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The Earth is now starting to get closer to being hospitable to people like us or animals like us. In the last video, we saw during the Proterozoic Eon, oxygen began to accumulate in the atmosphere. This actually caused this first snowball Earth and this mass extinction of all the anaerobic species. But it made conditions suitable for eukaryotic cells. And maybe even more important, these eukaryotic cells were able to form multicellular organisms. And we see where that starts right here on this chart, on this time clock. Multicellular life starts right over here. And I want to be clear. All of these things are a bit moving targets. As we discover more things in the geological record and we get more tools at our disposal, these numbers get tweaked. But they do give you a good sense, based on our current understanding, of when these things start to appear. And coinciding with multicelluar life, and this is interesting in it's s right because it has its own metalevel effect on evolution, you actually start also having sexual reproduction. And what's interesting about this, why this has such a big impact on evolution-- and we talk about it a lot in the biology playlist-- is before evolution, variation in DNA had to be completely dependent really on mutations, and just random movement around within DNA, or maybe some viruses. Now with sexual reproduction, you had kind of a systematic mixing of DNA so that you got more variation in the gene pool, which allowed more selection for-- or I guess you had more variance to select for. And so you kind of had an acceleration in the actual pace of evolution. So we're talking-- I've looked at a bunch of sources from, they say 1.2 billion, 1.5 billion, a little bit over a billion, if you call a little bit several million years ago, you start having these multicellular life forms and sexual reproduction. The other thing that we talked about in the Proterozoic Eon is the accumulation of oxygen allowed the ozone layer to build up. Ozone is just three oxygen atoms. It is O3. And by the end of the Proterozoic Eon-- so we're talking, I don't know, maybe 550 million years ago, give or take tens of, or hundreds, or maybe 100 million years-- these are all moving targets-- the ozone layer was dense enough to protect the land from UV rays. We talked about that in the last video, that the Earth is being bombarded with UV rays. And the ozone layer is the only thing that really keeps us from being seriously irradiated by the Sun and allows land animals to actually live. And so coinciding with that time period, around 550 million years ago, you start to have life colonizing, especially significant life, colonizing land. So life colonizes land, colonizes the land. And this was kind of an interesting-- when I first learned it, it was kind of an aha moment. You always assume that kind of trees and grasses are kind of part of the background. They come part and parcel with land. But it turns out that animals colonized land before plants did. Plants didn't come into the picture until about 450 million years ago, give or take a few tens of millions of years. And so we're now entering the end of the Proterozoic Eon. Life has started to colonize land. We now have an ozone layer. And what happens-- and actually there's another snowball glaciation or a snowball Earth near the end of the Proterozoic Era, Eon, I should say. And there's a bunch of theories about why it came about. And then why disappeared. Maybe there were volcanoes, greenhouse gases, who knows. But as we enter the end of that, we start seeing life began to flourish. And it starts to really flourish as we enter the Phaner-- I always have trouble saying this-- the Phanerozoic Eon. And it's not even labeled here. The Phanerozoic Eon is this chunk of time right over here. And let me write it out. So this right over here is the Phanerozoic, the Phanerozoic Eon. And so this chart, these divisions right here are eons. And then they jump into, instead of doing eons here, they then break into eras. Eras are subsets of eons. They are hundreds of millions of years. So this is the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. And that's actually our current era. But perhaps the most interesting-- well, I don't want to pick favorites here. But it's one of the most interesting times in the geologic era-- is the first period in the Paleozoic Era, which is the first era in the Phanerozoic Eon. And that's the Cambrian period. You might have heard of it before, the Cambrian period. That's about this period of time right over here, Cambrian. And during this period of time, the Earth experiences what we call the Cambrian Explosion. And that's because there's just this explosion in the number of species and genera that existed, the biodiversity, on the planet. And it might just be that we had the ozone layer protecting us, things were colonizing land. It was an oxygen-rich environment. We start seeing complex multicellular organisms. It's about that time, if you fast forward maybe a few tens of millions of years, you start seeing the first fish, the first kind of preamphibians or protoamphibians. You fast forward a little bit. As we get out of the Cambrian period, we start seeing the plants. So they actually draw it right over here on this-- land plants-- or at this point right over here. And, of course, these are moving targets, depending on what we discover in the fossil record. And for me, the big aha moment here is so many of these things that you consider fundamental to what Earth is are relatively recent phenomena, Plants weren't on land until about 450 million years ago. Insects weren't on land-- or did not even exist until about 400 million years ago. Reptiles didn't exist until about 300 million years ago. So we are about right over here now. Mammals didn't exist until about 200 million years ago. Birds didn't exist until about 150 million years ago. The whole dinosaur age, which we kind of consider in our distant past, that's essentially the Mesozoic Era right here. So this is the Age of the Dinosaurs right over here. When you look at your time clock, you can see it's a relatively recent time period. And it actually ends with, we currently believe, a huge rock, a six-mile in diameter rock, colliding with what is now the Yucatan Peninsula in Mexico, or right off the coast of the Yucatan Peninsula. And it destroyed all of the large land life forms, especially the dinosaurs. And to put all of this in perspective-- and actually the thing that really was an aha moment for me-- it's, OK, plants are 450 million years ago. Grass, I kind of view as this fundamental thing in nature. But grass has only been around for about-- I've seen multiple estimates-- 40 to 70 million years. Grass is a relatively new thing on the planet. Flowers have only been around for 130 million years. So there was a time where you had dinosaurs, but you did not have flowers and you did not have grass. And so you fast forward all the way. And so when you look at this scale, it's kind of funny to look at this. This is the time period where the dinosaurs showed up. This whole brown line is where the mammals showed up. So the dinosaurs started to show up along with the mammals. And then, of course, the dinosaurs died out here. Our ancestors, when the giant rock hit the Earth, must have been boroughed in holes and were able to stash some food away, or who knows what, and didn't get fully affected. I'm sure most of the large mammals were destroyed. But what's almost-- it's humbling, or almost humorous, or almost ridiculous, when you look at this chart is they put a little dot-- you can't even see it here, a little button. They say 2 million years ago, the first humans-- and even this is being pretty generous when they say first humans. These are really the first prehumans. The first humans that are the same as us, if you took one of those babies and your brought them up in the suburbs and gave them haircuts and stuff, they would be the same thing as we are, those didn't exist until 200,000 years ago, give or take. 200,000 to 400,000 years ago, I've seen estimates. So this is actually a very generous period of time to say first humans. It's actually 200,000 years ago. And just to give you an idea of how new we are and how new evolution is, it was only 5 million years ago-- and I mentioned this in a previous video-- it was only 5 million years ago-- so this is just to get a sense. This is 0 years. Homo sapien sapien, only around for 200,000 years. The Neanderthals, they were cousin species. They weren't our ancestors. Many people think they were. They were a cousin species. We come from the same root. Although there are now theories that they might have remixed in with Homo sapiens. So maybe some of us have some Neanderthal DNA. And it shouldn't be viewed as an insult. They had big brains. Well, they didn't necessarily have big brains. They had big heads. But that seems to imply a big brain. But who knows? We always tend to portray them as somehow inferior. But I don't want to get into the political correctness of how to portray Neanderthals. But anyway, this is a very small period of time, 200,000. If you go 2 million years, then you get to kind of the prehuman, the prehuman ancestors. And our family tree only diverged from the chimpanzees 5 million years ago. If you draw that on this clock right here, it would be like two pixels or maybe not even two pixels as when we diverged from the chimpanzees. So hopefully, that gives you a sense of things. At least for me, it really puts things in perspective.