What is convergent evolution? How many ways can life evolve?
Ard: One of the questions we’d like to ask you, Simon, is if you were to reel in the tape of life again, rerun evolution again, would something like ourselves grace the replay or nothing like us at all?
SCM: Well, the simple answer is we don’t know, because the tape of life has only been run once. So far as animals are concerned, the story more or less begins in the Cambrian explosion, about half a billion years ago, and the conceit of rerunning the tape of life goes back to at least Stephen J. Gould. He said, ‘Look, if any one of them had gone extinct, rather than another one, then it’s just as likely that our ancestor would have gone extinct, rather than somebody else, and therefore we would not be there.’ Therefore, his argument was, rerun the tape of life, there would be life, there would be animals, but nothing like ourselves. And I think this is wrong.
SCM: The number of opportunities, if you like, the number of solutions by which biology arranges itself are surprisingly limited. So, if we were to rerun the tape of life, my estimation would be that, indeed, there would be something pretty similar to a human, in fact something pretty similar to this conversation, but there would also be an entire biosphere. So I’m not restricting the argument merely to intelligent bipeds with skills in manipulation and higher cognition. There would be animals in the sea, there would be plants in the forest, but in each and every case, the sort of end from which we see today, from the beginnings of the Cambrian explosion, would be surprisingly predictable.
David: Given what you said about the traditional view, how does your view then take issue with it? How is it different?
SCM: Well, in one sense, the views of biology are unified, in as much as everyone agrees to the first approximation – Darwin was right: evolution happens, evolution is reality, and all the rest of it. Really the difference, I think, is those… such as most of my colleagues who emphasise the degrees of randomness in evolution, the uncertainties, the unpredictability about it. And at first sight, this seems very reasonable: mutations are more or less random, perhaps; correspondingly, mass extinctions are meant to be more or less by chance, and the survivors are the lucky ones. And there is certainly a lot of truth in that, I wouldn’t deny it for a moment, but on the other hand, if you stand back slightly, there seems to be recurrent patterns.
One has to also point out that not only is there recurrence in biology – this is what we call evolutionary convergence – but there is also evidence that, in a broad sense, things are becoming more interesting, almost more complex. So there are, if you like, trends towards greater complexity. But beneath that there is a sort of drum beat which more or less says, ‘How many ways can you do something? How many ways can you fly? How many ways can you swim? How many ways can you breathe? How many ways can you think?’ And it then turns out that if you look at evolutionary convergence, there might be a rather limited number.
David: Is convergence then… if you were to try and give it a definition, is it natural selection finding the best solution to a physical problem or an engineering problem?
SCM: Effectively, yes. I mean, the point about convergence is that one can show by mathematical modelling that it can, in principle, happen by chance. After all, evolution has to go somewhere. And you can also make the observation, in fact, if you’ve got a certain, what we call a body plan, then there are constraints on what you can possibly do.
Most people argue that it, in a sense, reinforces our idea of Darwinian adaptation. Some things work, other things don’t. I think the crucial difference is two things. First of all, convergence is ubiquitous. I can’t think of anything which has only evolved once – or very, very few exceptions. And the corresponding point is that if you look at the total number of alternatives that biology in principle could throw up – as Ard will know as a mathematical physicist – the numbers are stupendously, stupidly big: far, far more than the number of particles in the visible universe.
David: Ah, right.
SCM: And yet out of this immensity of possibilities, at least on this planet, the total number of solutions is a handful.
David: That’s essentially what caught your attention, is answering that. How come out of all those possibilities, it’s just these few over and over again?
SCM: Absolutely right. The degree of similarity, and the possibility of alternatives which have never been realised, begins to sort of tweak the imagination. It’s a sort of ‘what if… ?’ experiment.
David: So it’s random, but because certain solutions are just the solutions that work, then natural selection will randomly find them.
SCM: Yes, there’s nothing wrong with randomness: the world won’t work without it. But the world is also ordered, and it’s, in a sense, a paradox of how do you get this self-order emerging from what is originally, of course, just this sea of early particles and coalesces not only into planets and galaxies, and all those good things, but also life-forms which in a certain sense begin to step out from physics and chemistry into new worlds.
Ard: So what would be an example of convergence? What would be a classic example of something that’s converged more than once?
SCM: If you think of the classic example of convergence, it has to be the eye. You and I look at each other, and we are using what’s called a camera eye. In other words, we’ve got a double chamber; in the middle there’s a lens that focuses the light and impinges on the retina and then goes into the back of our brain, and the rest is history.
Now, it also turns out that something like a camera eye has evolved independently about seven times. I took the precaution of bringing along an octopus.
SCM: And if I show you this octopus, then what we have here is an animal which is now, unfortunately, dead. You’ll see there are many familiar things about it, including its body here. And although we’re not going to dissect it, just about here there’s a brain, and of course it’s got the famous tentacles. And I could tell you stories about convergence throughout this animal.
