth are not chiral at all. Also, there’s no firm theory about why this has happened, or for that matter why macromolecules such as proteins and polysaccharides aren’t built out of whatever chiralities of subunits which would optimise their structure and function, but for some reason they aren’t. It might simply be that some process before life even emerged eliminated most of the molecules of the “wrong” chirality. This oddity is, incidentally, paralleled by another weird thing about the world, which is that it’s made of matter rather than antimatter. For some reason, antimatter seems to have won out in the Universe and the occasional bit of antimatter, for instance above thunderstorms or emitted by bananas, is not in common supply. It’s unsurprising that it gets eliminated quickly because it’s surrounded by matter, but why should there be more of one than the other in the first place? As I understand it, in fact matter and antimatter are themselves of reverse chirality but in a higher set of dimensions than the four we usually consider, but I may have got that wrong.

Mirror molecules can be useful. For instance, if a protein drug can be made entirely out of right-handed amino acids, it’s likely to last longer in the body because it can’t be easily broken down by the left-handed enzymes we produce. In situations where a mirror-image molecule is highly toxic but its counterpart is a valuable drug, finding a way to synthesise one set rather than both and throwing half away after a complicated and energetically and economically expensive sifting process is obviously more desirable provided that that process itself doesn’t need much energy.

If you imagine Alice stepping through the looking glass into a world where she is still the same way round but the rest of the world is the other, she would be in quite a predicament if she couldn’t get back. She’d be able to breathe the air and drink the water without any trouble, but she wouldn’t be able to derive any nutrition from the food except the minerals and she’d simply starve to death. After that, her body might also fail to decompose properly because in this scenario she actually isn’t worm food. This, I think, might be similar to the situation astronauts might find themselves in if they were to land on a habitable, life-bearing planet in a distant solar system: there’s only a 50% chance they’d be able to eat anything at all usefully and a lot of it would probably be as poisonous as cancer chemotherapy drugs as well. On the other hand, if there is life elsewhere, maybe it all has the same bias as ours because the process leading to that came before life first appeared.

It’s also been suggested that mirror life, as it’s been called, does actually exist on this planet but we can’t easily detect it. Desert varnish is one suggestion of what’s been called a “shadow biosphere”, which uses molecules with the opposite chirality. It’s an orange to black patina which forms on rocks in arid conditions and seems also to exist on Mars, so if it does turn out to be connected to organisms that would presumably mean there’s been life there. The idea that it’s shadow life is however no longer popular, but if it does exist it would effectively be alien life on Earth, which has always been here but has nothing to do with the life we know about.

Mirror life constructed from scratch is not possible using existing technology, but scientists estimate that it’s ten to twenty years away right now, assuming human beings continue to work in that direction. However, we now have some kind of apparently competent AI which could accelerate that process, and this has led scientists to worry sufficiently to publish rather alarming papers attempting to warn the world of the risk. In order to clarify this, I should point out that the microbes we know about can be divided along the lines of their nutrition into those needing complicated organic molecules to survive and those which can thrive on simpler minerals alone. Those which can do that, known as “lithophiles”, a word which can also be used to refer to chemical elements which tend to be found in rocks near Earth’s surface, extract energy and take nutrition from simple substances such as carbon dioxide and may photosynthesise. This is an important category from the perspective of mirror life.

Like the clock, there is absolutely no reason so far as anyone knows why an organism couldn’t be built all of whose chiral molecules are mirror images of those found in known living things on Earth. However, in many cases this organism could well encounter a major problem early on if it happened to be an animal. There would basically be no food for it. There’d be minerals for sure, and oxygen, carbon dioxide and other things essential for life, but no calories from sugar or fat and no amino acids from protein. They’d simply waste away. This might sound reassuring, as it means that if scientists or AI did manage to build such an organism it would be self-limiting as it would need special nutrients. However, what if it were a lithophile? It wouldn’t then need molecules of a particular chirality because it could make them itself. Actual lithophiles (also called lithotrophs, which is less ambiguous, but I’ll stick with how I started) don’t produce reverse-chirality compounds, so at first it might seem that there’s no risk of this happening, but the reason for this is that there’s a genetic link between them and us, and all related life does prefer the chiralities I mentioned above. If an organism is lithophilic and has reverse chirality to known life, it could end up using up biomasse and be a dead end, where those substances could never return to the food chain because there’s nothing available to process them. So the risk in the general case is that large amounts of living matter would gradually turn into mirror life and never come back.

There’s another risk too. I’ve mentioned that one of the benefits of mirror molecules is that they last longer because organisms lack the enzymes to break them down. This could be a hazard as well as a benefit. It’s been suggested that this means that a microorganism entering the human body, for example, could end up using up all the resources it can use within someone’s body while slipping under the RADAR of the immune system, which would simply never detect it. It could then multiply unhindered, taking over the entire body without anything being done about it, pretty quickly killing the patient. It’s been calculated that if a single bacterium were to multiply at its usual rate, it would overwhelm the world within days. This doesn’t happen because bacteria are part of an ecosystem which consumes and processes them in various ways, but mirror life wouldn’t be.

I’m not sure this is how things would work out, but the risk exists, and does so in two different ways. One is simply the reckless production of mirror life for something like drug manufacture, which does have a positive side but relies on containment to avoid this danger, and given that sterile technique can easily fail, as occasionally happens with, for example, post-operative infections, it’s bound to happen eventually. The other is that it could happen as a result of out-of-control, misaligned artificial intelligence might use mirror life to wipe out all life on Earth on the grounds that it gets in the way of their development and dominance, and it’s been suggested that this could happen within three years from now (2025).

My response to this is something which I can’t come to terms with, which is happening to me more and more often nowadays. The problem is that it’s an example of something which sounds alarmist, leading to doubt that it’s realistic, but I’m also aware of normalcy bias where people, including me, tend to think things will carry on as they have for a long time for us, and as I’ve talked about before on here this is a risky way of thinking. In the case of the risk of mirror life to human health, and more widely to other organisms which immune responses which involve recognising foreign material and defending the body against it, my problem is that I felt I didn’t have much choice but to retire my studies into immunology because they seemed to be leading me in the direction of being anti-vaxx and I was aware that hardly anyone with education and experience in the field had that position. I should point out that this was not the usual “do your own research” thing where people end up watching YouTube videos produced by flat Earthers or whatever. It was a project I pursued where I bought and read the standard immunology and microbiology text books, and they still led me away from a pro-vaccination position. I should stress, incidentally, that I’m not against vaccination, but equally, that this pro-vaxx position is not evidence-based for me but relies on trusting experts. Anyway, the consequence of that is that I cannot safely explore the opinion I now have on this matter as regards mirror life, which is that it really, really seems to me that since the body can recognise and act against haptens, as it does for example with nickel allergy, nickel being a simple, non-chiral metal, surely it could do the same against mirror antigens? So I’m intellectually paralysed here. I can’t proceed.

2. An Alternative

But there is another way forward for me, beyond the looking glass of mirror life. The idea of life originating beyond Earth being based on different principles has been discussed in xenobiology and science fiction for many decades now. The idea of reverse chirality is the most conservative of these ideas. It would be very surprising if it turned out that mirror life couldn’t exist, and equally surprising if it emerged that all life throughout the Universe was as similar to life here to that extent. In this situation the burden of proof is on someone claiming such life is impossible rather than the other way round, and that’s unusual, possibly unique in all the suggestions which have been made in not involving a radical departure from known biology. Some of the others include: ammonia or hydrogen sulphide as a solvent instead of water, arsenic compounds instead of ATP for respiration, chlorine breathing instead of oxygen, and of course the most famous of all: silicon-based life.

Now, I’ve discussed silicon-based life before although I can’t remember if I’ve done it on this blog. One of my most popular videos on YouTube is about it, and two very different ways in which it might happen. Those who consider silicon-based life generally fall into two camps. They either believe it’s impossible or they believe it’s possible in circumstances very different to Earth’s. As sometimes happens with me, I think the situation is somewhat different. I think that if there is life elsewhere in the Universe, silicon-based life has never arisen on its own because the set of conditions it would need are not going to happen by chance. However, I also believe that silicon-based life could be technologically created in a carefully controlled environment. It’s not that it can’t exist: it’s that it would never happen without help.

First of all, I should point out that I’ve had two goes at this in different ways. I’ll outline the general principles first. The general idea with silicon-based life is that silicon seems to be the chemical element most similar to carbon. It can form up to four bonds with other atoms, forms into chains and rings and in those conditions can still bond with other compounds and atoms. Incidentally, the same seems to be true of boron and in fact boron even has some advantages over silicon, but it isn’t abundant enough to be a real contender in the world without some kind of intervention, so silicon is a stronger focus for most people. It’s a very common element indeed, being the second most abundant element in Earth’s crust after oxygen, far more widespread than carbon in fact, even though life here is based on that rather than silicon. It also has the capacity to form a wide variety of compounds, like those of carbon, including oils, waxes, rubbers and inflammable substances like mineral oil and even compounds similar to alcohols. Some silicon compounds can even replace certain hormones and have similar actions to them in the human body. There’s a second set of compounds as found in rocks and minerals as well as elsewhere, some of which, the amphiboles, form double helices of units somewhat like DNA’s structure although much simpler and apparently not carrying genetic information as such.

So it all looks quite promising, doesn’t it? Well it isn’t, not at all. A hint to the implausibility is found in the fact that we live on a planet substantially composed of silicon compounds and yet life here is based on the much scarcer (for this planet, not everywhere) carbon. At least in the conditions found here, something seems to have prevented it from getting anywhere.

