Alright, so let's talk about convergence. What is convergence? You know, I always think about things in evolutionary biology as sort of starting with a tree, and so I think of convergence as being something like that we initially we start off with maybe some sort of shape like A, and then we have an ancestor A, and a descendant A, something like that. So evolution is going from the bottom of the top here.
But then we see some feature, and that feature arises multiple times. It's not that the ancestor was a bee, it's that bee arose twice independently. And that's, very simply, that's the simplest sort of definition of what convergence is.
Now you're probably rather familiar with some classic examples of that. For example, um, uh, you know, actually let me tell you a story. Um, when I was in kindergarten, I got a toy of an ichthyosaur.
And you guys have probably seen an ichthyosaur before, but, you know, an ichthyosaur is... A long fish-shaped lizard like this. It's a marine reptile. It was around in the Triassic.
A famous paleontologist, Mariani, actually, was the one who first found fossil remains of ichthyosaurs. Now, an ichthyosaur is probably more closely related to, oh, I don't know, let's just say, you know, like a lizard. like so.
However, if we look over at the synapsid side of the vertebrate family tree, at us mammals, we see something that's rather similar to this. We see a dolphin. That is possibly the worst dolphin in the world. That's a really bad dolphin.
Well, anyway. All right, that's a dolphin supposed to be jumping out of the ocean. It looks like it is dead.
Yes, it looks like a rotting dolphin. That's what it looks like. I'm sorry about that. I apologize. But anyway, we're...
going to keep going on. All right, so we have a half-dead dolphin, and then there's, of course, us. All right, us and the lizard can be like, what is going on with our crazy cousins?
All right, now these two animals look very similar. All right, they both have a large fin on their back, a dorsal fin. Oh, you're probably wondering where the kindergarten stuff comes in. Well, I'll get there. Don't worry.
They actually all have sort of this long, narrow snout. Long, narrow snout. There we go.
Close enough. They have, you know, usually like big fins. The thiosaur had four.
The dolphin only has two lateral fins. And then they've got a tail portion or something that has a tail. Now in the thiosaur, it's going up from dorsal to ventral. like this, like a fish, and a dolphin, and this is the part that didn't really come out so great, is that it's laterally rotated.
But otherwise it looks like a pretty, I mean, to one's eyes you might say it looks like a pretty similar looking A pretty similar looking tail. So I was in kindergarten and I brought this in like the first week of classes. As a kindergartner, this is my toy for show and tell, and I tried to explain it was an ichthyosaur.
My teacher says, no that's a dolphin, David. And then told me to sit down. So, ichthyosaurs look a lot like dolphins.
And it's so striking, in fact. What we think an ichthyosaur looks like and what a dolphin looks like being so similar. You know, they clearly, lizards and us, did not evolve from something that looks like an ichthyosaur or a dolphin.
It looked like whatever it was, this sort of synapsid-diapsid ancestor was probably some sort of reptilian amphibian looking thing. Would have looked more like a salamander than either a dolphin or an ichthyosaur, certainly. So how is it that we get such similar looking forms evolving across the evolutionary tree?
How can this be? How can this sort of repeated evolution happen? Well, it's actually worse than this.
Because even though the ichthyosaur and the dolphin are the really striking examples, the fact of the matter is if we go further down the tree, if we go over to... Our friends, the ray-finned fishes, will find that there's the tuna. And if we go down the tree some more, we go over to what we call the elasmobranchs, or the cartilaginous fishes. And then we've got sharks.
And they all have this long body, big tails, big fins, the same sort of shape. It's the reason why you can easily mistake a shark with a dolphin. you know, that big fin on there.
So what's going on there? We actually call this shape of fish. Tuna form. What's going on?
Well, they're all different lineages that are adapted for doing the same thing, we think, right? Like the dinosaurs are extinct. We don't know what they were doing.
But we think they were probably doing the same thing that dolphins and sharks and tuna do, which is that they are big, powerful, swimming alpha predators out in the waters, hunting fish mostly that are much smaller than them. And so what do they need? They need speed because the fish that are out there in the open ocean, they like to swim.
