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We finished off the last video in the Hadean Eon. It was named for Hades or the ancient Greek underworld. Hades is also the name of the god that ran the Greek underworld, Zeus's oldest brother. And it was an appropriate name, although the idea of, the ancient Greek notion of the underworld isn't exactly the more modern notion of Hell. But it was a hellish environment. You had all this lava flowing around. You had things impacting the Earth from space. And as far as we can tell right now, it was completely inhospitable to life. And to make matters worse, even though the Earth started to cool down a little bit, maybe the crust became a little bit more solid. Maybe the collisions started to happen less and less, as we started to go a few hundred million years fast forward. After the Theia rammed into the early earth and formed the moon, there was something called the Late Heavy Bombardment. And right now, the consensus is that life, whatever we are descended from, would have had to come about after the Late Heavy Bombardment. Because this was a time where so many things from outer space were hitting Earth, that it was so violent, that it might have killed off any kind of primitive, self-replicating organisms or molecules that might have existed before it. And I won't go into the physics of the Late Heavy Bombardment. But we believe that it happened, because Uranus and Neptune-- so if this is the sun right here-- that is the sun. This is the asteroid belt. That's outside the orbits of the inner, rocky planets. That Uranus and Neptune, their orbits moved outward. And I'm not going to go into the physics. But what that caused is, gravitationally, it caused a lot of the asteroids in the asteroid belt to move inward and start impacting the inner planets. And of course, Earth was one of the inner planets. And I should make the sun like orange or something, not blue. I don't want you to think that's Earth. And it also impacted the moon. And it's more obvious on the moon, because the moon does not have an atmosphere to kind of smooth over the impact. So the consensus is that only after the Late Heavy Bombardment was Earth kind of ready for life. And we believe that the first life formed 3.824 billion years ago. Remember, g for giga, for billion years ago. And when we talk about life at this period, we're not talking about squirrels or panda bears. We're talking about extremely simple life forms. We're talking about prokaryotes. And let me give you a little primer on that right now, though we go into much more detail in the biology playlist. We're talking about prokaryotes. And I'll compare them to eukaryotes. Prokaryotes are, for the most part, unicellular organisms that have no nucleuses. They also don't have any other membrane-bound what we call organelles or these little parts of the cells that perform specific functions, like mitochondria. So their DNA is just kind of floating around. So let me draw this character's DNA. So it's just floating around, just like that. And prokaryote literally means before kernel or before a nucleus. Eukaryotes do have a nucleus, where all of their DNA is. So this is the nuclear membrane. And then all of its DNA is floating inside of the nucleus. And then it also has other membrane-bound organelles. Mitochondria is kind of the most famous of them. So it also has things like mitochondria. We'll learn more about that in future videos. Mitochondria, we believe, is essentially one prokaryote crawling inside of another prokaryote and kind of starting to become a symbiotic organism with each other. But I won't go into that right now. But when we talk about life at this period, we're talking about prokaryotes. And we still have prokaryotes on the planet. Bacteria and archaea are examples of prokaryotes. And just to give you a little bit of a tidbit right here, this kind of shows our current understanding of where we think things branched off from. So at this point of the tree is some common ancestor to prokaryotes and eukaryotes. So these are the prokaryotes right over here, the bacteria and the archaea. And here is the eukaryote. And this first living thing, or this first set of living things, we think might have just been some type of self-replicating molecules. And slowly, some membrane might have come around, and it became a little bit more organized. DNA, RNA-- maybe RNA was that original self-replicating molecule-- became the method of kind of transmitting information from one generation to the next. So it's really still an open question of exactly what that first life is or even how do you define that first life. But based on studying the genetic makeup of molecules of current organisms, this is how we think the tree of life came about. So we have one common ancestor. Then they broke apart. And then the archaea and eukaryotes have a common ancestor that's different from the bacteria. And we'll talk more about that in the future. And this right here, just so you can visualize it, this is an example of bacteria. This is E. coli Or Escherichia coli. It's just an example of bacteria. It comes in a bunch of shapes and forms. But it's a prokaryotic life form. And the earliest life forms, we also think were anaerobes. These are things that did not-- one, that they did not need oxygen. And they, for the most part, found oxygen poisonous. And the earliest life forms also probably did not perform photosynthesis. They might have gotten their energy from other sources, chemically, from this kind of extremely volatile environment that they were in at that time. So if we fast forward a little bit-- and this is actually a major event in the history of Earth. And these are huge time scales we're talking about. Remember, I'm kind of just nonchalantly saying, oh, 4.6 billion years ago to 3.8 billion. Oh, that's just 800 million years. Remember-- and I'll talk about this. Grass has only existed for 50 million years. This is 800 million years. Humans and chimpanzees only diverged 5 million years ago. This is 800 million years we're talking about, from ancient Greece to now, we're only talking about 2,500 years. You multiply that times 1,000, you get 2 and 1/2 a million years. And this is 800 million years we're talking about. So these are extremely huge periods of time. And that's why we call them eons. Eons are 500 million to a billion years. Now, the dividing line between the Hadean Eon and the Archean Eon-- and it's kind of a fuzzy dividing line, but most people place it about 3.8 billion years ago. It's kind of the earliest rocks that we can observe. And so we have rocks from, that are roughly 3.8 billion years ago. So we kind of put that as the beginning of the Archean Eon. And so there's two things there. One, rocks have survived from the beginning of the Archean Eon. And also, that's roughly when we think that the first life existed. And so we're now in the Archean Eon. And you might say, oh, maybe Earth is a more pleasant place now. But it would not be. It still has no to little oxygen in the environment. If you were to go to Earth at that time, it might have looked something like this. It would have a been a reddish sky. You would have had nitrogen and methane and carbon dioxide in the atmosphere. There would have been nothing for you to breathe. There still would have been a lot of volcanic activity. This right here, these are pictures of stromatolites. And these are formed from bacteria that are bringing in sediment particles. And over time, these things get built up. But the most significant event in the Archean period, at least in my humble opinion, was what we believe started to happen about 3.5 billion years ago. And this is prokaryotes, or especially bacteria, evolving to actually utilize energy from the sun, to actually do photosynthesis. And the real fascinating byproduct of that, other than the fact that they can now use energy directly from the sun, is that it started to produce oxygen, so starts to produce oxygen. And at first, this oxygen, even though it was being produced by the cyanobacteria, by this blue-green bacteria, it really didn't accumulate in the atmosphere. Because you had all of this iron that was dissolved in the oceans. And let me be clear. All of the life that we're going to be talking about for really the next several billion years, it all occurred in the ocean. We had no ozone layer now. The land was being irradiated. The land was just a completely inhospitable environment for life. So all of this was occurring in the ocean. And so the first oxygen that actually got produced, it actually, instead of just being released into the atmosphere, it ended up bonding with the iron that was dissolved in the ocean at that time. So it actually didn't have a chance to accumulate in the atmosphere. And when we fast forward past the Archean period, we're going to see, that once a lot of that iron was oxidized and the oxygen really did start to get released in the atmosphere, it actually had-- it's funny to say-- a cataclysmic effect or a catastrophic effect on the other anaerobic life on the planet at the time. And it's funny to say that because it was a catastrophe for them. But it was kind of a necessary thing that had to happen for us to happen. So for us, it was a blessing that this cyanobacteria started to pump out a lot of oxygen and eventually oxidized all of the iron and eventually released a lot of oxygen into the atmosphere and killed off all of this anaerobic bacteria, so that eventually we could-- us oxygen-breathing organisms could come about. But that's not going to happen for a while. We still have a few billion years before things start flopping around on the land. Anyway, see you in the next video.