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臭氧层和原核生物在元古宙时期的出现.大氧化事件(氧化灾变). Sal Khan 创建




we left off in the last video in the Archaean Eon and where art key comes from ancient Greek it means the beginning or origin and it's the eon in which either life first started to exist or at least it first started to somewhat flourish it's possible that maybe life first started to exist at the end of the Hadean Eon and of course this boundary is is vague and on the Archaean en 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 Archaean Eon it really has a pretty profound effects once we get into the Proterozoic Eon is that you started to have this ionic 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 and if you can see that I'll rewrite it Proterra 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 earlier earlier life and I'm not a Greek scholar so III you know any of you Greeks out there forgive me if I'm not getting the translation exactly right but what's really interesting at the brought the Proterozoic Eon is that that oxygen that was being produced by the cyanobacteria 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 happened to 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 you know these dates they might be moved around a few hundred million years you get more and more data but this is the current understanding of when things happen maybe we'll look at the geological record of the fossil record and we'll move these things around in the future I I could only imagine that 50 years from now or 100 years from now if someone's still watching this video a lot of this might say hey you know we found out later that oxygen started accumulated 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 2.4 billion years ago oxygen begins begins to accumulate 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 mark it on this right here 2.3 billion years ago or 2300 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 eukaryotic organisms where eukaryotic cell 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 it's the greatest greatest extinction event extinction event in Earth's history in history and I'll do history with the capital H in Earth's in the history of the earth and that's because you know the cyanobacteria is producing all this oxygen 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 that were anaerobic that did not need oxygen that actually found oxygen that actually found oxygen poisonous now but since we have oxygen there's two intra things that happened once that oxygen accumulated other than causing this mass extinction event actually three interesting things two of them are essentially 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 that have nuclear membranes around their DNA most eukaryotes have other organelles like mitochondria they need oxygen they they need they need oxygen you can go to the biology playlist we actually talked 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 you know how the DNA has changed over time and we'll do multiple videos of that we think we think that the first eukaryotes showed up about 2.2 billion years ago although there's some debate here there's some evidence that might have been a little earlier some evidence that might have been later I'm sure that number will be refined but about you know right give or take a few hundred millions of years one prokaryote got engulfed by another pro carry and said hey we do pretty well living together I mean a lot of the current theory is that mitochondria is actually descendent from the the current theory is that mitochondria is actually descended from a 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 our you're the human body we're not just one eukaryotic cell we're made up of trillions estimates are fifty to a hundred trillion eukaryotic cells so this was a super these are our ancestors that had to come into being at that time and once again all of this is happening in side of the oceans now the other interesting thing that happened remember we're being bombarded with UV radiation for the Sun so if you're on the land so if you're on the land let me draw the land in 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 violent 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 you were getting anything on the land would have just gotten irradiated this DNA would get mutated 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 that blocks most of the UV radiation from the Sun and now that oxygen began to accumulate we have the oxygen a catastrophy oxygen accumulates in the atmosphere some of that at 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 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 on the ozone oxygen cycle so this 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 this is also a pretty significant event we believe 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 it's a 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 and it entered its first and some people believe its longest snowball period so that's why that what they talk about right here on this diagram the first the first snowball earth it's sometimes called the Huron ik glaciation and that happened because we weren't able to retain our heat if if that theory is correct and so the whole the as the theory goes the whole earth essentially just iced over so as we kind of as we go through the proterozoic eon i guess the the big markers of it is this the first time that we now have an oxygen-rich atmosphere it's the first time that eukaryotes are can now come into existence because they they have they now have oxygen that to I guess we could say breathe and the other big thing is now this is where the ozone form 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