And so you need to swim really fast to trap them between the surface of the water and then catch them. And that's why you always see dolphins and sharks, etc., jumping out of the water. They're adapted for doing that sort of thing.
Even when, actually, it's a shark and it's chasing, you know, a poor seal or something in planet Earth. It's leaping out of the water. So these massive muscular machines have evolved multiple times because they've gotten, they have a similar sort of evolutionary pressure to become these big swimming predators. And so thiosaurus were probably adapted for that, just like dolphins are, just like tuna, just like sharks are.
The tuna forms. And this is the classic example of convergence. I'm sure that many of you have probably already heard about this sort of example of convergence before.
Now there's a lot other examples of convergence, I'll go through a few. One thing that's really important to sort of look for when we're defining convergence is that convergence, like adaptive radiations, it's kind of a vague term. It's not really very clean in terms of what is it we mean when we say convergence. Here we've got morphological features, similar morphological features, a whole suite of morphological features that are appearing over and over and over again.
Are eyes a feature that we can say is convergent? You know, if we look at you know, the head of an insect. They've got eyes, but they've got these complex compound eyes.
Meanwhile, we have eyes that in fact have a lens over them, just a single lens. So if we were to draw a little diagram of our eyes, we've got these eyes that have a lens that light passes through, it bounces off the back of your eye, and that's how your eye figures out what you're seeing. The insect eyes are quite a bit different actually. There are a whole bunch of little lenses all strung together and in each one it leads into a little cone, a little tube-like cone, and the light goes in there through all those different lenses and their brain puts it all together and figures out what things look like that way. That's a very different way of designing what an eye looks like.
Well, did the ancestor have an eye? We aren't really too sure. We don't think that the ancestor had an eye, not anything like a human eye or an insect eye.
However, if we want to talk about convergence eyes, we don't need to go, you know, we don't need to go too close to humans, in fact. We can still remain and we can we can still remain this far out, you know, talking about things just as distantly related to us as a fly is. We can talk about an octopus or a squid. So let's just draw a little squid here. All right.
There's a little squiddy friend. Two eyes on either side of its head here. Its mouth is in among its tentacles. Well, the cephalopod eye?
The squid eye, it's got a lens. Light goes through the lens, it's reflected off the back of the eye, and that's how the eye sees. So the cephalopod eyes are really, really close actually to how human eyes are designed. That's an act of real convergence there, right?
Now you could say that there's, you know, morphological convergence. You could also say that there's sort of functional convergence. You know, the arthropod eyes, maybe they're designed very differently. The eyes of the housefly or the ant or the wasp or whatever, they're designed very differently in terms of their structure from our eyes. However, they do the same thing.
Well, there's another thing actually that all these eyes have in common. The other thing that those eyes all have in common, and they also have it in common with flatworms. Flatworms have sort of a body like this and then they've got normally like two spots like this. Their eyes are actually just a bowl-like depression that light comes into.
Like that, alright? Cup eyes. Well, what do all four of these eyes actually have in common? Their placement is actually controlled by a very similar gene.
So there's a gene, and the version of it can be found across all animals. Well, most animals. Let's say all all bilaterian animals, okay?
Jellyfish, etc. need not apply. However, flatworms, bugs, insects, cephalopods, humans, they all have a particular gene, and that gene has changed a little in each of us, but for the most part it does the same stuff, and that gene is called... PAX6.
What does PAX6 do? That's actually an acronym. I'm not going to read out to you what that acronym stands for.
However, what's important is that PAX6 is really important when all four of us are just a lump of cells, basically. There's a different, there's a set of genes that turn on that tell that lump of cells, which are all the cells have just the same DNA in them, which tells them, ah, what you should be doing, what that part of the body is. PAX6 in all of these tells a particular part of your body that that's going to be sort of a head portion. That's important. It's important even when you don't have eyes in terms of making your neural system However, in all of these cases, let's just assume for the moment that all four of these, the eyes evolved independently, you have an eye where you want to put it.
