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:
- 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.
- 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.
A Box Of Chocolates 