Now let's take a quick look, and I really mean a brief look at the course of life on Earth. The different time periods of Earth's history have been divided into various subsections. And here in this diagram: Archaean, Proterozoic, Paleozoic, Mesozoic, Cenozoic. But I'm actually just going to make two big divisions in the course of Earth's history. What I'm going to cover is what is called Precambrian time and the Phanerozoic era, which is the modern era in which we live. Now, Precambrian time is the period of time between the formation of Earth and the appearance of most animal groups around the Cambrian Explosion, which was a period of rapid evolutionary diversification. During the majority of Earth's history, life has been unicellular. In fact, you can see from this figure, multicellular life is quite recent in terms of the billions of years the Earth has existed. Also, a very important thing to note especially about the early ages of Earth's history, is that oxygen was absent. In fact, it wasn't until the photosynthetic processes evolved that oxygen became abundant in the atmosphere. And in fact, way back in the time of the dinosaurs, for example, oxygen was far more abundant than it is now. Which is part of the reason that living organisms were so much bigger back then. It's in part because of the lower oxygen concentrations that life has gotten smaller over the course of history since the dinosaurs until now. A very important event occurred in Precambrian time, that is the formation of the first eukaryotes and they were formed through a process called symbiogenesis. What is symbiogenesis? Symbiogenesis, which you probably know as Endosymbiont Theory, is when two separate life forms fuse together to create a new life form. So what is endosymbiont theory? Well, remember, we had our big giant membranous cell that engulfed an itty-bitty little prokaryote which eventually became the mitochondria. That was a symbiogenesis event. So, the first eukaryotes formed through symbiogenesis, through one cell engulfing another and the two living symbiotically. Additionally, multicellular organisms appeared during this time. Another really important development in the course of life on Earth.
Now in the Phanerozoic era, we've had a couple of pretty major events. In the Paleozoic era, we have the Cambrian explosion which again kind of defines the boundary between Precambrian time and this modern era that we live in. The Cambrian Explosion created a great diversity of life. Also, during the Paleozoic era, we have the terrestrialization of Plants and Animals. Yes, before this time all life existed in the oceans and it was in this era where life actually finally made it to land. First plants, then animals. Then in the Mesozoic era, we have the dominance of gymnosperms and dinosaurs. Gymnosperms are plant organisms like pine trees, for example. These were the dominant plant life form on terrestrial Earth back then, and dinosaurs were the dominant animal life form on terrestrial Earth back then. Of course, we had a great or a mass extinction. See you later dinosaurs. And we move into the Cenozoic era, where now the plant scene is dominated by angiosperms, which are flowering plants. Like the ones that produce delicious fruits and vegetables that we so love to eat. And also, the dominance of mammals in the animal scene. And not to get too deep into it, but in many ways the dying out of the dinosaurs opened the door for mammals to become the next dominant animal form. So again, those mass extinctions, it's kind of like one door closes, another door opens.
The last idea I want to touch upon in this lesson is the idea of punctuated equilibrium. And this is essentially the theory that evolutionary changes occur rapidly, like in the Cambrian explosion. And these rapid, these events where there's rapid evolutionary change punctuate long periods of very little change. So basically, if you were to plot the course of evolutionary change on Earth, you'd have long periods where not a lot is going on, and then boom spikes of activity. And then again, long periods of not a lot going on and then some major thing causes, boom, another burst of activity. The Cambrian explosion is an example of one of those bursts, but the creation of eukaryotes represents another example of that, punctuated rapid change. Right? We have symbiogenesis, new life is created, and boom, it just explodes and takes off into all these different shapes and forms. So, yes, the course of Earth's history is not just a steady progression with steady change but in fact, more chaotic, you know, periods of rest with periods of evolutionary explosive chaos, basically. Alright. That's all I have for this lesson. I'll see you guys next time.