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寒武纪物种大爆发和显生宙生物多样性. Sal Khan 创建




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 masked 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 Clark 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 multicellular life and this is interesting in its own right because it has its own meta-level effect on evolution you actually start also having sexual reproduction sexual reproduction and what's interesting about this why this has such a big impact on evolution 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 it's sexual reproduction you had kind of a 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 very more variants to select for and so you kind of had an acceleration in the actual case of evolution so that's what we're talking you know 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 scroll a little bit several hundred 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 hundred or maybe even a hundred 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 there's that the land that the earth is being bombarded with UV rays and the ozone layer is what 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 that time period around five hundred fifty million years ago you start to have life colonizing especially significant life colonizing land so life colonizes land colonize is the land and this was kind of an interesting when I first learned it it was kind of an aha moment you always assumed that that kind of trees and grasses are kind of part of the background they come they come part and parcel with land but it turns out that animals colonize 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 is started to colonize land we now have an ozone layer and what happens and actually there's another snowball glaciation or snowball earth near the end of the Proterra Proterozoic Eon I should say and there's a bunch of theories about why it came about and then why it disappeared maybe they were volcanoes greenhouse gases who knows but as we enter the end of that we start seeing life begin to flourish and it starts to really flourish as we enter the funner I have all I always have trouble saying this the phanerozoic eon and this right it's not even labeled here the phanerozoic eon is this is this chunk of time right over here and let me write it out so this right over here is the venero Panero zoic the phanerozoic eon and so this chart they kind of these divisions right here are eons and then jump into instead of doing eons here they didn't break into eras errors are subsets of eons they're 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 Palio's in the peleas Paleozoic era era which is the first 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 this period of time right over here Cambrian and during this period of time the earth experiences what what we call the Cambrian explosion and that's because there's just this explosion in the number of of species and and genera that existed the biodiversity on the planet 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 pre amphibians or proto amphibians you fast forward a little bit as we get out of the Cambrian period we start seeing we start seeing 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 a relatively recent phenomenon 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 we're 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 you know 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 and are we currently believe a huge rock a six mile in diameter Rock colliding with what is now the Yucatan Peninsula in Mexico right off the coast of the Yucatan Peninsula it destroyed all of the large land life-forms especially the dinosaurs and to put all of this in in in perspective and actually the thing that really was an aha moment for me you know it's okay plants are 450 million years ago grass I kind of used 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 a hundred and thirty 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 they say okay this is where this is the time period where the dinosaurs showed up this Brown this whole brown line is where the man will showed up so the dinosaur started to show up along with the mammals and then of course the dinosaurs died out here our ancestors when the asteroid when the when the giant rock hit the earth must have been burrowed in holes and were able to staff some food away or who knows what and and didn't get fully affected I'm sure some of many of most of the large mammals were destroyed but it'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 bar they say two million years ago the first humans and even this is being pretty generous when they say first humans these are really the first pre humans the first humans that are the same as us if you took one of those babies and you brought them up in the suburbs and gave them haircuts the stuff they would be the same thing as we are those didn't exist until 200,000 years ago give or take me at 200,000 to thousand years ago I've seen estimates so this is actually a very generous a period of time to say first humans it's actually two hundred thousand years ago and just to give you an idea of of how new we are and and how new our evolution is it was only five million years ago and I mentioned this in a previous video it was only five million ago so this is just a good sense this is zero years Homo sapiens sapiens only around for two hundred thousand years the Neanderthals they were cousin species they weren't our ancestors many people think they were there were cousin species we come from the same route although there are now theories that they might have remixed in with the with with Homo sapiens so maybe some of us have some Neanderthal DNA and it shouldn't be viewed as an insult you know they had big brains well there didn't actually 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 as somehow inferior but well I don't to get into the political correctness of how to portray a Neanderthals but anyway this is a very small small period of time towards that if you go to million years then you get to kind of the pre-human the pre-human ancestors and our our our family tree only diverged from the chimpanzees five million years ago if you put if you draw that on this clock right here it would barely make it would be like two pixels or maybe not even two pixels is when we diverged from the chimpanzees so hopefully that gives you a sense of things at least at least for me it really puts things in perspective