You want to put it on the head portion of your body. Well, guess what? You already have, when you're just a clump of cells, you have this band on your embryo that that's going to become the head portion of your body.
So guess what? You're going to use that gene that helps tell where the head portion of your body is, and you're going to connect, you're going to use that gene to figure out where you want to put your eyes. The eyes do not look like that. Actually, the eyes don't look like anything at this stage.
The cells that are going to become your eyes get told that instruction really early on. And in fact, in all of these, that's true. The flatworm can't make eyes if it's not making Pax 6 because it doesn't know where to put the eyes. If you keep the drosophila, the housefly, or if you keep any sort of insect or crab or crustacean from having Pax 6, it ends up making an adult without eyes.
And it also happens in humans, and it happens in other mammals, and it's also true in cephalopods as well. Pax 6 is instrumental for you being able to figure out where to form particular parts of your body. And one of those parts of your body that it's instrumental in was not a part of the body of the ancestor, the eyes.
Instead, all developed different solutions. Well, us and the cephalopods ended up inventing the same solution as a way of seeing. And then, biologically, the best place to put it was to put it on the head portion of the body.
And how do you get to tell your body to do that? Go look for the Pax6 gene. So this is the sort of way in which the way that we grow and construct our bodies actually has a lot of interplay with how selection happens, right? We look for the best place. The best way to do things, the easiest way to do things, right?
Evolution does. And evolution doesn't actually look. It's looking blindly, right? It doesn't actually see any of the solutions it's actually trying.
It just tries them and it tries so many of them that it randomly ends up picking things up, seeing if they work, and then trying again over and over and over again for millions and millions of years. And it adds up to eventually seeming like that it's found a solution. So millions and millions of years allowed for the eyes to become really dependent upon Pax 6 as a way of figuring out where the eyes should go. And that was all probably figured out more than 500 million years ago when these things all started becoming complex enough that we start finding fossils of arthropods, vertebrates, cephalopods, etc. in the fossil record. Okay, so that's development and adaptation and how the two work together and this kind of leads into the next thing which is parallel evolution.
So some actually kind of differentiate convergence from parallel evolution. A way to think about it in this case is that convergence, a really good example of convergence is that morphological convergence between the cephalopod eye and the human eye. The structure of the eye is the same thing with one lens.
What is really interesting though is the parallel evolution, and we could say that the fact that all these different lineages of animals randomly decided, well, ended up deciding on using Pax6 as the gene that would help figure out where to put the eyes on the body. That's parallel evolution. And parallel evolution is a case of where development and the constraints of development mean that maybe the same solution gets found more often.
But also at the same time, it's really hard to differentiate that from just saying that there's just not many different things to do with the body that you have. All right. A really good example of this is Ammonites.
All right. And Ammonites, you probably have seen an Ammonite shell before. The Ammonites are named for Ammon, a ram god. The Romans thought that, you know, if you've got this big spirally form that looks like a ram horn coming out of the rocks, clearly that is evidence of the divine power of gods and that the ram god clearly has, you know, some sort of... He's left his imprint on nature because why else could it come out of rocks?
Hence ammonites. But ammonites are this really big, really diverse group of cephalopods that had shells, and some of the shells got really, really big. Now the most common form for that shell is something like this, what we would call a planospiral shell. So it's just a coil, and it's a coil all in one plane.
You've probably seen maybe some snails that have shells like this. There's also nautiloids, which are another cephalopod group that also has shells. But those two different groups, the nautiloids and the ammonites, actually they evolved They're shells independently. They're actually rather distantly related to each other.
Ammonites, we think they're more closely related to octopuses and squid than they are to the nautiloids. The nautiloids have a great fossil record, by the way, as well. Particularly back in the Paleozoic, they were known for making these long shells that we call orthocone nautiloids. Orthocone nautiloids. And orthocone cone nautiloids, basically they uncoiled their shell, and then they just lived in this long, straight cone.