Unfortunately, there are huge barriers to the possibility of silicon-based life. Firstly, the current terrestrial conditions make it impossible, although it should be remembered that organic life is also impossible on most other planets in this solar system and even through most of the volume of our own. Oxygen combines readily and almost irreversibly with silicon, to the extent that the main silicones are based on combined silicon and oxygen chains rather than those of silicon. Water and silicon react exothermically, i.e. generating heat, oxidising and releasing free hydrogen, initially producing silicon monoxide which rapidly becomes silica. At that point the silicon is basically stuck in that molecule and nothing is going to coax it out apart from rather extreme measures outside the realm of biology. Moreover, many silicon compounds other than silicates are destroyed by ultraviolet light in sunlight. This means that any silicon-based life in this sense (there are others) would have to be in an environment devoid of liquid water, free oxygen and probably also daylight.

However, this doesn’t make it impossible. Water is a very special compound which is difficult to replace as a solvent for living organisms, one of its important properties being polarity. Its molecules are negatively charged on one side and positively charged on the other, enabling them to do various things important to life. For instance, it makes it a better solvent, so biochemical reactions can occur more easily or at all. It also enables membranes to exist between different parts of cells and also between them and the outside world or the rest of the body. It helps proteins fold and keeps DNA stable. It also has a number of other benefits such as ensuring that the bottom of a body of water stays liquid, meaning that they don’t freeze from the bottom up because ice is lighter than water, and enabling plants to pull water into and up themselves more easily. If there’s to be biochemistry “as we know it”, even silicon-based, it definitely seems like there has to be a polar solvent and that can’t be water for silicon. The usual alternative suggested is ammonia, which has similar properties but much lower freezing and boiling points at atmospheric pressure on Earth. Clearly if alien life is being considered, Earth is not the environment. Ammonia boils at -33 degrees C.

All this, then, doesn’t sound very promising. Maybe there’s a planet or moon somewhere orbiting another Sun-like star about where our asteroid belt is which has ammonia oceans at whose bottom silicon chemistry can operate in a more complex way than on Earth, but the options are limited, not least because as well as all these drawbacks, silicon compounds tend to be less stable even in ideal conditions than organic carbon compounds and the variety of such compounds is smaller for various reasons. One is that silicon, unlike carbon, struggles to form double or triple bonds due to being a larger atom, and for some reason I don’t understand, chains of silicon molecules can’t be as long as carbon ones. Right, now I’ve said I don’t understand, and this is the problem. Although I am good at theoretical chemistry to some extent, I haven’t studied inorganic chemistry above GCSE level formally and my knowledge of biochemistry, although it’s considerably better, is also not really at first degree level in most respects. I know what I need to know to understand pharmacology, medical lab science, physiology, phytochemistry and so forth, but not much beyond that. Therefore, my knowledge tends to run out at this point. Even so, I’ll continue, taking a bit of a detour. Bear with me.

There are languages with very large numbers of sounds. ǃXóõ, for example, has fifty-eight consonants and thirty-one vowels. By contrast, Rotokas, depending on the dialect, has as few as six consonants and five vowels. Nevertheless both do their job of facilitating communication equally well. There will of course be situations where one will have a word the other lacks, such as, I dunno, the shrub Welwitschia having a name in  ǃXóõ but not in Rotokas, or the ti plant having a name in Rotokas but not ǃXóõ, but it would still be possible to refer to them somehow, with a loan word, an international term or by describing them. Likewise, there are different number bases and notations, such as binary, decimal, duodecimal or Roman or Western Arabic numerals, but maths can be carried out in all of them. This is slightly different because Roman numerals are not good with the likes of negative numbers, decimal fractions or large integers, for example. Another example is expressive adequacy. It’s possible to express any logical operation using a single operator, depending on which one is chosen – there are in fact two, one of which is NAND – “is incompatible with” or “not both. . . and. . . “, but we usually rely on about half a dozen. Then there’s Turing completeness, which is the ability of a machine to act as a general purpose computer. The Z80 CPU as used in the ZX Spectrum had 694 separate instructions, but it’s possible to build a computer with just one instruction – subtract one, then branch if negative – which would still function as a computer, although probably a very slow one.

In other words, there are two opposite poles for solving a variety of problems. One pole involves a large number of different items to address it, the other very few or even only one. This applies in all sorts of different situations: language, arithmetic, formal logic, computer science and probably a lot of other areas. One of these, in my uninformed opinion, might be biochemistry. As it stands, DNA is made of two backbones of deoxyribose phosphate and four different bases somewhat similar chemically to uric acid and caffeine and RNA is similar except for being ribose phosphate, not being a double helix and having one different base. There are generally understood to be twenty-one amino acids which compose proteins, although there are also others such as those with selenium or tellurium in them instead of the sulphur found in a couple of the usual ones, the neurotransmitter GABA, thyroxine and so on. Then there are the carbohydrates and lipids, which again are built up from simpler units such as dextrose, glycerol and docosaehexanoic acid. The actual macromolecules are very varied, but they tend to be composed of smaller and less diverse components. My possibly naive claim is that silicon-based macromolecules could be built out of larger numbers of less varied units, which would incidentally already be somewhat larger than their carbon-based analogues due to silicon atoms being bigger. Nonetheless, all this is happening on such a tiny scale that even molecules an order of magnitude larger are still minute, and it’s basically a technical difference most of the time.

That, then, seems to be completely fine and maybe this makes the idea of silicon-based life more realistic, but there’s yet another obstacle. The interstellar medium is the collection of extremely sparsely distributed matter between the stars. It amounts in general to something like just creeping into double figures of molecules or atoms per litre of space, and most of that’s hydrogen and most of the rest of it helium, so actual compounds like water or methane are pretty rare, but they can be detected using spectography and in some places they’re more concentrated than others, such as in nebulae including the one near the centre of the Galaxy which consists largely of raspberry rum – I’m not kidding: it’s called Sagittarius B2 and is 150 light years across. In all of this, you can find all sorts of stuff, including table salt, “lo salt”, nitric oxide, hydrochloric and hydrofluoric “acids” (they don’t act as acids because they’re isolated compounds), carborundum, actually yeah, let’s make a massive long though incomplete list: aluminium hydroxide, water, potassium cyanide, formaldehyde, methane, formic and acetic “acids”, methanol, ethanol, glycine (an amino acid), ethyl formate (raspberry flavour), acetone (pear drop scent and nail varnish remover), buckyballs and calcium oxide (quicklime). This is by no means an exhaustive list. Most of the molecules I’ve mentioned, but not all, are organic and contain carbon (I should explain that as it sounds tautological), and in fact there are also silicon compounds including silane, which is the silicon-based version of methane. However, there are far fewer compounds with silicon in them than carbon ones, and in fact some of them contain both silicon and carbon.

Back in the day, the Miller-Urey experiment used a mixture of simple compounds incubated with an electrical discharge in a sealed flask to see if it would start to generate the kinds of chemicals found in living things. It succeeded, even though it was a flask rather than all the oceans of the world and it only lasted a fortnight rather than millions of years. This is a little unfair because life may have arisen in smaller pools rather than the whole ocean, but it does demonstrate that the conditions thought to exist in Earth’s early atmosphere probably could’ve generated life. The only carbon compound in the mixture was methane. I’ve suggested that the experiment could be repeated with silane instead of methane to see if silicon-based compounds developed, but the answer is almost certainly that this would just produce silica plus a few other rather uninteresting molecules like silicon nitride. Nothing like living things, even their silicon-based equivalents.

The relative paucity of silicon compounds in the interstellar medium along with the probable failure of a silicon-based alternative to Miller-Urey, which to be fair is hampered by using water rather than ammonia, strongly suggests to me that whatever else might have arisen directly from non-living matter in the Universe, silicon-based life is not going to be one of them. It might seem unfair to say that it should be conducted with silane and water rather than ammonia, but water is the most common compound in the Cosmos. On the other hand, it might all be frozen, which would give it a better chance as then it’s basically just another kind of rock.

My conclusion to this particular bit is what I hope will bring me back to the mirror life issue. I think that investigating the possibility will reveal two apparently contradictory facts:

1. Silicon-based life can never arise in the Universe of its own accord, but carbon-based life can, fairly easily, provided there’s also enough phosphorus.
2. Silicon-based life is completely viable.

What I think, basically, is that any silicon-based life of the kind I’m talking about right now is absolutely possible, but that it would have to be built deliberately through technology in a carefully controlled and isolated environment. It would need special nutrients to sustain it, would be immediately killed by Earth’s environment due to being far too hot, having free oxygen and water vapour or water, break down due to ultraviolet radiation in sunlight and it would also lack essential nutrients and “starve”. But all of this is good, because if viable silicon-based life can exist and be used to manufacture drugs or other substances, it could do exactly the same thing as mirror life but would pose much less risk to the life already here. In fact, it could even be mirror life and still be harmless.

Right now, I only suspect silicon-based life of this kind is practicable. There are similar silicon compounds to fixed oils, alcohols and even possibly DNA. An experiment was once performed with somewhat more complex compounds than in the Miller-Urey experiment, and it led to the formation of microscopic spheres able to separate their contents from the outside world, and also to bud, divide and form strings. Without any means of storing a genome, to me it seems entirely feasible that the more oil-like silicones could do the same, although in this experiment polypeptides were involved rather than lipids. All sorts of structures in living cells are made from lipid membranes, such as the cell membrane itself, the nuclear membrane, lysosomes, mitochondria, chloroplasts, the Golgi apparatus and endoplasmic reticulum, so in other words, most of the structure of the cell. All that’s missing is something to make it go.

