What might aliens look like?

Applying what we know about life on Earth, to space
26 February 2021
Presented by Ben McAllister, Adam Murphy
Production by Adam Murphy, Ben McAllister.


Artists impression of an alien spacecraft resembling a flying saucer in space


This month on Naked Astronomy, we're waxing lyrical about aliens. What might they look like? Can we apply the principles of biology to other worlds? To do that, Ben McAllister and Adam Murphy were joined by Arik Kershenbaum to chat about alien evolution...

Adam - In the 19th century, people were aware that life disappeared and new life emerged. We had found things like dinosaurs. So people were wondering where new species and new kinds of animals came from. And there were loads of ideas about that, but most of them were wrong.

Ben - Yeah. There was a popular idea that started to emerge around that time called Lamarckism, which is, you've possibly heard of Darwinism. And we'll be getting to that in a moment. This was sort of a competing idea. The idea with Lamarckism was say, for example, you had a deer that spends most of its life reaching for food on a tree. The process of reaching for that food will cause its neck to stretch. And it'll pass down that longer neck as some kind of variation to its children. The theory being that after many generations of this, eventually you get a giraffe. We know now that this is very wrong.

Adam - But it did sort of pave the way for people like Charles Darwin. And he came along and got really interested in natural history and about animals. And eventually he got invited on the Beagle, which was a ship that went touring around the ocean. And while he was looking on new islands and new animals, he noticed that certain animals had different things about them, different traits that gave them an advantage. Like there were birds on the island and some of them had specialised beaks, bigger ones for cracking nuts, or longer ones for getting into flowers, that sort of thing. And with a lot of work, and it took them a long time to put it all together, he eventually published all his ideas in a famous book called On the Origin of Species in 1859.

Ben - Yeah. That example is Darwin's famous finches. It's a relatively famous concept in biology, but Hey, here we are talking about deep evolutionary biology again, as two physicsy type people. So, you know, we're getting weird with it this time. That's the idea of Darwinism. If we wanted to talk about it less phenomenologically, less about like seeing the way things evolve and, on a more, I guess you'd say fundamental level, the idea behind natural selection, as it is known, the process by which evolution occurs is that the DNA, the instructions that are inside any given body, any given bird, any given fish, any given deer, that DNA is fixed. And it doesn't change much throughout the life of that particular creature, but every now and again, when the creature has offspring children, that DNA can mutate slightly.

Adam - And we know when we're saying fixed, we know the angry biologists in our audience will be talking about epigenetics, but that is well beyond the scope of what we're talking about here.

Ben - Yes. Mostly fixed.

Adam - Now, usually that mutation won't make any difference. The animal won't even notice it, or it will be something actively harmful. It's sort of a genetic lottery, but it gives it a useful change. That means it can out-compete the other animals.

Ben - Yeah. Like maybe for example, it's a finch that lives on an Island and all of a sudden it develops a slightly different shaped beak due to a random tweak of its DNA. And then that beak helps it crack open seeds or whatever

Adam - That is what is selected for, that longer beak. Now, the thing is what's selected for can change. The famous example is to imagine a whole set of moths in a forest that are white, but every so often a mutation makes one of them dark brown, and the dark brown ones stand out in the forest and get eaten. But then we come along and we pollute the forest and the smoke changes the colour of the trees. Now the white ones get eaten instead. So those selection pressures, they're called, can change as well.

Ben - Yeah. So that means that over time, as the environment changes, then the kinds of moths that are likely to survive and pass on their DNA, changes to the ones that are brown rather than the ones that are white. And that is the process that creates the very commonly used expression, survival of the fittest. It's sort of important to note, like survival of the fittest evolution, natural selection, these terms, people seem to sometimes talk about them as if they're almost like sentient forces that are doing something to try and cause these mutations and cause the species to change. It really is just a kind of statistics in a sense. I mean, if you've got like a million birds and half of them have one kind of beak and half of them have another kind of beak randomly, the ones that survive and pass on their genes, the ones that had the randomly selected, turned out to be better beak.

Adam - That's the thing. It's not the best thing. It's just that thing that fits. Fits best in its niche. It's not fittest as in strongest, or best, or thing that deserves to live, like some people who twisted Darwin's philosophy would have said. The other thing that's probably important to know is while the mutations that happen are completely random, natural selection and evolution isn't. It is selecting the most useful traits by statistics.