All right. Well, here's the funny thing. In the Mesozoic, which is hundreds of millions of years later, ammonites start doing the same thing too.
And in fact, it's really one of the best known fossils in North America, very, very common. It's called Baculites. And it's just a shell that has no spiral to it, just a comb. It is broken up into multiple segments, which is just how ammonites made their shell. They had what we call septa, little walls that they made on the inside of the shell as they grew.
Something like that. Oh, I don't have to do it all, but I will because otherwise it doesn't look right to me. So something like that.
So you see they've uncoiled their shell and they've made a long tube. We could call this a case of convergence, right? The nautiloids and the aminoids are very distantly related to each other. But I don't know if it's really quite accurate to call this really convergence as much as the fact that there's not a whole lot you can do if you have a conical shell that's coiled in a big spiral.
Right? There's only so many things you can do with that shell, as long as you have that shell. Ammonites do end up doing a few other odd things about it. There's one species, Nypinites, which literally looks like someone decided to ask, what if a knot was alive?
And by a knot, I mean like a knot, like a really big knot was alive, and that was an ammonite. But Baculites and Orthoconautaloids, they seem to have both just sort of said, well, what if we just unwind the shell and we just have a long straight thing in the water? What if we just do that? We'll just swim around like this. Hello, I'm a nautiloid.
Hello, I'm a baculitis. And that's just what they seem to do. It's a very simple alteration to do once you already have sort of a planar spiral shell like that.
And that's maybe more of a case of what we would think of as parallel evolution, where similar sort of constraints about how to build a body leads to very similar looking bodies getting built independently. So there is convergence definitely between the nautiloids and the ammonites. Some would say that this is maybe more of a case of parallel evolution. And that's important to thread out, is that there's cases in which maybe species might just randomly end up looking like each other in some ways, but it doesn't necessarily mean that they have the same ecological function.
It could be just because they both have similar constraints on their body form. And those similar constraints might be because they're related to each other. Now, this is all great and gravy, but it's really covered up the board. You guys can hardly see me behind all these really badly drawn dolphins and ammonites and cephalopods, etc.
We've talked about the different types of convergence and what is truly good convergence. Let's talk about one really great example of convergence that I like because I don't really think we're gonna get time to cover it that well in this class, and that is that convergence can really reveal to us things about ecological niches that we don't have around today, right? So like, it's really important to take this into account. We live in a world where 12,000 years ago there was giant glaciers over most of the northern hemisphere. The world is still responding to, still reacting to, the fact there were giant glaciers everywhere 12,000 years ago.
Why is that important? Because it means that the biological communities we're looking at are still responding to that. And there's also lots of other stuff that happened too. We lost a lot of very large animals about 12,000 years ago. Either due to the fact they were adapted for living in a glacial interval.
or because a bunch of humans showed up in their territories. All right? One of these animals, which you may have heard of, was the giant ground sloth.
I believe the giant ground sloth is a character, one of the characters in Ice Age, but they make him very small, rather docile. The giant ground sloth was 20 feet tall. Here's an image of a skeleton of it.
Looks almost like a giant bear when you look at the skeleton. They were giant. They were bipedal.
They're truly giant, by the way. And they had these giant claws on their forelimbs, which you'd expect. They're sloths, right? And it was a herbivore.
We know pretty well that based on its teeth it was eating plants. Just a few other cool things about giant sloths. Some of them had armor because actually sloths are really closely related to armadillos, so some of them developed armor but that was in South America. All the giant ground sloths that came to North America came from South America when the South American and North America joined up in what we call the Great American Interchange. Mostly that meant North American animals went to South America and killed off all the South American animals that lived down there.
We lost some cool things guys, but like giant ground sloths. And there are places in South America where giant ground sloths built big cave networks, because they lived in caves. They would dig big tunnels underground.