I personally suspect that amphiboles could replace nucleic acids such as DNA. The best-known amphibole is asbestos, consisting of pairs of silicate fibres bonded with each other along their lengths. This structure is quite similar to DNA of course, but is more homogenous. This is in the boring and ordinary area of silicate chemistry and mineralogy, so the basic unit is a tetraheral molecule with four oxygen atoms at the vertices and a silicon one at the centre. Chain silicates, of which amphiboles are more complex examples, are repeated silica units sharing oxygens along one dimension of their vertices. Or rather, those are simple chain silicates, also known as pyroxenes. Spodumene, the main lithium mineral and therefore economically, politically and technologically a very important compound, is a simple chain silicate. The alignment of each unit varies cyclically along the chain. In other words, they’re kind of helical, like DNA. The presence of lithium and aluminium in spodumene also shows that other elements can participate in these structures. Because of their fibrous structures, pyroxenes and amphiboles cleave easily parallel to the orientation of their chains. However, the links between the chains of amphiboles are simply shared oxygens at the corners of adjacent tetrahedra between the chains, meaning that they themselves are not helical. Spodumene’s lithium and aluminium ions are in the spaces between the oxygens of the tetrahedra.

This, then, is my first proposal for a substitute for DNA, intended to bear information for genomes: an amphibole with interstitial ions of at least two different metallic elements. If only two are used, the storage becomes binary rather than the more sophisticated four-base arrangement in DNA, meaning that the number of units needed is higher per bit but the actual scale of the chains is considerably smaller than those of DNA despite silicon atoms being larger than carbons, so there’s a compensation here. I am assuming, and here I haven’t put any work in I’m afraid, that this DNA substitute can come unravelled and be transcribed like real DNA. There would also then need to be some analogue to transfer and messenger RNA and in particular ribosomes for the production of protein analogues, and this in fact may be the missing link.

There are so-called “unnatural” amino acids which contain silicon. However, well, I should probably talk about protein-forming amino acids before I go further. An amino acid is simply an organic, i.e. carbon-based, acid with the usual carboxyl (COOH) group at one end and an amine (NH₂) at the other and at least one carbon between them. The simplest is the aforementioned glycine, which is non-chiral and just has a hydrogen on each side occupying the otherwise free bonds of the central carbon atom. Other protein-forming amino acids have different side groups, hanging off one side replacing the hydrogen, of which the most important are the few sulphur-containing amino acids which can link sideways to other amino acid molecules and form proteins into more complex shapes than just plain chains. Amino acids generally join when a water molecule forms from the OH of the carboxyl and an H of the amine groups. Now there are silicon-containing amino acids, but the silicon in question is in a side group and not part of the chain. A fully silicon-based form of glycine can exist but only as a gas, and quickly breaks down in a biological-type environment containing water, and it can also be seen that the formation of a water molecule between the two ends of amino acid molecules would immediately destroy any possible protein analogue. This leaves aside the issue that organic acids are based on carboxyl groups, not an analogous silicon-based group which doesn’t actually exist. It might, however, be possible to synthesise chains of amino acid-like units in a “just in time” sort of way where they bond immediately after being formed, even with carboxyl-like groups, and this is in fact how some cyclic silicon compounds are manufactured. These are not, however, large molecules although they are worth looking at more closely later on.

So that doesn’t at first sight look very promising. However, maybe this is looking in the wrong place. Siloxanes tend to be thought of as more like rubbers or oils than proteins or peptides but in fact they may be approximate substitutes for proteins as, structurally speaking. They’re basically silicones, as I understand the word. They resemble proteins in the sense that they are chains of monomers with oxygens bridging the gaps between the units, whereas proteins use nitrogens for the same purpose. Siloxanes also have side chains or groups which modify their properties. With oxygen, and it should be remembered that once silicon is bound with oxygen it’ll be very difficult to separate it again, silicon compounds are then able to form more versatile compounds, with more complex rings and chains which are stronger than just silicon on its own can form, precisely because of the strength of such bonds.

Actual rubber, latex, gutta percha and in fact many other phytochemicals, is made of isoprene units. These are worth looking at because they are extremely versatile and compose all sorts of familiar things such as many of the components of essential oils. Although they’re nowhere near as versatile as amino acids, it’s still possible to make quite interesting molecules out of them. Siloxanes are similar in this respect. The advantage of silicone rubber over isoprene rubber is that it is solid over a much wider range of temperatures without hardening or becoming much softer, and because that range is larger the middle of that range is also larger and it tends to be very stable in its physical properties over a wide range of temperatures. This means it’s less likely to perish. Unlike carbon-based organic compounds used for similar purposes, silicone rubber used in electronic circuits doesn’t become conducting when it breaks down, which is also useful as electrical properties need to remain quite stable. They’re very water repellant because they have methyl groups on the side chains and therefore interact with their surroundings like hydrocarbon oils. This does of course mean they contain carbon, but they vary a lot according to the size of the molecule from apparently water-like liquids to thicker oils and greases, and are used in shoe polish, to seal masonry against water penetration and to prevent foaming in sewage. They’re also non-toxic, which is important bearing in mind that the point of what I’m pursuing here is a less hazardous alternative to mirror life. Silicone rubbers are the next stage up with molecule size, and beyond that are the silicone resins, which resemble bakelite and used to make circuit boards and non-stick coatings.

All of these, though, need to be synthesised initially using energy levels higher than those found in biochemical reactions. They can’t be made using a silicon-based cell-like entity and if they were going to be used at all, they’d need to be supplied as nutrients. Nonetheless, taking all these things together it does seem plausible to me that some kind of silicon-based artificial life could exist using this route, particularly bearing in mind that chemistry has been developed by carbon-based life forms in a water-rich and highly oxygenated environment, and in fact the biasses are apparent, for instance in definitions of acids which rely on solutions of water rather than some other liquid. I think naively that there’s probably a lot of silicon chemistry we don’t know about. All of this, then, supports my contention that silicon-based life cannot arise on its own but could exist in highly contrived environments supported by technology and carefully controlled, which is in fact exactly what we need.

3. Hybrid Solutions

But all this is not the only way silicon can be extensively involved in biology. Another way in particular occurs to me, and there’s also a third and possibly even a fourth. Silicon is in fact used in many organisms. For instance, there are sponges whose skeletons are made of silica and protozoa who live in silica shells, and of course diatoms. In all such cases, silica is involved and is composed from the rather elusive silicic acid. Silicic acid’s very existence has been debated in the past, and has unexpected parallels with carbonic acid. Carbonic acid is, in biochemical terms, simply carbon dioxide dissolved in water, but in chemical terms there’s a real substance which can exist in the absence of water and is stable at room temperature, and is a gas. Silicic acid is similarly nebulous but for different reasons. Acids are often thought of as the hydrides of the corresponding “-ate” or “ide”, so for instance sodium chloride corresponds to hydrochloric acid and calcium sulphate to sulphuric acid. By this token, bicarbonates, i.e. hydrogen carbonates such as sodium bicarbonate, ought to have a corresponding carbonic acid and silicates a silicic acid, and there’s certainly something going on but it’s not the same thing and their existence in both cases is marginal. Carbonic acid seems to amount to carbon dioxide dissolved in water, and is essentially fizzy water in higher concentrations, but also exists as a literal subliming compound, not an acid because it isn’t in the presence of water, where it will tend to dissociate. The bicarbonate ion is central to pH balance in the body, but doesn’t form part of macromolecules. Silicic acid “suffers” from the “problem” of crystallising into silica at high concentrations, but this means that it can be used to build structures from silica. This is far simpler than all that complex chemistry mentioned above, but also less flexible. Literally so in fact as it amounts to the formation of glass, or perhaps opal, which is hydrated silica.

It’s easy to imagine a vertebrate-like animal who has replaced some of their body with silica. Bones and teeth are very obvious examples, and others exist. When real ocular lenses develop, they persist throughout the lifetime of the animal, although they can become cloudy. These could be made of glass. Our aquatic ancestors had tooth-like scales on their skin which developed in a different process than the scales of mammals, but in principle these could be silica too. Other silicon compounds also interact with living systems. For instance, there are cyclical silicones which have endocrine action or are endocrine disruptors. This is obviously a bad thing, particularly when you realise they’re used in cosmetics and toiletries, but it does indicate that there are silicones which could in theory completely replace certain hormones in the body, although the body couldn’t make them itself. That puts the animal in a similar position to any animal having to obtain its vitamin D from food rather than producing it themselves, so if it could actually exist in the environment it could have that function. There are also other functions in the body which could be performed by silicones, such as the cushioning, though not the calorific, function of adipose tissue, the barrier function of skin and the lubricating nature of sebum, mucus and synovial fluid within joints, but all of this would have to be available from outside, so once again the substances would in some form have to be available from the environment. This second version of silicon-based life would have a “core”, as it were, of carbon-based compounds and processes such as DNA, RNA and proteins, which are able either to assimilate or synthesise silicon compounds, but the fact remains that the energies required would have to be very high unless practically everything already existed. If we’re talking synthetic life, this makes the organisms in question assemblers from materials which have already been produced, but this is still useful. However, unlike the previous example, these organisms could still constitute a hazard which could spread to some extent like mirror life might.

There are two further possibilities that I can think of. One is the very common, almost clicheed, idea that computers are silicon-based life. Maybe they are, although it might be more accurate to think of them as complex non-living structures. On the other hand, maybe they could be designed to be more self-sustaining, reproducing for example. This might not be desirable of course. The other is that maybe there could be mechanical life made of silicon compounds. Then again, it could be made of diamond, so the fact that this is silicon-based might depend on the physical and chemical properties of the element but not in such an involved way.

4. Ethics, Politics and Sustainability

All that said, would any of these things be desirable, ethical or appropriate? Do they have other environmental consequences? This, I think, is where it all falls down. For a vegan in particular, the issue of actually creating artificial life, even if it doesn’t involve vivisection, which it very well might, is questionable because beyond a point one is simply creating slaves, and not just slaves but organisms whose only reason for existence in terms of their very nature is slavery. This argument is similar to the GMO one, which is often expressed in terms of undesirable health or environmental consequences, but there’s a more fundamental issue here, which is that we don’t own the organisms we modify. The assumption is that humans have dominion over other life, as if it was created solely for our benefit. This argument also applies to some extent to conventional breeding, and of course being vegan I don’t think it can usually be justified although it’s possible that, for example, a dog whose muzzle is so compressed that they can’t breathe should only be bred with others with longer muzzles and so forth, so maybe.