Ben - The process of evolution is when many of those changes over a very, very long period of time, many of those random mutations that turn out to be beneficial and selected for accumulate through many, many generations. And eventually, say we're talking about birds again, or moths or whatever. Many, many generations later, you end up with a bird or a moth through these random changes, that looks so different from its ancestors that you don't even call it the same thing.

Adam - The other thing that's important to say about evolution is that it's not necessarily this forward march of progress. It's not necessarily always the animal getting better. It's not that Pikachu evolves into Raichu, and suddenly is better than it was. It is just different. It has just changed and better suited to that niche than it was before.

Ben - Yeah, that's right. Like there's no moral judgment or value judgment made on this kind of thing. Again, there's no sentience force that's using evolution to develop the traits of these bird beaks. It is just that they're getting better at surviving.

Adam - It is not, then, if you take those ideas and we see how they work on Earth, it's not completely unreasonable to say that the laws of natural selection ,and given they're just about surviving, would apply on other planets and other forms of life that we're not familiar with.

Ben - Yeah. I bet some of you are wondering, why am I hearing about evolutionary biology on my space science podcast? And it's because many astrobiologists believe that we can take the lessons that we've learned from Earth, the lessons about evolution and natural selection. And we can possibly use those lessons to deduce things about what alien life might be like.

Adam - We have to keep an open mind to the possibility that life elsewhere in the universe looks nothing like the life we know on Earth. There is a chance that new physics, chemistry, or biology changes things in ways we haven't yet understood.

Ben - But it's still reasonable to assume that principles of evolution and natural selection will apply. And for the sake of having an intelligent, not just completely rampantly speculative discussion about possible alien life, it makes good sense to draw in the context that we do have.

Adam - And indeed some astrobiologists believe that there's good reason to think that life on other planets probably isn't all that crazily out there different from the kinds of things we can conceive.

Ben - We chatted with one such expert, Arik Kershenbaum from the University of Cambridge. He's the author of the new book, the Zoologist's Guide to the Galaxy, about what life on this planet could teach us about life on other planets.

Arik - So zoologists really look at why life is the way it is, why animal life is the way it is. You kind of get this impression that zoologists go around collecting butterflies, and pinning them to pieces of cork. Really, what we're interested in is the mechanisms of evolution. Why some animals look one way, why some animals behave another way. And to do that, it's either a question of doing experiments in a very sort of like, standard scientific way. But a lot of the time, a zoologist like myself working with larger animals, or rare animals, we have to satisfy ourselves with observing them in the wild and trying to understand why they behave the way they do.

Ben - Great. So as well as just typically observing animals on Earth, trying to understand their behaviour, but as we've kind of flagged, you're a zoologist who's also interested in things a little bit more exotic. So what exactly is it that a zoologist can tell us about potential alien life in the universe?

Arik - Now, if the reasons the animals are the way they are on Earth, are the same sorts of reasons that apply elsewhere in the universe, then we can apply those rules to life on other planets as well. All life needs energy. We're certain of this. Whatever planet you're on, you will need some sort of energy. And you've got to get that energy. Well, if there's not enough energy around, then animals are going to be competing for it. And on Earth, you see that particularly in terms of predations, you have some animals eating other animals. And this is the kind of mechanism that we can look at how it evolved on Earth, and ask ourselves, if this is likely to be the same on other planets as well. Will there be predators and prey on other planets too?

Adam - Will there be the same kind of pressures? So will natural selection work the exact same way on another planet then, is that the idea?

Arik - Over the last 150 years, scientists have understood an awful lot about how life has become more and more complex on this planet. And the only reason that it could become more complex is through this process of natural selection. Even the very earliest organisms on the planet were incredibly incredibly complex, and that complexity has to arise through natural selection, but natural selection is actually quite a complex process. And we're only starting to understand in the last 50 years or so, a lot of the complexities and the subtleties by which natural selection works. One of the things that we need to understand is how many of those complexities and subtleties are specific to our planet, and how many of them are really general like natural selection itself. We can be confident that life on other planets arose through natural selection. We now need to think whether life on other planets complies with some of the other rules of natural selection that we understand on Earth.

Ben - Okay. So you're interested in taking the laws that we've seen, or the rules, I guess you could say, that we've seen that relate to how life developed here on Earth and applying it to other kinds of planets. You've just flagged that some of those, we expect to be quite similar in general. And some of them may in fact, be quite specific. How do you think that different environmental conditions or planetary conditions, for example, like a planet with really high gravity, or a planet with a really toxic atmosphere might affect the way that life evolves on different planets.

