. . . but not all! There’s Skippy, Lassie and Judy from ‘Daktari’, and there are even non-living actors such as that soft toy in ‘The Double Deckers’. Most writers are also human, as TVTropes observes. Generally, then, you get the choice of depicting aliens on telly or film by using human actors or nobody, although they do sometimes show alien “animals”, which are probably dogs most of the time. ‘Star Trek’ is of course a major offender in this area, but ‘Doctor Who’ not only shows human aliens a lot but they also all seem to be English. Then there’s ‘Star Wars’. Even when aliens are supposed to be non-humanoid, they can end up looking pretty much like us. But how realistic is this?
First of all, how realistic is the idea of life anywhere at all apart from Earth? I know I’ve been into this many times, and it’s important not to be guided by optimism or pessimism here, but realism. Many people claim that Earth has just been exceedingly lucky in retaining its life and evolving complex life, and even in our own history there’s the issue of not much at all happening until the Cryogenian at least, then a huge flurry of activity from the Ediacaran onward, along with a series of mass extinctions which at their worst wiped out 96% of all life, at the end of the Permian. In case you’re not familiar with these geological periods, it amounts to the 4600 million years of this planet’s history having no life, then apparently simple and mainly microscopic life, for seven eighths of its history. This could mean that it took evolution most of the time life has been around to stumble upon some event which accelerated it into the more complex forms which include our species, and even then it was subject to catastrophes such as being hit by asteroids and having gamma ray bursts convert much of the atmosphere into concentrated nitric acid.
The trouble is that we have just one known example, and all it’s really possible to conclude from it is that life exists in the Universe because it exists here. Unfortunately that doesn’t mean it exists anywhere else. The Rare Earth hypothesis focusses on the various things which seem to make life unlikely. For instance, although there are 125 thousand million galaxies in the observable Universe, the Milky Way may be unusual due to being unusually “quiet”, with fewer collisions and an optimally active central black hole. The Sun’s orbit round the Galaxy is particularly circular and mass extinctions have tended to coincide with the Solar System crossing a galactic arm. The distance from the centre is also optimal in that there are more heavier elements in the arm stars the closer they are to the nucleus and so life as we know it, which incidentally is what I’m talking about here rather than, say, possible plasma-based life, is more likely here, and the denser packing of the stars towards the centre makes collisions more likely, so it’s possible that as you get towards the centre the planets are being constantly pelted with comets and asteroids all the way through their existence rather than just at the beginning as happened here. Then there’s a problem with the orbits of planets in other solar systems. It’s clear that there are many “hot Jupiters”, although the method of detecting planets by looking for transits (basically eclipses) is biassed towards finding large, close planets. There are still a lot of them, and if, for example, they migrated inward they would probably have disrupted the orbits of Earth-like planets in the process of doing so. It may also be that the planets in this solar system have unusually circular orbits. Mercury has quite an elliptical one, and Pluto, though it is not currently considered a major planet, has about the same eccentricity, but on the whole they’re quite close to being circular, particularly Venus which is even closer to a circle than Earth. More elliptical orbits are likely to be less stable as well as leading to climatic extremes.
Clearing all that aside though, Isaac Asimov and others estimated that there were probably about six hundred million habitable planets in this Galaxy, or rather, planets which would become habitable at some stage. Many of these would be too young. It’s also possible that oxygen would not be produced in their atmospheres by photosynthesis. It’s been worked out that a mutation to release chlorine from sea salt instead is another possibility, and that may or may not be suitable for respiration, and a planet with no breathable atmosphere is still compatible with life, since that was Earth for most of our history. One problem with chlorine is that it’s a “dead end”. Its atoms can only form one bond, so the situation here where oxygen is part of a ring or has two bonds with another atom couldn’t exist in that kind of biochemistry. Chlorine would mainly be an oxidiser for respiration and wouldn’t contribute much to variety among organic compounds. Also, it would make the ocean extremely alkaline for this to happen, which renders a lot more compounds acidic. Asimov’s estimate may be obsolete because rather surprisingly, the most common type of star in the Universe, the red dwarf, has been found to be a suitable abode for life as we know it because a planet orbiting close enough to have locked its rotation, leaving one side in constant daylight and the other eternally dark, turns out to have a likely zone of temperatures hospitable to life in its twilight zone, so this could bump the numbers up a lot, or even multiply them. However, the fact remains that in our random sample, Earth, we find ourselves orbiting a yellow dwarf star at a distance of around 150 million kilometres, so the question arises of why we are here rather than living in the twilight zone of a planet orbiting near a red dwarf. Therefore I want to assume there are 600 million potentially habitable planets in the Galaxy and ignore the red dwarfs.