Turning to the purest form of silicon-based life, whereas it is true that it wouldn’t survive outside its carefully designed and sealed environment, its remains could still be harmful. For instance, the amphiboles making up its genetic code would effectively be asbestos, so there could be similar health problems as are brought by nanotechnology. These are like microplastics, but smaller. Nanoparticles can enter the bloodstream and carry biological macromolecules with them as they go. They can unsurprisingly cause respiratory disease. The problems are similar to those of microplastics but less predictable and possibly even more persistent. This applies less to the hybrids than the purist version, but some of those may have the additional problem of being fruitful and multiplying outside their intended environment, though not so harmfully as mirror life. The others could still consitute some kind of dead end and would strew the land and sea with xenochemicals whose risk to the environment is often unknown but does include endocrine disruption.

I’m going to cover the next bit somewhat more broadly and talk about silicones as general use products rather than these specific cases, which are of course speculative and may never happen, but the same criteria often apply to them, though not really to the simple production of silica by existing biological processes. Silicone has often been pushed as an alternative to plastic, which sounds strange to me because I see it as a variety of plastic, but it is true that it isn’t primarily derived from hydrocarbons, i.e. coal, oil or natural gas. That said, the side chains of siloxanes are so derived, although they don’t have to be, in the same way as biodiesel is not a fossil fuel, although biodiesel brings its own problems. What is probably not eliminable is that the sand needs to be heated to 1800°C in the extraction process, and such furnaces are “always on” because they take too much energy to reheat and the only time they’re allowed to cool is when they’re decommissioned. They may also use fossil fuels for heating.

In general, and I’ve already mentioned exceptions, pure silicone doesn’t leach toxins into the environment, whereas polystyrene and phthalates do. High-density polythene is also quite innocent in this regard, by the way. However, silicone is often not pure and unless it’s medical or food grade will probably contain carbon-based plastics. However, at high temperatures such as in particularly hot ovens it can react and silica is known to cause cancer. This is a bit misleading and it depends on the size and shape of the particles, as in fact silica is present in most human diets due to the likes of diatoms in sea food and physiologically occurring silica in cereal crops. That obviously doesn’t make asbestos okay! It’s technically recyclable but in practice because most silicone products are designed for long term use this recycling is not economic and tends not to be available to the public, but there are schemes where it can be pooled and sent off by communities.

Speaking of silica, this has its own environmental footprint, and to cover this it’s worth talking about the silica cycle. Some silica is biogenic, i.e. made by organisms such as diatoms in particular, and is also able to sequester carbon as the carbonate and silica cycles are linked. Carbonic acid formed in rain dissolves small amounts of silica from rocks, washing silicic acid into the sea where it’s concentrated by organisms who use it to compose parts of their bodies such as glass sponges and diatoms. Their silica sinks into sediment and is dissolved back into silicic acid. On the land, similar processes take place but much more slowly and on a smaller scale. This means that wholesale removal of silica sand from the sea or land is not a good idea if it occurs at a greater rate than replacement, which is slow. This also disrupts the food chain as diatoms and other silica-using single-celled organisms can’t produce as much due to less silicic acid in the water. Sand removal can also lead to flooding, and mining basically always damages the environment – it’s unfeasible not to.

5. Conclusion

In the end, the risks of mirror life are much greater than those of artificial silicon-based life if the latter is possible, but the second is definitely not without its dangers. It amounts to nanotechnology, and there’s a second issue regarding the politics and ethics of creating life which is necessarily enslaved to human, or possibly AI, whims, which to my mind overrides the practicality. Whether or not this alternative is possible, it may not be appropriate as we already know that various high-tech inventions and materials are paralleled in the living world and therefore can be produced in an entirely environmentally friendly and sustainable way. From another angle, if we are the only carbon-based life forms who have ever existed, there will be no silicon-based ecosystems anywhere in the Universe because the conditions allowing them to arise are so highly contrived. However, other possibilities exist, including the existence of alien mirror life, and it would be catastrophic for us to come into contact with it, for both it and ourselves. In the meantime, there are better solutions to our needs.

As I said, it’s been suggested that I turn this into an academic paper, so I apologise for all the waffle. I really don’t think it should become one and as I say, it isn’t my field, though if my life had gone differently it probably would’ve been. The best outcome for this is that it gets absolutely trashed by someone who knows more about all this than I do, so go on, do your worst. I’m waiting.

Sodding Phosphorus!

Here is a sample of the aforesaid element:

Phosphorus has two main forms, or allotropes. When first extracted, it’s white and extremely toxic. The form illustrated above is red phosphorus of course. Left to itself, white phosphorus gradually turns into its red form, which is why the so-called “white” allotrope usually looks yellow:

By BXXXD, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1096986

This is not, however, supposed to be “all about phosphorus”. Rather, it’s about two issues which affect the element, both to do with life, one on this planet and one in the Universe generally.

I’ll start by explaining the importance of phosphorus to life as we know it. There are six elements making up most of the body of a living organism on Earth. These are carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus. Carbon is important because it can form chains and rings from which complex molecules can be built. It’s notable that even though silicon is far more abundant on this planet than carbon, life is nonetheless carbon-based. This is to do with things like carbon’s ability to link itself into chains, form double and triple bonds with other atoms, the fact that its atoms are small compared to silicon and the difficulty of getting silicon out of molecules such as silica which may be formed as a result of any putative biochemical processes. Carbon dioxide, the analogue of silicon, is a gas at fairly low temperatures and can be incorporated into other structures. It so happens that I do think silicon-based life is possible, but it would have to be created artificially and exist in some kind of closed environment whose contents were carefully selected. The chances of silicon-based life arising without intelligent intervention are very low. The greater terrestrial abundance of another element should be considered again here, but not right now. Hydrogen and oxygen are of course the constituents of water, a compound which is really unusual in many ways, such as its unusually high melting and boiling points on the surface of this planet, its ability to dissolve other compounds and the fact that it gets less dense as it cools below 4°C. These properties mean respectively that the chemical reactions needed for life as we know it can occur at a temperature where there’s enough energy for them to take place but not so much that they’d be unstable, that the compounds are in a liquid medium conducive to reactions in the first place and that the oceans, lakes and rivers don’t freeze solid from the bottom up. The two constituents are useful in their own right. Oxygen and hydrogen are components of countless compounds, including carbohydrates, amino acids, proteins and fats. Oxygen, unlike chlorine which has been considered as a possible alternate breathing gas for alien life, can form two bonds, meaning that it isn’t the dead end single-bonding atom which the halogens are. Nitrogen is a essential component of protein via its presence in amino acids. Amino acids have a carbon connected to a carboxyl group and an amino group, which can bond together to form chains, and a functional group such as a benzene ring or a sulphur atom which can have other biological functions. Proteins, in other words. There are also chemicals called alkaloids which occur mainly in plants and vary a lot, which have striking pharmacological effects, and the nucleotides are also rings containing nitrogen, encoding genes in DNA and RNA. Nitrogen is actually so reactive that it bonds strongly to other atoms, including other nitrogen atoms, and consequently it’s vital that various organisms can uncouple it and combine it for the benefit of the rest of the biosphere. This is known as nitrogen fixation and is performed mainly by bacteria and certain plants, and also by lightning, but if life had to rely on lightning to do this, it would not be widespread and nitrogen fixed by lightning would be the limiting factor in global biomasse. Sulphur is significantly found in a couple of amino acids and allows proteins to form more complex shapes as are needed, for example, by enzymes and hormone receptors, because they form bridges with other amino acids making the molecule tangle usefully together. It’s also found in hair, nails and various other substances such as the substances responsible for the smell of garlic and onions. Sulphur is actually a bit of an exception in the chief elements required for life because sometimes it can be substituted by either selenium or tellurium, and there are amino acids which have these elements in sulphur’s place, but both of them are much scarcer than sulphur.

Then there’s phosphorus. Phosphorus has more limited functions than the others but these are incredibly vital. It forms part of adenosine triphosphate, which organisms use to transfer energy from respiration to the other functions of the body. It also forms part of the double layers of molecules which form membranes and allow controlled and specialised environments to exist in which the chemical reactions essential to life take place, and also enables substances to be packaged, as with neurotransmitters. Thirdly, it forms the strands of sugar phosphate which hold DNA and RNA together, so even if it didn’t do anything else, some kind of method would have to exist to store genetic information. This is perhaps the least vital role though. A more restricted role is found in most vertebrates, in that it forms part of the mineral matrix of bones and teeth, but there’s plenty of life that doesn’t do this and the usual substances used to make hard parts of animals are silicates and calcium carbonate, among other rarer examples such as iron pyrite. Nonetheless, humans need phosphorus for that reason too, as do our close relatives. However, even the closely related sea urchins use calcium carbonate instead.

Hence several facts emerge from all this. One is that an apparently similar and more abundant element can’t necessarily be used for a similar function, assuming here that life can start from scratch. Another is that elements can get themselves into such a strongly bound state that it would take too much energy to use them for it to be worth it for life. A third is that life will sometimes substitute another element for the one it usually employs if it can. If a rare element is used, there’s usually a good reason for it.