Arik - One of the nice things about natural selection, is it's pretty much independent of the mechanism. A lot of astrobiologists, a lot of scientists are thinking about things like what kind of life could exist on a frozen moon like Titan, what kind of life could exist on other planets with different atmospheres, different temperatures and pressures and so on, and what biochemistries are possible. But the nice thing is that whatever biochemistry is possible, it's going to evolve through natural selection. So the natural selection itself is independent of the underlying chemical mechanism. And that means that some of the conclusions that we draw from natural selection are going to be general, irrespective of the environmental conditions on that planet. Some things won't of course, some things will be heavily dependent on the environment, particularly the way the aliens look. So the way the extraterrestrial life forms are shaped will really be strongly affected by the various environmental factor on a particular planet. But at least we can be confident that they will have achieved that shape through natural selection

Adam - Because Ben and I are both very much astrophysicists, could you give us an example of what that would look like? What pressure would change, how a thing looks. If you know what I mean?

Arik - We can look at the laws of mechanics and how they work on Earth and try and extrapolate them to other worlds. You can think, for example, that legs are incredibly common on this planet. Legs are very common on this planet because life evolved on the surface. Life evolved, presumably, on rocks underneath the ocean. And when you're slithering across a rock, there's a lot of friction, and you could just move a lot faster by lifting yourself a little bit up off that rock. If life evolved on a planet with no rocks, with no surface. So perhaps in an underground ocean, you might find that legs are completely unnecessary. Another example, which is very useful is flight. So we know from the laws of physics, that there are not that many ways to fly. Okay, there's ballistic flight. You can jump like a grasshopper or a cricket. That's not particularly effective. You can fly with aerodynamic flight, which is what we're familiar with on this planet. Birds, and bats, and insects have wings and they generate aerodynamic lift. The other way of flying, of course, is to be buoyant, and to fly like a zeppelin. And we tend to think that doesn't happen on Earth, but of course that's not true. All the fish in the sea fly through buoyant flight. We don't think of fish as flying, but they're moving through a fluid just like birds are. And the fact that that fluid is extremely dense and viscous, really is not the point when it comes to how that flight's affected. So you can have planets with a wide variety of atmospheric densities, ranging all the way from a very sparse atmosphere like on Mars, all the way through to perhaps even a liquid atmosphere, like in our oceans, but the principles of flight remain the same across all of those environments.

Ben - What I'm hearing from that is two things. One, which is from the perspective of a crab, a fish may as well be a bird. And the other thing is that when you're swimming through the water, you can really say you're kind of flying, which is something that I find pretty appealing as a concept.

Adam - And it means all penguins as well, they do fly just not through the air.

Ben - So I know that you like to spend a lot of time researching the way different animals communicate. Have you given any thought to how aliens of different kinds might communicate, and how their evolution on different planets might impact the way they communicate?

Arik - Communication is really a key issue here, because one of the things that we think is really fundamental to the way that life evolves on any planet is that sociality is likely to evolve. Okay? Animals are always interacting. They're always interacting with each other, of the same species, of different species, but sooner or later, there will be advantages and disadvantages to animals working together. And the moment that there's any connection, any contact between individuals, then there must be some form of communication, all animals on this planet communicate. So communication is going to be something that's going to be common throughout the universe. The way that animals communicate of course is heavily constrained by the environment. If you are in an underground ocean with no light at all, then there's not much point in communicating using sight, which of course, visual signals are a very important way of communicating on this planet. Sound is generally an extremely good way to communicate. It tends to work terribly well in most environments, there are all kinds of different media, different channels through which communication could take place. And that's going to be constrained by the environment. But once animals start working in groups, once animals start cooperating with each other, for various reasons, which are also defined by the rules of natural selection, then that communication will become more and more complex. That's something that we see on Earth, but it's not anything special about it. It's just because animals are living in groups and working together.

Ben - So do you think if we then expect that any kind of alien life out there that was going to reach any kind of complexity is going to, uh, needs to communicate in one form or another? What do you think that means for our prospects of potentially communicating with them? Are we ever to encounter them?