Animal life became possible on the land on this planet around 500 million years ago, although the likes of tardigrades were probably around before this accidentally, for instance if they were in a body of water which dried up seasonally they would be technically on land but dormant. It’s estimated that life will be wiped out on this planet by about 2 800 million years from now, by which time protected environments such as lakes on top of mountains or water deep underground will have boiled away, but long before that, complex life will have become impossible, so it’s thought, although I do wonder because it seems like evolutionary pressure will be extreme as Earth becomes more hostile and that something new would emerge. Leaving that speculation aside, photosynthesis will cease by around 800 million years from now and therefore any surviving life will take on very different forms even if it remains complex. I have to confess that I don’t fully understand why this will happen although I know it’s to do with carbon dioxide falling below the point where chloroplasts can use it, because I don’t know why this fall would take place, but I’m just going to accept that. The Sun will become a red giant 5 400 million years from now, giving the Earth a total life span of ten thousand million years. Over that time, it appears that complex terrestrial animals will exist for 1 200 million years, which is an eighth of that period. Consequently the currently viable number of planets falls from 600 million to forty-eight million at any one time assuming the Sun is average for a star with life-bearing planets or moons nearby. This assumption, however, may be wrong because smaller stars are more common than larger ones and they last longer and age more slowly, so it may be that most complex life is found in systems whose stars are somewhat smaller and cooler than ours. Again the question arises of why we are here, but again the answer is unavailable due to the minute sample size of one.
It’s now feasible to consider the likelihood of humanoid aliens. Up until recently, I’ve always assumed it was practically impossible for this to happen even if the Universe is full of intelligent life. The problem can be stated as follows. Suppose every beneficial mutation has two equally probable possibilities of happening which are almost equitable in improving fitness in a given situation. On forty-eight million worlds, that would be enough to provide a unique life form after only twenty-six steps. If evolution results in tool-using sentient terrestrials as a result of a random walk like the meanderings of a particle undergoing Brownian motion, this idea has more validity, and it would be supported by the possibility that sentience is a “bad idea”. Sentience for humans requires small brood sizes and a long childhood, which reduces the ability to populate an unexploited environment quickly, and it’s also been argued that sentience is self-defeating because it leads to environmental change incompatible with the survival of the species concerned. This would mean that the ability to become technological may not be particularly selected for and could therefore have a more random element to it. But we can look around at the animals on our planet and see perhaps three dozen phyla representing body plans, only a quarter or fewer of which are currently represented by more than a handful of species, and even in our own phylum there are markèdly alien-looking forms such as sea squirts:
These animals start off as tadpoles.
The question is, therefore, what are the chances of even producing vertebrates, let alone humanoids?
I’ve mentioned many times before that there was a time, getting on for 600 million years ago now, from which only one chordate fossil has been discovered as opposed to a large number of priapulids. This was Pikaia:
Description
English: Life reconstruction of Pikaia gracilens
Date
12 July 2016
Source
Own work
Author
Nobu Tamura email:nobu.tamura@yahoo.com http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/
This is a living priapulid:
These were much more successful at the time than Pikaia, and Stephen Jay Gould suggested that it was pure happenstance that the ancestors of fish survived and the priapulids went into decline (they had a reputation of being the smallest phylum of all until recently).
Nonetheless, the very form of a priapulid suggests that certain shapes of animal are more likely than others. Humans have, of course, an organ which is similar in shape and there are also acorn worms:
These too are, incidentally, fairly close relatives of vertebrates, and the three-part body form still exists in our internal anatomy – the three parts of our brains are probably related to their own body shapes. Acorn worms are to us like someone took the genes we have as humans and tried to make a completely different kind of animal out of them. They have gills like fish, and like humans as embryos, sometimes hundreds of them. They smell of iodine compounds because like us they secrete them, but on the outside rather than in the thyroid, and the genes that lead us to develop a forebrain, midbrain and hindbrain instead in them lead to, well let’s be frank, the glans, the foreskin and the shaft, as it were. A basic underlying similarity has gone in drastically different directions here.
There are plenty of repeating independent patterns in evolution. One of the most striking ones is the remarkable similarity between brachiopods and bivalves. Brachiopods are now a minor phylum but used to be much more widespread. Here are some examples:
These are not molluscs. They have nothing to do with molluscs in fact. We are about as closely related to scallops and cockles as they are. Nonetheless they look remarkably similar because their lifestyles are similar. They’re sedentary filter feeders. There are also a remarkably large number of animals, and even a few plants, that look like flowers. It’s a fairly good bet that if multicellular life is common in the oceans of Earthlike planets, so are bivalve and sea anemone like animals. Convergent evolution is a thing.