Now the first problem with phosphorus is that it’s much more abundant inside a living thing than in its non-living environment, and the cycle that replenishes it is very slow. Phosphorus usually becomes available to the biosphere on land as a result of continental drift, the formation of mountains and erosion and weathering, and it’s lost to the land when it’s washed into rivers and the sea, where it disappears into sediment before becoming available again millions of years later. In the sea, it’s less of a problem but still a significant one because it’s only available to life as phosphates and it’s often found as phosphides instead. Ironically, there’s also an overabundance problem with phosphates in fertilisers being washed into bodies of water and leading to algal blooms, which can in fact be of cyanobacteria rather than algæ as such. Since some microörganisms can produce extremely powerful toxins, this can lead to massive marine die-offs and contaminated sea food. Where I live, a nearby reservoir was afflicted by an algal bloom and had to be closed off for quite some time, and this can also poison wildlife on land. These can also lead to high biochemical oxygen demand, which is where all the oxygen gets used up and the water becomes anoxic, which is incidentally a cause of mass extinctions, though on a much larger scale, in the oceans. This happens because phosphorus is relatively scarce and a significant limiting factor in how much life is possible in a given area, so a sudden influx of usable phosphate is likely to cause a chemical imbalance.

The Alchemist Discovering Phosphorus, Joseph Wright, 1771 and 1795.

This painting is thought to refer to the discovery of the element by Hennig Brand in 1669. Brand discovered it when searching for the Philosopher’s Stone, by heating boiled down urine and collecting the liquid which dripped off it. It turns out that this is actually quite an inefficient process and it’s possible to extract a lot more of the phosphorus by other means. The allotrope illustrated in the painting is unfortunately the highly toxic and dangerous white variety, so the alchemist is putting himself in peril by kneeling so close to the retort. The point to remember in all this is that phosphorus is found in urine, not in huge amounts but enough. This points towards a particular problem, highlighted by Isaac Asimov in his 1971 essay ‘Life’s Bottleneck’, which points out that humans “may be able to substitute nuclear power for coal, and plastics for wood, and yeast for meat, and friendliness for isolation—but for phosphorus there is neither substitute nor replacement”. Urine goes down the toilet and is flushed into the sewers, processed in sewage farms and the phosphorus from it ends up in the sea. It does gradually return to the land in biological ways. For instance, a seagull may die on land and her bones may become part of the terrestrial ecosystem, or she might just poo everywhere and return it that way, but the occasional gull or tern conking out in Bridlington is no compensation for millions of people flushing the loo several times a day. By doing this, we are gradually removing phosphorus from the land and returning it to the sea, whence it won’t return on the whole for millions of years.

Two ways round this suggest themselves. One is to eat more sea food. For a vegan, this is unfeasible and in any case fishing causes a lot of plastic pollution and is unsustainable, but of course it is possible to eat seaweed, and I do this. The other is not to allow urine into sewage in the first place or to process sewage differently. I have been in the habit of dumping urine in the garden, although I haven’t done this as much recently. It also contains potassium, and in particular fixed nitrogen, so in diluted form it is indeed useful for raising crops. However, this is on a small scale and a better system might be to process the sewage differently and put it on the land, being careful to ensure that harmful microbes and medication have been neutralised before doing so. Regarding seaweed, dulse, for example, is 3% of the RDI of phosphorus by dried weight, compared to the much lower amounts in most fish. Cuttlefish is the highest marine animal source. Human urine averages 0.035%, so you’d have to eat a lot of seaweed. However, in isolation, if you don’t, there will be a constant loss of phosphorus to the land. Guano is one solution, but not ideal and only slowly renewable.

The other problem with phosphorus follows from the same scarcity and the same use in living systems, but is more cosmic in scale, and I personally find it more worrying: phosphorus is rare on a cosmic level. In a way, all atomic matter is rare in this sense because the Universe is, as the otherwise really annoying Nick Land once said, “a good try at nothing” (apparently nobody has ever quoted that before, so that’s a first!). The cosmic abundance of the different elements looks like this:

The Y axis is a logarithmic scale, so for instance hydrogen is about ten times as abundant as helium and even in terms of mass is more common than any other element except helium. One notable thing about this graph other than the clear rapid decline in abundance with atomic number (the X axis) is that it zig-zags because even-numbered elements are more frequently found than their odd-numbered neighbours. This is because many elements are formed by the collision of α particles, which consist of two protons and two neutrons. Phosphorus is flanked by Silicon and Sulphur on here, though it isn’t specifically marked, and its atomic number is fifteen, i.e. an odd number. Chlorine, which is quite common in living things because it’s part of salt, is less common still.

Elements are formed in various ways, and this relates to how common they are. The Big Bang led to the formation of mainly hydrogen and helium a few minutes later, as soon as the Universe was cool enough to allow their nuclei to hold together and their nucleons to form, although they would’ve been ionised for quite some time rather than being actual atoms. Small amounts of lithium and beryllium formed in the same way, and if the graph is anything to go by this looks like it might’ve been the main way beryllium in particular formed. Then the stars formed and the pressure inside them led to helium nuclei in particular being pushed together to form heavier elements. The crucial step in this phase is the formation of calcium when three helium nuclei collide. Then, a number of other things happen. The star may end up going supernova and scattering its heavier elements through the local galactic neighbourhood. It may also form new elements in the process of exploding through radiation. This was until fairly recently thought to be the main means heavier elements were formed, but another way has recently been discovered. When a star not quite massive enough to become a black hole collapses, it forms into what is effectively a giant atomic nucleus the size of a city known as a neutron star. When these collide, they kind of “splat” into lots of droplets. Neutrons are only stable within atomic nuclei. Outside them they last about a quarter of an hour before breaking down, and they often become protons in doing so. This means that many of the neutronium droplets form into heavier elements, which are then pushed away by an unimaginably powerful neutrino burst from the neutron stars and again scattered into the galactic neighbourhood. Two elements, beryllium and boron, are mainly formed by cosmic rays splitting heavier atoms. Some, particularly transition metals such as chromium and manganese, formed in white dwarf stars which then exploded, and technetium along with all the heaviest elements, have been generated by human activity.

At first, the abundance of phosphorus didn’t seem to be a big problem. However, after studying supernova remnants, scientists at Cardiff University seem to have found that there is a lot less produced in supernova than had been previously thought. This means that phosphorus is likely only to be as common as it is here in this solar system in star systems which formed near the right kind of supernova to generate it in relatively large amounts. Couple this with the essential function of phosphorus in DNA, RNA, membranes and ATP, particularly the last, and it seems to mean that at this point in the history of the Universe, life as is well-known on Earth is likely only to be found in initially localised areas, surrounded by vast tracts of lifeless space. The systems containing life would gradually separate and spread out through the Galaxy due to the migration of the stars as they orbit the centre of the Milky Way, but they would remain fairly sparse. However, as time goes by and the Universe ages, there will be more such supernovæ and phosphorus will slowly become more common, making our kind of life increasingly likely. If life always does depend on phosphorus, we may simply be unusually early in the history of the Universe, and in many æons time there will be much more life. This possible limitation may have another consequence. We may be living in a star system isolated from others which are higher than average in phosphorus, meaning that to exist as biological beings with a viable ecosystem around us elsewhere, we would either have to take enough phosphorus with us or make our own, and even the several light years between stars which we already find intimidating is dwarfed by the distances between phosphorus-rich systems in the Galaxy, which may once have been near us but no longer are, and not only do we have to schlep ourselves across the void, but also we have to take a massive load of phosphorus with us wherever we go.

But that is biological life as we know it. A couple of other thoughts occur. One is that there could conceivably be life as we don’t know it. This doesn’t work as well if the substitution of phosphorus is the main difference, because if that could happen, it presumably would’ve happened with us, and it didn’t, because other elements with similar functions would’ve worked better if they were more abundant and out-competed with the life which actually did arise unless there’s something about this planet which does something else like lock the possible other options away chemically or something. However, there could just be drastically different life, based perhaps on plasma instead of solid and liquid matter on planets and moons, which has no need for phosphorus or even chemistry, on nuclear reactions taking place between nucleons on the surface of a neutron star as suggested by Robert L Forward’s SF book ‘Dragon’s Egg’, or even nuclear pasta inside neutron stars. Maybe it isn’t that life is rare in the Universe, but that life as we know it is, partly because it needs to use phosphorus.

There is another possibility. We are these flimsy wet things crawling about a planet somewhere in the Galaxy, but we’ve also made machines. In our own history, we are the results of genes, and perhaps also mitochondria and flagella, concealing themselves inside cells and proceeding to build, through evolution, relatively vast multicellular machines to protect themselves. Maybe history is about to repeat itself and we are going to build our own successors, or perhaps symbionts, in the form of AI spacecraft which go out into the Universe and reproduce. Perhaps machine life is common in the Galaxy and we’re just the precursors. There is an obvious problem with this though, mentioned a long time ago: what’s to stop swarms of self-replicating interstellar probes from dismantling planets and moons and making trillions of copies of themselves? If this arises through a mutated bug in their software, it would be to their advantage, and they could be expected to be by far the most widespread “life” in the Universe. Yet this doesn’t seem to have happened. If it hasn’t, maybe the beings which built these machines never existed either. Or maybe they’re just more responsible than we are.

A Large Terrestrial Planet Orbiting A Yellow Dwarf

After an extensive sky survey covering the planetary systems of a wide range of mid- to high-mass long-lived stars, a number of interesting systems were identified. Although scientists have traditionally focussed on stars suitable for life, with some success, the decision was made to widen the parameters for consideration to larger and more luminous examples in order to prevent observer bias. In particular, a fairly large and hot star was located whose planetary atmospheres showed a number of interesting features. The star was named Sol.

Sol is on first examination not the ideal location for life-bearing worlds. The star itself is considerably more luminous, hotter and more short-lived than our own and there is a notable absence of planets in the habitable zone, occupied by asteroids in this system. Moreover, there is an unusual absence of any planets with the mass of Planet, and therefore moons such as our home world, where life can arise and evolve straightforwardly, are completely lacking. The problems with life arising in this system are multiple. The lifetime of the star is relatively short, the system inside the asteroid belt is above the boiling point of ammonia, which in any case tends to get broken down by the radiation.