It is in many ways almost an honorary fish, but let’s concentrate just on the eyes. Now the eyes are located just about there. To cut a long story short, if I was to dissect out this eye and look at it, its construction is almost identical to ours, but, and this is the crucial thing: this is an octopus; this is a mollusc, and in fact one of its close relatives is an earthworm.
SCM: You and I are chordates, and some of our closest relatives, and I don’t have them here because they’re not very edible, are starfish. If we consider the common ancestor of ourselves and the octopus, it probably had extremely simple eyes, what in the trade we call ‘icebots’, but it would have had nothing like the complexity of this, the camera eye. So in essence we can say that these arrived at the same solution completely independently.
David: How long ago was our common ancestor?
SCM: That’s a very good question. Most likely, it would have been a worm-like creature, and it would have been living perhaps 500, 600 million years ago.
David: So they diverged, and then they developed eyes and we developed eyes completely separately?
SCM: Precisely: they diverged, and the eyes converged.
SCM: But, in fact, when you go into what are the associative properties of camera eyes, one of the most interesting is intelligence. And it is no accident that the octopus is arguably by far the most intelligent of all the invertebrates.
David: Is there more than one kind of eye that natural selection has converged on?
SCM: Yes. There are many, many sorts of different eyes, and in many cases they show convergences, but there are some nuances to this. Perhaps I can briefly explain.
Here, for example, we’ve got eyes – models, I’m pleased to say – which are from different sorts of mammals, as it so happens. But if we look amongst these sorts of eyes, they have the same arrangement as ours. They’re sort of effectively camera eyes, but there are all sorts of differences. Some, for instance, have a reflective layer at the back. You see that in a cat: it’s call the tapetum. In certain cases they can see into areas of the electromagnetic spectrum which we can’t, including ultra-violet light and so on and so forth. And all of these are convergent. So in other words, there’s much more to an eye than an eye. So convergence rules.
But there are more, wide points to make. If we look at this shrimp here, now this is a relative of the insects: it’s an arthropod; it’s technically a crustacean – and, by the way they’re delicious – but at the front we’ve got eyes, and these are very different from the camera eye. They’re so-called compound eyes – that is they have very many lenses. And there are two things which matter about this. First of all, this so-called compound eye arrangement is also convergent. It’s evolved independently four or five times in different groups of animals.
And there’s another point which is, in its own particular way, the compound eye is very economic. Really all you have to do is make a lot of lenses, put them on a hemisphere and plug it into the brain. It’s a bit more complicated than that, but what matters is you can say, look at the amount of light which this compound eye can collect. Yes, the eye is quite small, but if we scaled it up to the size of our eyes, it turns out that the compound eye, if we had to have a compound eye, rather than a camera eye, would be an enormous balloon-like structure, which would be several metres across, above our heads.
Ard: Like this?
SCM: Exactly, colossal.
David: Not terribly practical.
SCM: Not terribly practical. And this matters again, because if we’re looking at intelligent aliens, for the sake of argument, then if they ever visit us, which I think is very unlikely, then they will have camera eyes. They will not have compound eyes.
David: Right. This gets back to your engineering solutions again, doesn’t it?
SCM: That’s right. There’s a rule of engagement here which says there’s nothing wrong with a compound eye, and on those alien biospheres there will be compound eyes. Don’t worry.
It’s not rocket science; it’s evolution.
'When Richard Dawkins refers to ‘The Blind Watchmaker’, I absolutely agree with him. Evolution, per se, does not know where it’s going. But that does not necessarily rule out the possibility that the organisation of the universe at large is predisposed to life.'Transcript
Ard: Do you think convergence strengthens the case for Darwinian evolution?
SCM: I hope so.
Ard: Yeah, exactly.
SCM: Absolutely. I think the slight risk for Darwinian evolution is that – unlike physics, where I’m told that those that practice cosmology have successfully lost most of the visible universe in the last 20 years, busy looking for dark matter and dark energy, and suspect strongly that as and when this ‘material’ turns up, it’s going require a radical rethinking of our understanding of the basic structure of the universe – biology, I think, without sounding offensive, is slightly rested on its laurels. Whereas, in point of fact, the things I’ve been hinting at, with regards not only to convergence, but the integration of form, and the possibility that the number of outcomes is much more restricted, might point to a deeper structure of biology.
What I’m arguing is that natural selection, by in large, is a process. There’s this interconnection – this inter-conversation of different parts of the organism with each other. But there’s another aspect of this which only now is coming into full fruition: it’s a topic of so-called self-organisation. So, for instance, if you look at the way an embryo develops, of course you can see which genes are being turned on and off, which proteins are being made, which cells are dying, which cells are proliferating. But in point of fact, there’s a sort of… almost a flow in the way in which the embryo creates itself.