Arik - One of the jobs of scientists who are looking for signals from outer space, signals from potential intelligent alien civilizations, is to understand how we would recognise such a signal. So space is very loud. There are radio signals all across the spectrum, all kinds of noises. And when we're listening to those signals from outer space, almost all of which are of natural origin, we need to think how to differentiate between something that's intentional, an intentional signal, and just the regular sort of static, and noise of space. One of the things that people look for of course, is regularity. Some kind of indication of perhaps a technological origin, but we also have to think about how alien civilizations might encode information into signals that they're sending out into space. And one of the things we need to do to understand that, is to think about what's fundamental about our language. What's fundamental about how these aliens must communicate. If they're going to convey large amounts of information, complex information, I always draw the analogy of how can you build a radio telescope, or some device to send signals out from your planet towards planet Earth, if you can't ask your neighbour to pass your screwdriver, right? They must have some form of complex communication. Otherwise they wouldn't be able to achieve that level of technological complexity. So there's a great interplay between understanding what language must be and looking for signals from outer space,

Adam - Then say it happened, and something landed, a spaceship landed here and the door opened and something came out. How much could we expect that thing to have the same sort of structure that we do? Or would it be completely, for lack of a better word, alien?

Arik - There are some things that we can be quite confident will be shared by technological intelligent alien species. Look, every alien planet that is inhabited, that has life on it, will have a diversity of life. There will be some less complex and some more complex. But if we're talking about that kind of life which is sufficiently complex, either to come visit us, which is, you know, more complex than we can imagine ourselves being, or complex enough to send a signal, then we can speculate about some of their features, some of the things about them. The evolutionary history of Earth has been heavily constrained, not just by the sort of environmental conditions and astronomical coincidences, like the asteroid that sent the dinosaurs extinct, but it's been heavily constrained by the fact there are simply not that many good solutions to how life can be. So I mentioned already, legs. They're not that many different ways that you can move quickly across the solid surface. They're not that many different ways that you can fly. And so some of the evolutionary innovations we see on Earth, like for example, a very important one is segmented bodies, right? So much of the complexity of animal life on Earth at the moment, arises from the fact that many organisms are made up of separate segments. Each segment can be slightly different, so some can have arms, some can have legs, some can have mouth parts, or wings and so on. And that kind of diversity in the ecosystem is driven by the fact that, you know, there's not all that many ways that you can arrive at that complexity. So if we're going to speculate, there are a few things that we could pick on. A technological alien species will almost certainly have appendages, right? It needs to manipulate the environment, some form of appendage. It will probably have a complex body. And so probably be segmented in some way, be made up of repeating units, which is an easy way for evolution to arrive at that kind of outcome. So this is straying into speculation to some extent, but I think that at the very least we can say if that were to happen, the door to open and the alien were to come out, we would be able to look at them and say, Oh, okay, that's how they got to be the way that they are.

Ben - What do you think about the more exotic ideas that have been proposed in the past about aliens that might look absolutely nothing like something that we would have a concept of, like something with a very kind of segmented form with appendages, I've heard various proposals for maybe there'd be an alien species that was more kind of distributed, and less rigidly defined to one physical form, some kind of gas like intelligence or something like that. I mean, that's obviously a bit harder to understand from our own context of evolution on Earth, but do you have any thoughts on that?

Arik - The most important thing to remember when thinking about the evolution of alien species, is that complexity can only arise through natural selection, and natural selection only works if there is an actual fitness advantage at each stage. So it's very common for people to think of fantastic solutions to life's problems, without thinking whether it's even plausible, or possible that such a form could evolve. So just because it sounds like it would be a great thing to have a gas intelligence, you still have to explain how that intelligence could have become intelligent in a way that involved a step by step increase in its fitness. And that just seems really unlikely in that case. Okay. If we can't explain how a slightly intelligent gas alien could become a slightly more intelligent gas alien, then that kind of solution is simply not possible. So intelligence, for instance, we know that intelligence evolves because it provides organisms with a means to solve problems. If you're a gas alien, what are your problems? The simple precursor of this intelligent gas alien, what problem is it trying to solve, that it would be an advantage to become more intelligent? So it's okay, and it's absolutely right to keep an open mind to things that we don't see on Earth that may be possible, but we have to remember, they must have evolved. And that means we have to understand how they could have evolved in a sequence of small steps. One of the examples I give a lot is that of telepathy, why don't we see telepathy? Why is there no telepathy on Earth? Why is there probably no telepathy in the universe? All animal communication evolved from sensing. We sense the environment. We speak because we have ears. It's not the other way around. We could sense the sound in the environment. And therefore we used sound to communicate. No communication system evolved on its own, simply because there's no point in talking if no one's listening. So the reason that telepathy doesn't evolve is because there's no evolutionary advantage to be able to sense people's brainwaves, if they're not transmitting their brainwaves. And there's no point in transmitting their brainwaves if no one's listening. So it's always important to remember that just because something looks like a good idea, doesn't mean that it can necessarily evolve.