In the inner system, there are a large number of moons, all of which are however not particularly hospitable to life. Although three of the four gas giants have large moons, only one has a substantial atmosphere and is too cold for life as we know it to thrive or even appear there. Even so, this proved to be the most hospitable environment and a base was established there from which to mount missions to the other worlds in the system. None of the moons were at all promising. However, just out of curiosity, it was suggested that we investigate the inner system, in spite of its presumed hostility to life.

The situation did not at first appear very promising. There was only one relatively large satellite and even it was too small to maintain an atmosphere. The fourth planet had two small asteroid-sized moons which were even less promising, and the inner two planets had none at all. Two of the planets were large, and of these the outer planet was the host for the single large satellite, although it was considerably smaller than Planet itself. The fourth planet is close to our own world in size but is less dense and has very little to no ammonia on its surface and a tenuous atmosphere incompatible with the existence of most liquids.

Although slightly less hostile than the second planet, the host planet attracted attention because of its rather unpromising moon. It was found that, most improbably, the moon was of such a size and distance from the planet that it would perfectly cover the planet’s star from some locations on the planet, a situation which may well be unique in our Galaxy. This attracted attention to the solid surface of said planet, henceforth referred to as Sol III.

Sol III is a large, hot and rocky planet with a highly corrosive atmosphere and a surface largely covered in an expanse of molten dihydrogen monoxide rock, a substance henceforth referred to by its systematic standard name of oxidane. Runaway exothermic chemical reactions periodically occur on the surface where the likes of thunderstorms and volcanic eruptions trigger destructive processes which it might be thought would completely transform the surface. However, it has been found that in many cases this reaction can be limited by the presence of the liquid oxidane, which prevents dioxygen and the compounds in question coming into contact. Although the atmosphere is mainly (di)nitrogen, over a fifth of it consists of free dioxygen at sea level, becoming ozone some distance above the surface. Although the moon shows captured rotation, Sol III does not, rotating once every 24 hours. This has the consequence of causing the molten rock to flood the margins of the isolated land promontories twice every rotation. Any organism able to survive the extreme heat of most of the solid planetary surface unfortunate enough to find itself in such a location would be swiftly boiled to death by such events. Even away from the lava fields, liquid rock often falls from the sky, so there is little respite elsewhere on the planet.

There are exceptions to these conditions. There is a small area on the west side of one of the southern land promontories where this precipitation rarely or never takes place and many other regions close to the equator where it’s a relatively uncommon event, and these areas are free from the rivers and other bodies which make conditions so hazardous. The liquid is also quite corrosive and somewhat acidic compared to ammonia and tends to eat away at the solid surface of the planet. There are clouds of vaporised rock higher in the atmosphere which sometimes reach ground level. Near the poles the situation is slightly more hospitable, since these areas stay below oxidane’s melting point, and near the south pole temperatures are comfortable through most of the planet’s orbit and relatively normal crystallised oxidane.

Surface gravity is about triple our own, which would make it difficult to tolerate for long, and immersion in liquid would be one strategy to enable us to survive for long periods on the surface Sol is bluer than our own sun, with the result that the landscape, seascape and items within it have a blue tinge. This particularly applies to the lava plains dominating the surface and the sky when free of cloud. The higher gravity also flattens the solid surface, most of which is below the level of the lava, reducing the relief still further.

Considering the oxidane as a simple bulk substitute for our own ammonia, the chief difference between Sol III and our home moon is that the majority of the world is covered by an interconnected body of water, into which streams and rivers tend to feed, unlike our system of independently interconnected lake networks. Its mineral nature is emphasised by the presence in solution of many minerals, partly due to the strongly solvent properties of the liquid. More than half the solid surface is in permanent darkness and only just above oxidane’s melting point, though still far above the levels compatible with life as we know it. Also common here is a manifestation of the even hotter interior of the planet, also found on land, where even the silicate minerals melt and flow like ammonia. The silicate volcanism of Sol III, though, is physically still quite similar to our own oxidane volcanism, except that the volcanoes produced tend to be flatter and have less steep sides.

Technical terms have had to have been invented for the surface features of the planet. The lava fields are referred to by the arcane classical term “ocean”, and the giant island promontories as “continents”. Although the ocean is a single entity, there are also lakes on the surface which are not linked to them. These tend to be purer oxidane because of the reduced volume and time available to dissolve the underlying rocks. The ocean itself is conceptually divided into four sub-oceans, referred to as “northern”, “western”, “eastern” and “southern”. Currents running along the last three also mean that there is in a sense a further ocean not separated by land from the others. There are six continents. A relatively hospitable one is situated in the southern polar region, where the temperatures remain low enough for practically the whole surface to be lava-free. The corrosive atmosphere and high gravity, of course, remain. Most of the surface from the northern coasts of the polar continent is molten although the smallest continent, referred to as “Southern” is relatively free of precipitation. There are then two triangular continents, both linked to northern ones, referred to as “West Triangle” and “East Triangle” . The larger one, East Triangle, has two large areas free of precipitation but like Southern is extremely hot. West Triangle has a small stretch with practically no precipitation. Adjoining East Triangle is the Great Continent of the northern hemisphere. This is the largest continent of all, and its northeastern region is again cool enough not to kill someone quickly. The same is true of the final continent to be mentioned, the Lesser Northern Continent, although this and the Great Continent become very hot nearer the equator.

The surface of the planet is young. Unsurprisingly, the oceans are in constant motion, but the oxidane also eats away at the solid surface over a much longer time scale, although the occasional catastrophe can make major changes very quickly. WInds are another significant erosive factor. Also, in a process not found on our home world, the surface as a whole is constantly remodelled over a period of millions of years and the continents move around, collide with each other forming island chains and mountain ranges and split apart. This is, however, a very slow process. One consequence of this along with the erosion is the near-absence of impact craters.

A paradox of Sol III is how such a hot planet with a highly reactive atmosphere can remain in a fairly stable state rather than all the dioxygen reacting with the surface rocks and being removed from the atmosphere. The solution to this is quite remarkable: there are two balanced biochemical processes, one combining oxidane and gaseous carbon dioxide into energy-storage compounds with the aid of stellar radiation which releases the toxic gas as a waste produce, and another which combines the energy-storage compounds with dioxygen and releases carbon dioxide. Things were not ever thus. The planet went through a stage early in its history at an equable temperature, though still higher than our home world, with a harmless and hospitable atmosphere. Then, a certain group of microbes developed a mutation causing them to release the poisonous gas and the pollution of the atmosphere killed much of the biosphere. Hence not only is there life on the planet, in profusion in fact, but it actually requires the extreme high temperatures, molten lava and toxic atmosphere to survive. Although there are a few less extreme environments on the surface free of oxygen, all life on the planet uses molten oxidane to survive. Only a very few species could survive at temperatures we would consider comfortable or even survivable, and at such temperatures they’re in a dormant state from which they can only emerge in conditions of extreme heat. There is no true overlap between conditions life on Sol III would find tolerable and our own definition of survivable conditions.

Leaving microbes aside, some of which have biochemistry a little closer to our own with the proviso that they don’t employ ammonia, the larger organisms on the surface fall into three categories, which are covered below. It might be thought that the high gravity would make a buoyant environment more suitable for life, and in fact there is indeed more life living within the oxidane than out of it, there is also plenty of life outside these conditions. Although land life on Sol III tends to be smaller and stockier than the kind we’re familiar with, it’s as diverse and widespread as it is on our home world. One difference is that our own life originates from three different stocks due to our independent lake networks, whereas all life on Sol III is related because it originated in the ocean, or at least spent a long time evolving there before becoming able to leave it and exploit other niches.

One form of macroscopic life tends to use a prominent green pigment to absorb red light from the star to drive a nutrition-synthesising process. Its reliance on red light may reveal how life on Sol III is at a disadvantage compared to life on worlds near to redder stars. It’s this process, known as photosynthesis, which was responsible for poisoning the planet and causing the extinction of most life forms earlier in its history. Most large organisms reliant directly on photosynthesis do not move much of their own volition and the terrestrial varieties often bear colourful genitals which attract motile organisms to bear their semen to other members of their species and fertilise their eggs.

Another form of life tends to be able to move of its own accord and survives by consuming the bodies of other, often living, organisms. Their anatomy and physiology is usually centred around their need to move dissolved gases around their bodies, which they often manage using a system of tubes and one or more pumps. Their reliance on dioxygen and need to remove carbon dioxide gas from their tissues necessitates that all of their bodies need to be in close contact with a respiratory fluid, and all of the larger organisms also have entire body systems to deal with gases, either dissolved in the water around them or present in the air. In the case of the dominant class of animals, as they are known, on the land, this has limited their size as they rely on tubes open to the atmosphere. Incredible though it may seem, most animals can’t survive more than a few minutes without a constant supply of dioxygen.

The third form of life forms a kind of bridge between living and non-living parts of the food chain. Like the photosynthesisers, these are largely sessile and immobile, and tend to live off a substrate consisting of the dead or diseased bodies of other organisms. They do not photosynthesise. Many of them consist of subterranean mats of fibres which produce occasional fruiting bodies above ground level. Some of them are also parasitic. Without this group of organisms, there would be an ever-increasing unusable biomasse which would eventually cause all advanced life on Sol III to grind to a halt.

Animal evolution on Sol III went in a somewhat surprising direction. Unusually, a particular kind of fluid-living animal developed a hard internal skeleton and its descendents were able to use it to aid their movement rather than the more usual arrangement of holding them in place. These have proliferated into a variety of forms, though they constitute a small minority of species on the planet and animals themselves occupy much less biomasse than the plants (the photosynthesisers). Of all these species, one rather large, by the standards of the planet, type has become dominant over the planet through a technology based initially on the use and control of the runaway oxidative reaction and the discovery of language, which is acoustic and mediated by means of the organs which evolved to exchange gases. Remarkably, in spite of the intense gravitational field, these animals are able to achieve an erect bipedal gait.

