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Ozone layer and eukaryotes show up in the Proterozoic eon

Ozone layer and eukaryotes show up in the Proterozoic eon. Great oxygenation event (oxygen catastrophe). Created by Sal Khan.

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Video transcript

We left off in the last video in the Archean Eon. And "arche" comes from ancient Greek. It means beginning or origin. And it's the eon in which either life first started to exist or at least it first start to somewhat flourish. It's possible that maybe life first started to exist at the end of the Hadeon Eon. And, of course, this boundary is vague. And the Archean Eon is also the first eon where we still have rocks from that time. So we are able to find rocks that we can date to be roughly 3.8 billion years. Now, the other really interesting thing that happened in the Archean Eon, and it really has pretty profound effects once we get into the Proterozoic Eon, is that you started to have cyanobacteria produce oxygen. And we said in the last video that they were producing oxygen, but most of that oxygen was being absorbed by iron in the oceans. But what happens is we enter into the Proterozoic Eon. And Proterozoic Eon, it's right over here. I don't know if you can see it . I'll rewrite it, Proterozoic. We're now in the Proterozoic Eon that starts up about 2.5 billion years ago. And "proterozoic" comes from the Greek for earlier life. And I'm not a Greek scholar. So any of you Greeks out there, forgive me if I'm not getting the translation exactly right. But what's really interesting about the Proterozoic Eon is that that oxygen that was being produced by the cyanobactera, and at some point begins to saturate the iron and any other molecule that could have absorbed it before. And once it saturates, it starts to get released into the atmosphere. So the oxygen starts to get released and accumulate in the atmosphere. And we think this started to happen about 2.4 billion years ago. So 2.4 billion years ago, oxygen begins to accumulate in the atmosphere. And, of course, these dates, they might be moved around a few hundred million years as we get more and more data. But this is the current understanding of when things happened. And maybe we'll look at the geological record, or the fossil record, and we'll move these things around in the future. I can only imagine that 50 years from now, or 100 years from now, if someone is still watching this video, a lot of this might say, hey, we found out later that oxygen started to accumulate in the atmosphere earlier or later or that eukaryotes occurred earlier or later. But this, as far as I can tell, is our best current understanding. But the 2.4 billion years ago, oxygen begins to accumulate in the atmosphere. And what's interesting about this is once it accumulates and once it gets kind of a critical amount of oxygen in the atmosphere-- and I touched on this in the last video-- about 2.3 billion years ago, we have something called the Great Oxygenation Event, sometimes called the Oxygen Catastrophe. And this is right here. They marked it this right here, 2.3 billion years ago or 2,300 million years ago, atmosphere becomes oxygen rich. And it's not as oxygen rich as our current atmosphere. But it becomes oxygen rich enough that at least the environment becomes suitable for eurkaryotic organisms or eukaryotic cells. Now the other interesting thing, and we might not care so much about it because we needed the oxygen, is that we think that this was actually the greatest extinction event in the history of Earth. That's why it's called the Oxygen Catastrophe. So this right over here, 2.3 billion years ago-- I shouldn't giggle about it-- this is a serious matter. This is the greatest extinction event in Earth's history, in History. And I'll do history with a capital H, in the history of the Earth. And that's because the cyanobacteria is producing all this oxygen. It eventually saturates the iron. It accumulates in the air. Once it gets to enough concentration, it begins to actually suffocate. It's poisonous. It's poisonous to most of the other organisms on the planet that were anaerobic, that did not need oxygen, that actually found oxygen poisonous. But since we have oxygen, there's two interesting things that happened once that oxygen accumulated, other than causing this mass extinction event-- actually three interesting things. Two of them are crucial to us eventually showing up on this planet. The first is is that it became suitable now for eukaryotes to exist. Eukaryotic organisms, remember, these are organisms that have nuclear membranes around their DNA. Most eukaryotes have other organelles, like mitochondria. They need oxygen. You can go to the biology playlist. We actually talk about respiration that occurs in the mitochondria. And that's obviously a process that needs oxygen. So one, we have, now that oxygen is in the atmosphere, we're starting to have an environment where eukaryotes could at least exist. And based on the fossil records, and when we look at how the DNA has changed over time-- and we'll do multiple videos of that-- we think that the first eukaryotes showed up about 2.2 billion years ago. Although there's some debate here. There's some evidence it might have been a little earlier, some evidence it might have a little later. I'm sure that number will be refined. But give or take a few hundred millions of years, one prokaryote got engulfed by another prokaryote, and said, hey, we do pretty well living together. I mean the current theory is that mitochondria is actually descended from kind of an ancient prokaryotic cell, an ancient bacteria. It actually has its own DNA. And actually, your mitochondrial DNA is passed down from your mother, and your mother's mother, and your mother's mother, so on and so forth. So it's kind of like another little animal living inside of a larger cell. And we are eukaryotes. We needed this to happen. The human body, we're not just one eukaryotic cell. We're made up of trillions. The estimates are 50 to 100 trillion eukaryotic cells. So these are our ancestors that had to come into being at that time. And once again, all of this is happening inside of the oceans. Now, the other interesting thing that happened-- remember, we're being bombarded with UV radiation from the Sun. So if you're on the land-- let me draw the land and the ocean. So here is the ocean and then here is the land. Here is the land, right over there in yellow, constantly being bombarded with UV radiation. And "UV" stands for ultraviolet. So I'm drawing it in purple. But it's even more violet than purple. So it's constantly being bombarded by with ultraviolet radiation from the Sun, which is very inhospitable to DNA and to life. And so the only life at this point could occur in the ocean, where it was protected to some degree from the ultraviolet radiation. The land was just open to it. Anything on the land would have just gotten irradiated. It's DNA would get mutated. It just would not be able to live. So what happened, and what I guess has to happen, and the reason why we are able to live on land now is that we have an ozone layer. We have an ozone layer up in the upper atmosphere that helps absorb, that blocks most of the UV radiation from the Sun. And now that oxygen began to accumulate, we have the Oxygen Catastrophe. Oxygen accumulates in the atmosphere. Some of that oxygen goes into the upper atmosphere. So we're now in this time period right over here. It goes into the upper atmosphere. It actually really reacts with the UV light to turn into ozone, which then can help actually block the UV light. And I'll do another video maybe on the ozone/oxygen cycle. So this oxygen production, it's crucial, one, to having an ozone layer so that eventually life can exist on the land. And it's also crucial because eukaryotic organisms need that oxygen. Now, the third thing that happened, and this is also pretty significant event, we believe that that oxygen that started to accumulate in the atmosphere, reacted with methane in the atmosphere. So it reacted with methane. And methane is an ozone-- not an ozone. It's a greenhouse gas. It helps retain heat in the atmosphere. And once it reacts with the oxygen and starts dropping out of the atmosphere as methane, we believe the Earth cooled down. And it entered its first, and some people believe it's longest, snowball period. So that's what they talk about right here on this diagram, the first snowball Earth. It's sometimes called the Huronic glaciation. And that happened because we weren't able to retain our heat, if that theory is correct. And so the whole-- as the theory goes-- the whole Earth essentially just iced over. So as we go through the Proterozoic Eon, I guess the big markers of it is it's the first time that we now have an oxygen-rich atmosphere. It's the first time that eukaryotes can now come into existence because they now have oxygen to, I guess we could say, breathe. And the other big thing is now this is where the ozone forms. So this kind of sets the stage for in the next eon, for animals or living things, to eventually get on to the land. And we'll talk about that in the next video.