Now, I think that’s almost as far as people have got, because biologists don‘t tend to say, ‘Well, what makes self-organisation possible?’ It doesn’t happen by accident, and one can only assume that there are physical or chemical factors which are governing these outcomes in biology.
Ard: And these are giving the deep structures?
SCM: That’s the real possibility.
David: And that’s something outside of just the textbook version that the genes are a recipe for everything that happens? This is saying there’s some other level of organisation which constrains the genes, perhaps?
SCM: It’s very likely so. We can’t manage without genes, thank you very much indeed.
David: Well, of course.
SCM: But, of course, one’s also entitled to say, ‘Well, what is a gene?’ And we do know perfectly well that a gene is much more than simply a strand of DNA. The same gene can do different things at different times; the same part of the DNA can do different things at different times as well. So, this rather particulate view of evolution is one which, in a way, is too reductionist.
David: Again that would be contrary to the standard textbook that it’s all random, and there’s absolutely no direction. You’re suggesting there may be, not the idea of a purpose or a goal, but a direction? Would you tell us a little bit more of what you mean by that? I think I know what you mean by it.
SCM: This is tricky territory. To begin with, of course, there’s always a danger of trying to smuggle in a sort of teleology, and this is an area which biology actually struggles with continuously. I think what one can say with some fairness is that there are trends and there are a number of interesting observations suggesting that there are limits to what can be actually achieved.
Ard: You were saying earlier that there’s a directionality towards higher complexity. That’s a trend.
SCM: Yes, I think there is very good evidence that through geological time things become more interesting, if you like, more complex. The reason why people are so suspicious about trends is that, to begin with, there is this older idea of so-called orthogenesis: that evolution was doomed to go in certain directions. I mean, point of fact, that patently isn’t the case in as much as one sees a sort of self-fertile system from a rather uninteresting biosphere some 3 billion years ago to now one which is coruscating with diversity.
Correspondingly, when you see animals in particular, but in fact the argument does extend to plants at least: it’s difficult to avoid the idea that they’ve got some sense of intentionality. They know what they’re doing, and it’s tempting to extrapolate this into ideas of purpose, and I think the problem here is that it’s a philosophical discussion.
So far as biology is concerned, so far as Darwinian evolution is concerned, it is completely and utterly blind. When Richard Dawkins refers to ‘The Blind Watchmaker’, I absolutely agree with him. Evolution, per se, does not know where it’s going. But, that does not necessarily rule out the possibility that the organisation of the universe at large is predisposed to life, is predisposed to evolution, and as I’ve said in a number of other contexts, in one way, evolution is simply the mechanism by which the universe becomes self-aware.
David: So that major trend you’re talking about… when you talk about trends, it’s the trend towards greater complexity and greater mind, isn’t it? That’s the one you keep coming back to.
SCM: In part, but I think Ard, as a physicist, would agree that if we look at the physical organisation of the universe, it is very, very highly ordered indeed. And the paradox, and I think it’s actually an interesting question, is what is it about life, what is this thing, this sort of extraordinary thing which hovers between being chaotic, gas-like behaviour, where nothing ever settles down, to an immobile crystalline-like form?
And life, in this sort of metaphor, sort of describes this incredibly narrow line. It’s sort of tip-toeing all the way along like this.
SCM: And yet it’s that expression of the universe which then looks back at the stars and says, ‘What on Earth are we doing here?’ And this is non-trivial.
David: That description of life being in between too rigid and too chaotic… that obviously appeals to you as a biologist. Does it work for you as a physicist?
Ard: Life is very different from anything we see in physics. So in physics we have things like crystals that are solid and gases that are chaotic and other chaotic systems, and life is kind of on the edge of chaos and order. And there is something amazing, really completely in a different category than anything we have in the physical sciences, which is what makes it beautiful and interesting.
So I think we understand some really important things, like the Darwinian way it develops over time, but there are bigger questions of, why did it develop this way not that way? Convergence does point towards there being some kind of deeper structure; it has to. There’s no way, given the number of possibilities that it could theoretically go down, that there isn’t some other principles that are…
David: Yes, I was going to say… both of you keep using this ‘deeper structure’. What do you mean by this ‘deeper structure’? It’s like you two know something that I don’t.
Ard: Well, we don’t. I got that word from Simon. The ‘deeper structure’ basically means we know something is there, but we have no idea what it is.
David: But that it has rules or structure, or rules that we haven’t discovered yet? Is that what you’re saying?
Ard: I think it tells us that we don’t know the whole story yet, but we know parts of the story.
David: It’s more than that, though?
SCM: Well, in my limited experience, the rules are the things you formulate at the end. Scientists don’t go out there and say, ‘This is a rule and now I’m going to set out and prove it.’ And if we think about what life is, as Ard says, it’s got this sort of fantastic balance between total disorder and over-order.