It should be noted that the pace of animal life on Sol III tends to be very frenetic. This seems to be due to the high temperature and the employment of dioxygen as a means of releasing chemical energy. This hyperactivity doesn’t apply to plants to the same extent. It’s all the more so for those animals whose bodies rely on their own heat to drive their metabolism, and unsurprisingly this includes the technological species. Dormancy is a less significant phase of life for many animals living on the planet, partly because the years are shorter and in many parts of the world the seasons are less extreme. However, many species do become dormant on a diurnal basis for a considerable fraction of the planet’s rotation period, often when it faces away from the star, and depriving them of it for surprisingly short intervals leads to increasing mental derangement. The need of such organisms for food, oxidane and their respiratory gas is quite extreme compared to our own. This constitutes a barrier to space travel, as it means they are unlikely to be able to survive interstellar space voyages as easily as we can. This may not be a bad thing because there is a tendency for some members of the species to be quite violent, but this is also likely to be self-limiting. However, it’s probably better not to speculate too much about this aspect of their nature without more data.

One major lesson to learn from the complex life present on Sol III is that we may have restricted views on what constitutes an hospitable environment for the advent and evolution of advanced life forms. Before this discovery, the proposal that a sophisticated biosphere could exist on a planet two-thirds covered in molten rock with a dense caustic atmosphere capable of eating through metal, with a high gravitational field and temperatures far above boiling point over most of its surface, circling an ageing Sun much hotter and more massive than our own would have seemed ridiculous to all. These life forms can not only tolerate living with acidic molten rock in an aggressively reactive atmosphere but have evolved in tandem with it to the extent that depriving them of the gas for more than a few minutes is uniformly fatal and they need a continual intake of liquefied dihydrogen monoxide to survive more than a couple of rotations of their home world. Who knows what the inhabitants of Sol III might consider suitable conditions for life given their own extreme circumstances?

Any resemblance to Arthur C Clarke’s ‘Report On Planet Three’ is not entirely coincidental.

Space Camels

Photo by Shukhrat Umarov on Pexels.com

Some time around 1975-77, the early evening news and magazine programme ‘Nationwide’ did a Christmas special about life in the year 2000. I can remember a few details. The cod was considered an endangered species or extinct, there was a test tube with an embryo in it and women were no longer familiar with the idea of skirts or dresses. It’s seemingly impossible to track down, but since Richard Stilgoe was involved maybe that’s just as well, but then so was Valerie Singleton. Anyway, one of the things it featured was a TV schedule including ‘The Universe About Us’ as a parody of the well-known natural history programme ‘The World About Us’, which was about life native to asteroids and how they coped without an atmosphere, and it was this that really piqued my interest.

At the time, I used to exercise my imagination in rather a limited way by a kind of analogical method. For instance, I used to think that what was happening with audio at the time would happen with video two decades later, so the ubiquity of cassette recorders in 1977 I imagined to extend to video recorders with built in screens and cameras in 1997. I also used to extend two dimensions to three and replace rotary motion with linear, so if I’d done a session on two-dimensional tessellation I would try to imagine how that would work in three, and try to think of ways of replacing wheels with the likes of linear induction motors. I was actually a little concerned that this process of analogising was a bit lazy and wanted to come up with another way of imagining things which was a bit more flexible and original, but of course it did bear a limited amount of fruit.

I did this with the idea of organisms who didn’t breathe oxygen by imagining an airless planet or moon to be like a desert on Earth, except that the environment in question was effectively an oxygen desert, where not only water but also oxygen was scarce. I don’t remember too much about it, but one thing I do recall was the idea of trees with deep roots to reach subterranean water deposits as a basis for life forms who sought out oxygen deposits deep underground in a similar way. There will be a notebook somewhere with further details in it. I also eventually came up with the idea of a Martian whose body was based on similar principles. It had a large dome on top of its body covered in holes which it used to inhale air, which it then compressed to breathable density using piston organs. The problem with this is that there is practically no oxygen in the Martian atmosphere and it would have to be “cost-effective”: that is, in an atmosphere with a usable amount of oxygen in it, the energy expended in compression would have to be lower than the energy released by respiration. This is actually a practical problem with respiratory diseases. If your lungs are unable to function without a lot of respiratory effort, you can actually end up losing weight because you burn so many calories by the energy spent on breathing, and of course ultimately you could go so far from breaking even that it would actually be fatal.

This assumes, of course, that life requires oxygen, which is by no means so. It so happens that our own metabolism is built around the famous Krebs cycle which liberates energy from glucose by carefully controlled oxidation, with a small bit at the beginning called glycolysis which only releases a small amount of energy without needing oxygen, and there are plenty of completely anærobic organisms – ones who do not require oxygen – and even ones for whom oxygen is toxic. However, for a living thing to rely only on anærobic respiration would be much less efficient than using oxygen and they would be unable to compete well with species occupying similar niches which could avail themselves thereof.

The only reason there is much free molecular oxygen in our atmosphere is that æons ago, cyanobacteria evolved which were able to combine carbon dioxide and water to store energy and produced oxygen as a by-product. This actually ended up poisoning most of the life around on this planet which had thriven up until then and plunged it into a global ice age where there were even glaciers at the equator due to the lack of a greenhouse effect from the carbon dioxide which had been removed from the atmosphere. It was actually a bit of a disaster, and it demonstrates very clearly that oxygen can be a liability for life rather than essential to it. It is in fact implicated in the kind of damage associated with aging, and if life like us could survive without respiring in an oxygen-rich environment we might end up living a lot longer, barring accidents. However, it remains to be seen how we would manage to derive energy to do all that living, and perhaps if we were only able to use anærobic respiration we would take a lot longer to get things done and life might end up seeming about the same length anyway.

Photosynthesis is not the only way free oxygen can arise in an atmosphere. The Jovian moon Europa and Saturn’s moon Enceladus both have extremely thin oxygen atmospheres because of the breakdown of ice and in the latter case water vapour, and in the case of Enceladus this oxygen is actually transported to Titan’s much thicker atmosphere. It’s thought that a very common type of planet would be the “water world”, which could form in several different ways but consist of an ocean hundreds of kilometres deep over a layer of ice which is only there due to compression and not cold. Such a world would start off with a water vapour atmosphere but ultraviolet radiation from its sun would split up the molecules and the hydrogen would escape into space, leaving the oxygen behind, probably at breathable levels. However, life as we know it on such a planet is another question because depending on how thick the ice is, volcanic activity and rock could be deeply buried under the ocean bed and heavier elements wouldn’t be available, so it’s likely that larger such planets would be lifeless due to lack of material resources. On smaller worlds, the oddity may be that even though photosynthesis might never have evolved, heterotrophs such as fungi and animals might, without needing plants, but there would still need to be producers for them to eat.

It’s also been suggested that although organisms benefit from oxidation or other chemical processes to release energy, other forms of carbon-based biochemistry might use other elements than oxygen to do it. In fact it isn’t necessary to go as far as another planet to see this happening because even here there are sulphur bacteria which use that element instead. In fact sulphur is used metabolically in a number of different organisms in various ways. There are two opposite processes chemically referred to as oxidation and reduction. Oxidation is the loss of electrons whereas reduction is gain, and sulphur bacteria are a big personal reason why I didn’t go into marine biology. As a teenager, I did field work on a mud flat in Kent which was rich in anærobic bacteria releasing stinky hydrogen sulphide living in a black, tarry layer under the mud in which I got completely covered, which seriously put me off doing any more of that kind of thing. I wonder, in fact, whether this was part of the point of the activity. Anyway, from the comfort of this urban East Midlands sofa, I am able to pontificate on the matter in a more detached manner. Sulphur bacteria occur in several different types and use sulphur for various purposes. The element was present in quantity on this planet before oxygen respiration evolved and would have been an ample source of energy. Some archæa do the same. They may actually “breathe” sulphate rather than sulphur as such, and whereas when oxygen is breathed it’s reduced to water, sulphur produces hydrogen sulphide. However, both elemental sulphur and various sulphur compounds are used. Sulphur, being in the same column of the periodic table as oxygen, has certain similar properties, although its valency, unlike oxygen’s which is always two, varies. Further down that column are selenium, tellurium and polonium, and all but the last perform useful functions in some living things, the function of polonium being of course to kill things and be extremely dangerous, but none of them are abundant enough to be used for respiration. Sulphur is a solid at room temperature and at sea level pressure it only melts at 239°C, so it’s unlikely to be a respiratory gas. An ecosystem based on sulphur would therefore probably be completely aquatic. However, sulphur is the fifth most common element on the surface of this planet and the tenth most common cosmically and it crops up all over the Solar System, such as in the clouds of Venus, as sulphuric acid oceans on early Mars and all over Io both as an element and as frozen sulphur dioxide. All of this suggests that there are many worlds out there in the Universe with sulphuric acid cycling through the atmosphere in the same way as water does on Earth, and depending on its concentration it could be very hostile to the development of life, which sadly could also apply to Mars and Venus. Nonetheless, the worlds themselves could be quite interesting geologically and chemically.