Nonetheless, vertebrates are unusual. It’s unusual for animals to have hard internal skeletons. They’re much more likely to have shells of some kind. Of those that have, namely sponges and I can’t think of any others, the function of that skeleton is not to aid movement but to hold them in place. And of course we’re the only animals with spines by definition. There are plenty of animals among the minor phyla who are similar to each other in spite of not being closely related, so for example hard exoskeletons are common and have evolved independently many times.
It isn’t looking very likely that there could be humanoid aliens. But I’m not so sure.
It’s very likely that there are various complex structures on other planets which are also present here. There are less complex ones such as volcanoes, lava tubes, rivers and lakes, and the valleys and hills they carve out. There are also plenty of crystals, pebbles and the like. Structures as complex as these desert roses:
These are of course very like cultivated roses, my point being that inorganic processes can throw up similar structures. It’s likely that there are places all over the Universe with desert roses. Another aspect of these is that they’re baryte whereas many others are gypsum – the precise composition is not the only possibility. And then, of course, there are actual roses made of water and various organic compounds. What’s happened here is that the nature of this Universe and matter within it dictates the form of the structure. Other Earth-like planets would have oceans, thunderstorms, auroræ and the like. The shape of landmasses on this planet, such as the vaguely triangular Afrika and South America and the large number of islands which are roughly the shape of Sri Lanka, Madagascar, Sardinia and Corsica, even suggests that an Earth-like planet with land covering, say, a third of its surface, might even have something like a vaguely Afrika-shaped continent with a Madagascar-like island offshore.
The lives of identical twins separated at birth can sometimes be spookily similar, even to the extent of having the same breed of dog with the same name. This is clearly due to various social pressures and trends interacting with their biology, and although it looks like a paranormal connection it probably isn’t. We’re not so used to these things in the human world as elsewhere, although it is remarkable, for example, that both the Mayans and the Ancient Egyptians were pyramid-building cultures which worshipped the Sun and wrote in hieroglyphics, so these things do happen. Among hominids, there’s an example of an ape who evolved in the Balkans who was thought to be related to us humans because of having so many features in common but turned out not to be. In this situation, the pre-existing conditions were also in place. Apes had already evolved.
It may be that pre-existing conditions predisposing to humanoid evolution would exist elsewhere. The physical conditions of the Universe, and of planets where terrestrial life forms evolve technology, assuming they exist, may be similarly dictatorial. In particular, to us it seems that limbs with fingers and thumbs are particularly useful, and these have evolved independently a few times, in monkeys and ourselves, koalas and dinosaurs. Koalas have two thumbs, which is an unusual condition among humans. However, an elephant’s trunk would seem to do the job pretty well too, as would a series of prehensile extensions to the lips, which in fact could even be more likely as it would go along with having speech organs. There are a few other things which make it more likely.
We are bilaterally symmetrical, bipedal, live on land, speak, have largely hairless skin and hard internal skeletons. We also have four limbs and forelimbs with opposable thumbs. If there is, out there somewhere, a world with a whole class of bipedal animals with erect posture, and that’s not too far-fetched as even here we have the bipedal birds, it seems likely that any technological species which evolved in that class would have that head start in approaching a humanoid appearance. If it was also ectothermal, requiring heat from the external environment, it could have naked skin like ours. It’s a little less likely that it would have arms, and the probability of it being bilaterally symmetrical, which is more or less implied by it having two legs, is unknown. It might well be neckless and have a different respiratory pigment such as the blue haemocyanin, which crops up a couple of times independently. Given a Universe where life is common, it’s possible that technological sentient life would look humanoid at least some of the time, particularly if it’s from a world covered in hairless bipeds anyway.
This, then, is what I currently think:
It’s fair to conclude that there are around fifty million planets and moons in this Galaxy which have started out with a good chance of being suitable for life as we know it. If these survive catastrophes, which may actually stimulate evolution rather than suppress it as with, for example, our snowball Earth period which may have given rise to complex organisms, they may just have the likes of bacteria on board and we could be the exception. On the other hand, the path evolution has taken here may be common on worlds with large oceans and substantial land masses, with large complex life forms colonising the land. Given that that happens, the most improbable step to my mind is the evolution of vertebrates and of there being a major phylum including them as opposed to them being a mere taxonomical footnote. Given that that happens at all, the development of humanoids becomes much more probable. Hence what I think probably obtains in this Galaxy, provided that complex biochemical life is common on Earth-like worlds and that intelligent technological life evolves often, and those are all big ifs, is that there are a huge range of different intelligent species, but among them will be the occasional humanoid species. And if there really is an organised Galactic federation which selects species for first contact, the chances are that they will be the first species we meet. But that’s a lot of ifs. For all we know, the Universe is a barren place where only Earth has life.
A Box Of Chocolates 