As we think about the evidence for evolution, how we know that species have changed over time, one of the clearest pieces of evidence that we have is going to be fossils. And fossils are the remains of organisms that lived in the past that we find in layers of rock or sediment in the Earth. This happens because as Earth goes through time, sediment gets laid down, and organisms that die get trapped in that sediment. And then that sediment gets compressed, and it forms these layers of rock that we can then go dig in and find the remains of those organisms. Now, this means that the geologic layers provide evidence for broad patterns in evolution.

So when you dig into these sorts of layers of the Earth, you're going to create a timeline for earth, where the deeper layers are older. And when we do this, when we look through these layers, we see this very clear pattern. The deeper, or we could say the older layers, have very different organisms than we see on Earth today. Alright. So we have this sort of simplified version here that we're going to go through.

We can imagine this as like, you know, a cliff wall or a canyon or something where we can see these exposed layers. Well, if we went digging here, near the surface, you might expect to find fossils, but near the surface, you're going to expect those fossils to be similar to things that you see today. Maybe you find something like this woolly mammoth skeleton. Right? Woolly mammoths, they're not around anymore.

That's an extinct organism, but they're very similar to the elephants that we see on Earth today. Now, if you go in deeper sediments, you're not going to find woolly mammoth skeletons anymore. Right? When you go deeper, you might find something like dinosaur skeletons. Now dinosaur skeletons, you don't find in those higher-up sediment, but you know there are layers where you find these dinosaur skeletons all over the earth, so it's very clear that these things were once common, living at a very specific time in Earth's history.

Now you go deeper, you don't see these dinosaur skeletons anymore. Maybe you see something like trilobites. Hundreds of millions of years ago, trilobites lived all over the ocean, and then they didn't. Right? They are definitely not around anymore, and they haven't been for a very long time.

So these layers in the fossils that we see demonstrate both extinctions, right? Things that were once alive aren't alive anymore. And we can also track these sorts of broad evolutionary changes. But we can also use fossils to track very specific evolutionary changes. So here we're going to say that fossils demonstrate specific evolutionary histories and transitions.

For this, we're going to use whales as an example, and whales are mammals. And the idea is that whales evolved from 4-legged animals that lived on land. Alright. So we're going to start with that as our hypothesis. Whales evolved from 4-legged, we're going to say here, terrestrial.

Terrestrial just means living on land. Terrestrial ancestors. Well, if that's the case, when we go digging and looking for fossils, we should be able to find fossils that show whale-like fossils with, we're going to say, intermediate intermediate leg structures. Alright. So we can test that hypothesis.

Right? So if you look at modern whales, modern whales have been around for basically the last 34 million years. Now they haven't looked exactly like this for the last 34 million years, but a characteristic in modern whales is this vestigial pelvis. Modern whales have a pelvis where you would expect the legs to connect to, but they don't have any legs, and only a couple of muscles connect to that pelvis. It's really not doing a lot for the whale.

Well, if we look back in time, if we look down in the layers and dig for fossils, what do we find? Well, in layers from about 40 million years ago, you find this organism called Darudin. Now, Darudin looks a lot like a whale, but it's got all its leg bones. Now, this is clearly a vestigial leg. It's not using this to walk on.

It's quite diminished from something that you could use to walk, but all the bones are still there. Now you go deeper. Well, you find this animal, Rodicetus, that was alive about 47 million years ago. Now, this is an organism that swam a lot, but it has a full leg. This is a swimming leg with probably webbed feet.

You can sort of imagine this maybe swimming kind of like a very large otter or something like that. Well, you go even further into the past, deeper in the layers, from 50 million years ago, you find this organism Indohyas. And Indohyus clearly has a leg that was used for walking mostly on land. Now we know that these things are related to each other from all the different homologies that they share, But we can see this transition in leg structure over time. Now it's important to remember, Inohayas wasn't trying to become a modern whale.

None of these organisms are trying to evolve in that direction. But if we play this sort of back in the other direction, we can see how it could happen. Indohyas, people think, lived by the water's edge, maybe hunted in the water. And so maybe it does a little bit of swimming, and then over time, variation that makes swimming easier becomes more common. You end up with this swimming leg in the Rodicetus.

This organism is spending more and more time in the water. It's swimming more and more with its tail. It ends up really not using its leg. It's not going on land anymore. The leg becomes even more diminished until eventually variation that doesn't have any leg bones, that seems to work the best for swimming.

That's what you find in the modern whale. All right. So remember, fossils can show us those very broad evolutionary patterns, but we can also test hypotheses for how species have changed through time. All right. Like always, there's examples and practice problems to follow.

You should give them a try.