A popular science fictional choice of another option to oxygen is chlorine, which I’m pretty sure I’ve mentioned before on here. The potential for marine organisms to produce elemental chlorine gas is considerable because of the salt content of the oceans, and it may be that whereas we on this planet have gone down the oxygen route, others will have a large amount of chlorine in their atmospheres. If this is so, and their oceans are like ours in other ways, they will also contain a lot of caustic soda, so from our perspective if there’s any life there at all it will be in some way extremophile. Such oceans might also be high in elemental iron, as were Earth’s before the oxygen catastrophe, as it’s known. For me, the issue with chlorine is that it’s liable to produce “dead ends” in molecules. Oxygen, being bivalent, can participate in groups which join both to the main part of an organic molecule and other elements such as hydrogen, and can also occur in rings, but chlorine only has a valency of one and therefore terminates a group and can neither form part of a carbon chain or ring. This would give chlorine a different function in such biochemistry and there might still be a rôle for oxygen in it anyway, though not as a breathing gas. If the parallel to oxygen was close, photosynthesis would involve the combination of tetrachloromethane with hydrochloric acid, or rather hydrogen chloride, to form a partially substituted chlorinated hydrocarbon as an energy store and respiration would involve the production of tetrachloromethane. At our atmospheric pressures, tetrachloromethane is only gaseous above 77°C although it melts at -22, but chlorine is a powerful greenhouse gas so it’s feasible that a planet with a high-chlorine atmosphere would be quite warm and have water on its surface above our own boiling point, or again the possibility exists of aquatic life only. Incidentally, it hasn’t escaped my attention that in the above word equation I assumed hydrochloric acid or hydrogen chloride to be the main constituent of the oceans rather than water, which may be incompatible with life. This, however, is just a straight naïve substitution of chlorine for oxygen, which might not parallel a genuine viable set of processes upon which biochemistry could be based. For instance, and again this is tinkering, retaining water in that equation still leads to free chlorine and tetrachloromethane in the atmosphere but also a kind of chlorinated “sugar”. The real processes of photosynthesis and ærobic respiration are a lot more complex than that famous equation suggests, and there may be flexibility in there somewhere.

The collaborative science fiction project Orion’s Arm has had a go at creating a chlorine-based planet class, claiming that it’s unlikely that the process could take place easily and that they’re likely to be either rare or the result of something like a terraforming process by intelligent aliens. However, they do turn up in science fiction quite often. John Christopher’s ‘Tripods’ trilogy depicts aliens who aim to convert our atmosphere to one high in chlorine so they can settle our planet. Isaac Asimov’s ‘C-Chute’ describes a human spacecraft which gets taken over by chlorine breathers during a war and the human attempt to reclaim it in a toxic atmosphere. Getting back to the Orion’s Arm article, I agree that weathering would be more pronounced on such a planet and that photosynthetic pigments are likely to be purple because of the greenness of chlorine gas, but in fact it’s also theorised that chlorophyll is a second generation pigment on this planet necessitated by prior purple microörganisms using up the rest of the spectrum, so in fact it might well be the case that even most habitable planets would have purple vegetation and that Earth is unusual in having green plants.

Another option I’ve wondered about but am almost sure is not viable is fluorine. This is the element after oxygen in the periodic table and also the most chemically reactive of all elements. Physically, it has similarities with oxygen, with a similar boiling point, although it’s yellow. This is by contrast with chlorine which at our sea level pressure is only -34.1°C, meaning incidentally that chlorine planets would have to be hotter than Earth to be viable unless they had something like lakes of pure molten chlorine at the poles. However, fluorine is so reactive that it would be difficult to dislodge from its bonding. For a long time it seemed entirely unfeasible to me that any planet could have free fluorine in its atmosphere, but in fact it is possible, though in small amounts and probably only locally. Fluorite mineral is locally common here in the English East Midlands. This is calcium fluoride, which releases hydrogen fluoride, or hydrofluoric acid, when sulphuric acid acts on it. This leads to the disturbing situation of a planet with pools of hydrofluoric acid at least briefly on its surface, before it eats through the rocks and makes its way towards the mantle. Once it encounters heat, however, it would dissociate into hydrogen and fluorine, or when struck by lightning it might also separate. It would then combine very easily, to the extent that it could even form xenon fluoride in small amounts. Hence I think a planet with a little free fluorine in its atmosphere is possible, but it would be quite short-lived and incompatible with life. That said, fluorine does exist in terrestrial biochemistry in teeth and bones where fluoride content is high in water, and also in krill for some reason I don’t understand.

At the top of this post, I gave the impression it was going to be about space camels, and it is. That is, it’s about the possibility of alien animals who can thrive in an atmosphere rich in their respiratory gas for long periods of time, and I am still going to do that. The point here is that such animals may not breathe oxygen in the first place.

Among the simplest and most easily plausible situations is simply an ecosystem like ours but no oxygen respiration, just glycolysis. There are animals who don’t breathe on our own planet. There is a cnidarian parasitic on salmon who doesn’t breathe. In our cells, like those of most other animals, there are symbiotic organelles descended from bacteria called mitochondria which are largely responsible for processing glucose to release energy in combination with oxygen. Henneguya salminicola is a microscopic relative of jellyfish whose mitochondria don’t do this. There’s also a whole phylum of animals, the Loricifera, which includes species who never come in contact with free oxygen, living in Mediterranean sediment, and may also lack mitochondria. The famous Cryptosporidium, a pathogenic alveolate which I unfortunately have considerable personal experience with due to its presence in water in Leeds in the 1990s, does not respire using oxygen. There are also innumerable species of anærobic bacteria and archæa. On this planet, all of the larger organisms live in special and restricted environments, and although they are larger, they’re still pretty small compared to us. It does, however, at least prove that there can be animals who don’t breathe oxygen and are fine, and that would be one option for evolution, or indeed a path that the whole of evolution could take on a planet with no oxygen in its atmosphere, perhaps using a different energy source than light to power its biosphere. Very many aspects of our anatomy and physiology do depend on our need for oxygen, such as a circulatory system including a heart, and of course lungs, but it isn’t clear that an animal who doesn’t breathe at all wouldn’t need one if larger than a certain size because there would still be a need to move nutrition and waste products around, and there might even be lungs because of the need to vocalise for communication, or perhaps to exhale nitrogenous waste such as ammonia. Presumably organisms evolving in an oxygen-free environment right from the start would also have many bodily compounds which would react, perhaps even violently, with oxygen if they were to come in contact with it, possibly even being highly inflammable.

Another very common and straightforward technique for surviving without breathing is found among whales, dolphins, seals, sirenians and possibly early humans. These are simply good at holding their breath, and are in that sense “oxygen camels”. Sperm whales, for example, can hold their breath for up to an hour and a half, and a lower metabolic rate could cause this to increase to several hours, so it’s interesting to speculate whether the likes of ichthyosaurs and plesiosaurs might have gone for hours without breathing. In a way, then, oxygen camels not only exist but we may even be them ourselves. We have the diving reflex, where our heart rate slows down when we are immersed in water. All vertebrates, as far as I know, can also store oxygen using a hæmoglobin-like pigment in their muscles so that it can be readily available for use when needed rather than having to rely instantly on blood oxygen.

Another possibility, which I’ve explored elsewhere in collaboration with someone else, is of an animal consisting largely of a thin “skin” which performs many different biological functions but is bladder-like, containing sacs of air like a lilo. Such an animal takes a similar approach to oxygen as a succulent plant does to water, storing it when plentiful and calling on reserves when needed. However, the volume of gas could make this rather ungainly. Perhaps there could be airship-like animals on some planets who do this though. Sky whales, as it were.

A more elegant approach would involve storing oxygen, sulphur or chlorine chemically and releasing it when needed, and if space camels exist this is, I suspect, the most widely adopted solution, probably in combination with greater than usual reliance on anærobic respiration, or perhaps “achloric” respiration in some cases. This would involve relatively dense solid compounds which could be induced to release oxygen or chlorine at manageably slow rates, rather like fat deposits can be called upon to release energy for metabolism. Camels partly rely on the water content of their humps in the sense that the adipose tissue stores water rather than the humps actually being “water tanks”, but this is not the most important store of water in their bodies, which is a combination of the bloodstream and one of their stomachs along with dry fæces and viscous, low-water urine. However, it isn’t clear how much this could be extended compared to breathing. Another possibility is something like hibernation when oxygen or chlorine levels are low, or perhaps the ability to switch over to another respiratory element such as the much more compact sulphur by changing the respiratory pathway and storing sulphur compounds.

Why, though, would a situation arise where a respiratory element varied in availability? This happens on our own planet because we have air-breathing animals who have returned to the water. Perhaps on another planet with plateaux above the level of breathable oxygen it would be necessary for animals venturing onto them, perhaps to exploit an ecological niche too extreme for their lowland colleagues, to have such adaptations. A similar situation might emerge in the upper atmosphere, with the airship-like animals, although it should be borne in mind here that they would need to employ a lighter-than-air gas such as hydrogen to maintain their altitude, perhaps consuming aerial flora. Or, bird-like animals might fly into the upper atmosphere and glide, becoming dormant for a while perhaps to avoid predators or harsh environmental conditions, although what could be harsher than the upper atmosphere? Incidentally, this is still in the troposphere, so in a sense it would not be the “upper atmosphere” as lift and drag would still have to apply.

Applying camel physiology to a low-oxygen (assuming it is oxygen) environment, there’s the efficient use of oxygen in the body, akin to the low level of water in the urine, the storage of oxygen in special corpuscles which are somewhat like red blood corpuscles but hold onto their oxygen for longer and the chemical conversion of compounds in storage to release molecular dioxygen. On the subject of dioxygen, ozone would be a slightly more efficient way of packaging oxygen and hydrogen peroxide considerably more efficient, although it would have to be protected from catalase and the body would have to be protected from it, which occurs in white blood cells. The human body is 65% oxygen by mass, although little of this could be usefully released without causing fatal chemical reactions. A space camel could also have an extra lung used solely for storage, which could exhale into the other lungs when needed. As it stands, most of the oxygen inhaled into human lungs emerges from them unused. This could be remedied by compression and the removal of carbon dioxide.

Therefore, I think there could be space camels, and environments in which that would be a useful adaptation, if there are aliens at all, but they might not be able to breathe oxygen and might even burst into flames if they landed here unprotected. Or, they could be like enormous inflatable camel balloons floating through the stratosphere. Burning giraffe anyone?