That's really weird, isn't it? That sounds like it's like out of a fantasy novel or something. But anyway, so we've got these giant 20-foot ground sloths with huge claws on their hands that eat plants. Okay, great.
Well, here's the weird thing. Go back about 15 million years. You're in the Miocene.
Alright, there's an animal, there's actually a few different species of a type of animal we call calicothyrs. What are calicotheres? Calicotheres are a relative of rhinos, but if you saw it, you'd probably say to yourself, that looks like a horse. At least if you saw it from the neck up. It looks like a horse, and in fact, horses and rhinos are actually part of the same group.
Okay, but it's probably more closely related to rhinos than it is to horses. But head-wise, horse. Okay?
If you saw it from the neck down, you'd probably say, that looks like an evil gorilla with claws. Why? Because it was bipedal.
It was a bipedal horse. Not quite bipedal. It had really, really long arms with j- big claws on those arms, and it seemed to walk on its knuckles like a mountain gorilla does or a chimpanzee. So just imagine that. So we've got a giant 20-foot ground sloth, and we've got also these quasi-bipedal rhino horse relatives with claws on their hands, also herbivorous.
That's weird. That sounds a little like convergence, doesn't it? We've got these things. They seem to be adapted for standing up on their hind legs. They've got really big claws, and they eat plants, right?
Let's keep going. We go back further, we go back to the Cretaceous. All right.
In the Cretaceous, across North America, Western North America, pretty much, and in Asia, we have a group of dinosaurs, non-avian dinosaurs, called therinsosaurs. Therinsosaurs had really long claws, and they were probably a group kind of related to Maniraptorans. We'll talk more about them when we get to theropods.
All right. But therinsosaurs were probably feathered. They had long necks. They had beaked mouths that were for mostly eating plants. And they had these really long claws.
Okay, and they were bipedal by the way, like all theropods are generally bipedal. Okay, so we've got another case of that. Go back a little bit more in time. In the 1970s, two giant arms were dug up from the early Cretaceous.
Those two giant arms look like the arms of what the group of animals we think of as the ostrich-like dinosaurs, the ornithomimus. Remember the gallimimus from Jurassic Park? Same group of animals, okay?
But there were these giant person-sized arms with these giant claws on them. No one knew what the owner of the claws looked like. Alright?
There was a big mystery. They were called Dinoceras, the terrible claw. And, you know, I saw this as a child in dinosaur books.
And I thought that was the coolest thing in the world, let me tell you. Giant person-sized claws. No idea what the rest of the animal looked like.
I thought of all sorts of terrifying things. Turns out Dinoceras is a giant hump-backed dinosaur. Literally with a hump.
Big hump on its back. Not like a cool, like, you know, like fin, like the spinosaur or whatever. Just a big hump on its back. And it's got this weird flat-looking skull with sort of like a duckbill on it. It's like a, it's like a, you know, if a platypus dreamt of being a dinosaur, maybe this is kind of what it would kind of be like.
It was probably covered in feathers as well, so really weird looking animal. So herbivorous, we think, based upon the duck bill. Really big arms with claws, bipedal and humpback, and seems to be adapted for doing something with those claws.
So again, really odd set of circumstances here, right? We've got bipedal, big. Giant claws and herbivorous. There's nothing like this today, all right?
Now, you only have to go back 12,000 years to get to the giant sloths. But they're extinct now. We can't study what they were doing ecologically. But probably all four of those things were doing something very similar. Dinoceras, the therizosaurs, the calicotheres, and the giant sloths.
And that, I think, is a really exciting convergence because that's telling us about a niche we can't see today because all the things that might have occupied that niche have died out. But we can study the fossil remains of those guys to understand something that we cannot go out and see in the world today. That there is some sort of ecological space for a bipedal giant thing with claws.
And isn't that cool? I think that's really cool. Convergence, you see, is key.
If we want to understand what alien life is like, we need to look at the cases where we have the repeated evolution of features across life. And that's the idea I want to leave you with today. Think about convergence and what it can tell us about aliens.