Adam - I suppose there's no point being the only telepathic pigeon, if none of the other pigeons are as well. We were mentioning that we can expect things to have a lot of certain similarities, but are there any ways, potentially, something could evolve to communicate or move or anything like that, that is completely unlike anything that we've got here at the moment? Are there any cool ways that could happen?

Arik - When it comes to communication, the one form of communication that hasn't been widely adopted on Earth, although it does exist, is electric field communication. So there are a few species of electric fishes that can convey relatively complex signals by varying the electric field that they generate around their bodies. They use this electric field to sense the environment. So it's a communication channel that has evolved from sensing as it must, but they've adapted it to send signals as well, on individual identity, and perhaps even the state of the individual that's transmitting. But it's a very difficult type of communication to use. It uses an awful lot of energy. You have to devote pretty much all of your body to generating these electric fields. So it only evolves in very, very specific ways, but you could certainly imagine on another planet, where conditions were different, where the advantage to using electric fields to sense the environment was much greater, where energy perhaps was much cheaper, that this could evolve to be a much more dominant mode of communication. And what organisms on a planet like that might be like, is very difficult for us to imagine, because we only have a couple of species of electric fishes to look at. So that's something that really could be very different.

Adam - And then when you look at, say the gamut sci-fi can run from Star Trek and where all aliens are just palette-swapped humans with something stuck on their foreheads, to where it gets really out there. Are there any that you've seen or you come across, that you say are reasonable or no, that's absolutely unreasonable.

Arik - Yeah. I think we have to be generous to the sci-fi sector actually, because on the one hand, you know, a lot of science fiction aliens are not about alien life at all. They're about a reflection of human life, and that's absolutely fine. I'm not interested in whether the planet Vulcan has super logical aliens on it because the purpose of Spock and the Vulcans is to examine what a lack of emotion means for humans. So that's okay. We need to respect the fact that limited budgets mean that perhaps a little forehead ridges is the best that we can do. I think that the alien life forms that strike me as being the most unreasonable would be something like Star Trek's Q. Aliens that have super powers that don't seem to have a realistic evolutionary origin. It's when you create an alien that has traits that are good for their own sake, rather than good because they could have evolved through a sort of chain of goodness.

Adam - So can you tell us about that idea of panspermia, what thoughts do you have on that?

Arik - The real question facing astrobiologists,I suppose the biggest question, is how likely is life to evolve? Because we now know that the galaxy is absolutely jam packed with Earth-like planets, right? There are billions of possible places where life could exist, but that doesn't mean that life does exist. We don't know how rare an event it is for life to arise on its own. And it's very difficult to answer that without finding life. So an interesting thing sort of comes up now. If we were to find evidence that life once existed on Mars, what does that mean? Does that mean that life evolved independently on Mars and on Earth? Or does it mean, as a lot of the people think possible, that life evolved either on Mars or on Earth, and was transferred from one to the other, which is a distinct possibility. It would be incredibly difficult to prove one way or another, no matter what we discover on Mars, but let's say that there was evidence somehow that life evolved independently on the two planets. Well, if that were the case, then that would just throw the doors wide open, because if life can evolve independently on two planets in the same solar system, then there must be, life must be incredibly, incredibly common throughout the universe. But I think it's going to be a very long time before we can answer that, we're going to have to get samples back from Mars, even if they have anything. And I think it's going to be occupying people for an extremely long time.

Ben - What do you think about phosphine in the atmosphere of Venus as a potential biomarker?

Arik - The story of phosphine in the atmosphere of Venus is so exciting and so interesting, but perhaps for reasons that aren't often mentioned. It's exciting for me because it shows that we are looking for and detecting bio-signatures on other planets. I mean, this has been an idea that's been around for a long time. Oh, we can look at other planets and see if there are chemicals in their atmospheres, which might indicate life. But now we're doing it. It's admittedly planets in our own solar system, but it's really, it's the first step towards the ability to look for life on planets, orbiting other stars. So whether or not phosphine itself in the case of Venus is a biosignature, is an indicator of life on the planet, I don't know. But merely the fact that we're doing it and we're finding things, and we're making progress there is, I think, tremendously exciting.


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