Jonathan B. Losos: So this question about the inevitability of evolution, the extent to which the outcomes we see in the world today were destined to occur, has a number of implications.
I mean just most generally it tells us whether how fated evolution was to occur, how the outcome today was destined in a way.
But it has other implications as well that people have long speculated about, and that is: what would life be like on other planets if it is evolved? Would it be like the world today here on Earth or would it be completely different?
And this question has taken on some increased urgency or at least interest in recent years because we now realize that there are many planets out there that are like Earth. We used to think that Earth was perhaps unique and so perhaps life as we know it is unique, because we’re the only place that it could evolve.
But quite the contrary we’ve now discovered that there are lots of what are called “habitable exoplanets”, Some people estimate millions, even billions just in our own Milky Way galaxy. So if that’s the case, if there are that many Earth-like planets – and by Earth-like I mean about the same size, temperature, atmosphere somewhat similar, running water – roughly similar conditions. If there are really that many Earth-like planets many people think that it’s very likely that life has evolved on them.
And so the question is what will that life look like? Well there are those who argue that from the argument of convergent evolution they argue that species facing the same conditions here on Earth evolved the same solutions by natural selection.
They extrapolate to say if conditions on other planets are similar to here then we would see very similar lifeforms, that you arrive on whatever planet you’ll see animal and plant-like organisms that look very familiar. Some people have gone so far as to say that, in fact, human type organisms, humanoids will occur on other planets. So there will be intelligent beings that if we saw them they would be recognizable which, of course, is what Hollywood tells us. If you watch almost any science fiction TV show or movie the intelligent lifeform is bipedal, a couple of arms, a mouth. Maybe they only have three fingers and pointy ears and they’re green, but they’re pretty humanoid.
And so some people say yes, that’s actually very likely that humans are a very successful lifeform here on Earth that we are extremely well adapted to our environment which ancestrally was occurring on the plains of Africa. But we have adapted so exquisitely that we now dominate the world. And so if this is such a good adaptation here on Earth it would similarly be a good adaptation on another planet and evolution would be likely to take the similar course. That is the argument that is being made in some quarters.
Not everyone is convinced by this argument that evolution is deterministic. We recognize that convergent evolution does occur more than we used to realize but still it is argued and I agree with this viewpoint, it’s not inevitable. And the reason is that there are often multiple ways to adapt to the same environmental circumstance. And so even though species are faced with the same conditions they may find different ways to adapt to them. And my favorite example of that has to do with a bird that everyone knows – the woodpecker. And everyone’s heard the tat-tat-tat of a woodpecker on a tree or on your garage siding or whatever. People don’t actually know what the woodpecker is doing.
This is what the woodpecker is doing: It is using its beak to pound on dead wood, listening for a hollow space, the echo indicating there’s a hollow space in the wood which is where a grub, a larval beetle or some other insect is eating the dead wood. And so it listens for the sound of a hollow space. When it hears it, it then starts tapping very hard – tat-tat-tat-tat-tat-tat – using its beak as a jackhammer to dig into the, to chisel or to dig into the deep wood to get to the tunnel. Once it’s there—the woodpecker has an extremely long tongue. So long, in fact, that it wraps around its brain case. But it sticks out this long tongue that has little prickles on it, and the tongue goes in and it snags the grub which looks like a mealworm or a – snags the grub and pulls it out and eats it. And that is how they capture the food that they eat.
Well woodpeckers are found on almost every continent in the world. They’re very successful, but they don’t fly very well across water. They don’t like to fly across water. And so isolated islands tend not to have woodpeckers. And in their absence other species have evolved to fill the same niche.
And the most extreme example of a different way of doing the same thing is an animal on the island of Madagascar called an aye-aye. Now an aye-aye is a type of lemur. Now people know lemurs from the TV show Zooboomafoo and maybe they’ve seen them, the ring-tailed lemur. They hop around, very cute, and so on.
The aye-aye is not very cute. It’s about the size maybe of a small housecat and it’s kind of demonic looking. It has these big leathery ears and these bright yellow eyes and a face that only a mother can love. And the native people of Madagascar had all kinds of taboos and myths about them because they look – and they only come out at night. But their most extreme feature that I think really kind of freaks people out is that their third finger is long and extremely thin. It looks skeletal, and it can rotate in any direction. It’s this kind of finger that can wiggle around.
Anyway, they live the same lifestyle as a woodpecker. They’re looking for the same grubs in dead wood. But they do it in a completely different way. Instead of tapping with their peak they tap with their finger. They go around the wood going tap-tap-tap and their big ears are rotated forward listening for the sound of an echo.
And when they hear the echo of an empty tunnel they have these teeth that are – these teeth are kind of sticking out like this, these chiseling incisors that are very strong. And they bite their way through the wood and bite into the wood until they get the tunnel. And then once they get there they then use their finger again. They stick it in there and they snag the larvae and pull it out. And so they’re doing exactly the same thing that the woodpecker is doing but they’ve evolved a completely different set of adaptations. And so that’s just one example of how species can adapt to do the same thing in very different ways.
And we see many examples of that in the world. Conversely we also see many examples of species that have no evolutionary parallel. What we call an evolutionary singleton. That is a species that is very well adapted to where it lives but no other species has done the same thing. And my favorite example of that is the duck-billed platypus, this extraordinary animal in Australia. Now people like to make fun of the platypus but they don’t realize that the platypus is exquisitely adapted to living in the streams in eastern Australia. And so it has very lush fur that allows it to swim in water that’s basically almost at freezing; It has a powerful tail; It’s got webbed feet for swimming; And then most extraordinarily it has this duck bill. Now it’s not actually – it kind of looks like a duck’s bill, but it’s not hard like a duck’s bill.It’s actually leathery.
But more importantly it’s covered with thousands of little receptors and these receptors there’s two types of them. One of them can detect slight variation in ripples of water. And so if something goes swimming by they can detect it in the water But in addition they have electro receptors. They can detect very slight electrical discharges. And so when an animal moves its muscles there’s a little bit of electrical activity, and the platypus can detect that. And so when it’s swimming under water it closes its eyes, its ears and its mouth, but based on the receptors on its bill it can find its way around and it can locate its prey—Remarkable adaptation to eating crayfish and other food items like that.
Well so, the platypus lives in these streams that are in no way remarkable. There are streams like that behind the house I grew up in in St. Louis and they occur around the world. Yet nowhere else has a duck-billed platypus evolved. Why is it that it evolved in Australia and nowhere else? Well there are many examples of species extremely well-adapted but no parallel. Things like the chameleon, elephants, giraffes, many types of plants. Many evolutionary singletons. In fact, humans are an evolutionary singleton. If we are so well adapted to our environment, why didn’t something like us evolve anywhere else in the world? Why didn’t they evolve on Madagascar or in South America where monkeys colonized 40 million years ago?
And so this is the counterpart to the argument of evolutionary determinism and convergence. We could probably make a list just as long of species that have not converged. And so in many cases species – in many cases evolution seemed not to be deterministic. That problems posed by environment may elicit different evolutionary solutions.