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.