If there is one adaptive radiation that you're likely to need to know about, it's going to be the Cambrian explosion. Now, the Cambrian explosion is an adaptive radiation that introduced nearly every animal phylum. And a phylum, that's the level of organization just under kingdom. So these are major animal groups that are coming around at this time.

So, some examples of animal phyla that arrived in the Cambrian include things like the chordates. That's anything with a nerve cord running down its back; you're a chordate. Things like the mollusks, which include snails, clams, squids, and octopuses, for example. Things like the arthropods, which include crustaceans and insects. So these are major animal groups that sort of burst onto the scene about 530,000,000 years ago. Now, we think this Cambrian explosion takes something like 10,000,000 years, but it's sort of centered around that 530,000,000-year mark. Alright. Now this starts the Paleozoic Era.

So, the Paleozoic Era, if you remember when we zoomed in on our timeline of life, starts with the Cambrian and goes until the end-Permian extinction event. So, what was life like before the Cambrian? Well, animal life was mostly soft-bodied. There were a few known predators. Most animals were probably just fixed and stuck on the ocean floor. And there were many sponge-like creatures. Now sponges are animals, and sponges were around before the Cambrian. And what you have around a lot are things that look like this is the classic Precambrian animal there.

Now, when you look at that, you probably don't look at it and think it's an animal. Right? But then the Cambrian comes, and all of a sudden, life is in color. All of a sudden, animals have hard bodies and hard parts. All of a sudden, you have many predators on the scene. You have much more complex body types, things that you would recognize as animals because they have limbs, arms, and legs. They have jaws. They have the first nervous systems. And now, when I say that life was in color, I almost mean literally because we have the first eyes at this time. This is all sort of shown in this trilobite here. Trilobites came around in the Cambrian.

They were this group of animals that were really numerous in the oceans for a long time until the end of the Permian. But they have these hard bodies. They have those first eyes that we find in the fossil record. And when you see them, yes, they look a little weird. They're not like what's around today, but they're clearly animals.

Alright. So, what caused the Cambrian? Well, remember, we say adaptive radiations can come from either ecological opportunity or new traits that increase fitness. But we think the Cambrian is probably from both of those, and almost sort of working together in almost like a feedback scenario. So first off, before the Cambrian, in the millions of years before, there was this big rise in algae, as I'll indicate there with an up arrow.

Now algae are photosynthesizers, and so they make a lot of oxygen. So that increase in algae caused a rise of oxygen in the atmosphere. Now more oxygen means that animals could potentially get bigger. More oxygen means that animals could potentially be more mobile. And now, this algae also could serve as a food source. So now you potentially have a bigger base to your food chain. Also, we've already noted the rise of predators and predators—animals eating other animals— drives diversification because animals now have to protect themselves. Classic Cambrian animals tend to be hard-bodied and really spiky because they have these first predators that they're evolving defenses against.

Now, we also are just going to say that diversification itself could create new ecological niches. As things are mobile, now they could swim up into the water column instead of just being stuck on the ocean floor. That's a whole new ecological niche. And well, predators could chase them up in the water column. That's a new ecological niche for those predators. You could have predators of predators, which couldn't have existed before. It's just this idea that the more organisms are around, well, that creates new niches for other organisms to evolve.

And then finally, we want to note that there was this new set of animal developmental genes that evolved either early in the Cambrian or slightly before the Cambrian. And these genes are present in many of these animal phyla that burst onto the scene at this time. Now, we're going to look at this in more detail in a future video, but the idea here is that these new genes enabled complex body plans to evolve, or at least enabled these complex body plans to evolve more rapidly. Alright, so to take the takeaway here: right before the Cambrian, animal life is pretty boring. And then we get the Cambrian, and animal life gets crazy, right?

We got animals eating animals. Life is mobile. It's a party. Alright. With that, we've got an example and practice to follow. I'll see you there.