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从恐龙的灭绝到人类的诞生. Sal Khan 创建




if we were to rewind the clock back about 70 million years you would see dinosaurs roaming the earth and this is a very nice picture here of a dinosaur enjoying a sunset at the beach but unfortunately for the dinosaurs about 65 million years ago we believe that a huge meteorite struck the earth and essentially wiped out the dinosaurs and they probably wipe it probably wiped out a bunch of other species with it because you could imagine the shock wave itself would would just exterminate tons of species then you would have the tsunami of unimaginable size that would just envelop the continents for some period of time and then you would have all of the soot that would go into the air and maybe make it impossible for most of the plant species to live because it would be blocking out all of a sunlight and so in an environment like that we could imagine that an animal like this would be well suited to survive it's sitting there underground maybe it can hibernate in some ways so it doesn't need food for long periods of time maybe it has its own food stash under there someplace and so we believe that our ancient ancient ancient ancient ancient ancestors after this mastics mass extinction event might have been something like this kind of a mole looking rodent animal that was protected from all of this craziness that was happening on the surface because they like to hang out underground and you know have all their food nearby them and maybe they could hibernate in some way so you could imagine once the everything settled down and now we're talking you know who knows hundreds of years thousands of years even millions of years some of this guy is descendants start to poke their head out of the ground and like you know what there's there's food in trees and there's no one else in the trees and trees are a good place to maybe get away get away from some of the other predators that have managed to survive this mass extinction event and some of its ancestors that or some of its descendants I should say that we're good at climbing trees decide hey let's let's try this tree thing out and so you started to have some selection for the descendants of this rodents that could climb trees well they were able to find food where these were their ancestors couldn't they could find protection in the trees where their ancestors couldn't and so you could imagine that some subset of this guy's descendants evolved into something that might have looked like this guy and all the pictures I'm showing you these are of modern animals for of course the dinosaur I'm sure this was kind of photoshopped in in some way this is a modern Bush baby but I show this picture because it could have been what some of these primitive primates look like because the Bush baby it kind of you know climbs trees it's able it kind of looks like it's starting to get a hand here to to start climbing the trees but it also has rodent-like qualities but this is of course a modern version of it so this bush babies ancient ancient ancient ancestor might have been that primitive primate or that species of primitive primate that that was a descendant of rodents that started saying hey let's see if we can climb these trees and find some food and then some of its descendants might have had just the right adaptations found their own little niche in the right ecosystems and they would have evolved into into monkeys once again this is a modern monkey but you could imagine some type of primitive monkey and then some of those primitive monkeys descendants they turn into these modern monkeys eventually but some of them they grow larger in size they spend more time outside of trees they lose their tail they don't need it as much for balance maybe it's actually a bad thing to have because someone else could grab it when they when you're in a fight or something like that and they evolved into Apes and in particular the great apes so one of the great apes the great apes involve gorillas and and chimpanzees and chimpanzees and the ancestor or really the great apes and also include humanity so let me just review back on this timeline just so that we don't get confused I'll review what we just we just talked about so before this mass extinction event 65 million years ago you had all these types of species here maybe this right up here maybe this was actually if I'm talking about species maybe this was Tyrannosaurus Rex because the dinosaurs involved a whole bunch of so this right might this might have been t-rex and then we there's a bunch of species that we could list over here but after that mass extinction event that was an endpoint for a ton for a ton of species except for maybe this primitive rodent mole-like thing that was something you know maybe a lot of them got died in the in this in this event but just enough of them survived because they were underground or just in the right place or in there in a mountain someplace who knows where they were and some of them were able to evolve to primitive primates and some of those primitive primates and this is once again these are pictures of primitive primate primates some of those primitive and when I say primitive these are modern versions of them so primitive doesn't necessarily mean worse because obviously these guys were able to find even in today's world they have a niche for themselves they're able to find food and reproduce and in ways that you know don't get in the way of other people another way people don't get in the way of them when I talk about primitive primate I'm just talking about kind of an ancestral primate maybe something that's not there today although maybe some of us descendants look very much like it but anyway some of those primates evolved into primitive monkeys some of those primitive monkeys descendants become modern monkeys so this is I'll call it em monkeys for modern monkeys and some of them involve into primitive Apes and apes their distinctive characteristic is that they're are like monkeys but they don't have tails and they're larger than most monkeys and so these primitive Apes some of their descendants are modern gorillas at some point they break off some of these descendents ra ra ancestor of both modern chimpanzees and of human beings and we think just looking at at the at the DNA evidence we think that this departure right here and the fossil evidence was about 7 million years ago that's our best guess for when we as human beings had a common ancestor with the chimpanzees now you have that common ancestor some of that common ancestors descendants became modern chimpanzees and some of them maybe they explored the right ecosystem where it was more advantageous to do so started to walk on two on two legs on two legs and the most famous fossil of this is the australopithecines fossil of Lucy that was discovered 3.2 million years that's it was discovered more recently it's 3.2 million million years old so the whole genus and genus is kind of one level of categorization above species the whole genus of Austria Australia pentacene these were four to two million years ago four to two and we never know you could always find a fossil that's older than this maybe newer than this I read one account that says maybe 1 million years ago but give or take the Lucy fossil which is the most the most well established australopithecines fossil is about 3 million years old and this is a reconstruction I have over here of Lucy so this is probably what Lucy looked like and once again there were many Lucy's it wasn't just there was one Lucy and we're all descended from Lucy's and it's actually not even clear that we are even descended directly from australopithecines we might be a cousin we might be a cousin species from or a cousin genus I should say genus is the category right above species so if you fast forward a little bit more you go to about 2.3 to 1.4 million years ago 2.3 to 1.4 million years ago we see fossils that they're standing upright the brain size is bigger because if you look at the australopithecines aussolas they are standing upright but their cranial capacity isn't that different than chimpanzees you fast forward a 2.3 million to 1.4 million years ago we start to see fossils where the they're standing upright still and the cranial capacity has grown and you're starting to see primitive stone tools around the bone fossils and so we believe that this is these are one of the first and this is really just how we categorize it but these are some of the first fossils that we categorize as belonging to the same genus as ours and the genus is homo and homo homo just means man so it's the it's the group right above species of man and we call them similar to man because it looks like they're trying to make primitive stone tools and they stand upright up like us and they have larger cranial capacities than the australopithecines fossils or modern chimpanzees and once again we never know if each we don't know if Homo habilis which literally means so the homo part means man habilis means handy because he like to I guess make tools or whatever else we don't know if Homo habilis is a descendant of Lucy's species of Australopithecus or maybe a cousin species maybe they're both descendents from some common ancestor we're not quite sure then you fast forward a little bit more we're talking now about 1.8 so now we're talking about 1.8 to 1.3 million years ago and we start seeing fossils where the cranial capacity larger than Homo habilis getting closer in size to kind of what we know what our notion is of kind of a modern person's cranial capacity at least relative to body body size and this is homo homo erectus and once again we don't know if homo erectus is the descendant of Homo habilis maybe they have a common ancestor who knows and it looks from the fossil evidence that there was especially when you look at this range here that there was some overlap where you had both Homo erectus and Homo habilis living on the same planet at the same time now you fast forward even more and we think about 600,000 to 300,000 once again you know where all of these are constantly being modified as we get better at finding new fossils or interpreting the fossils we have or we look at DNA evidence or whatever about six hundred thousand three hundred thousand years ago you have the Neanderthals appear and the and Earth alls are in the same genus as humans so it's really Homo neanderthalensis I always have trouble saying this so this is still part of part of Homo and a common misconception is that the Neanderthals are somehow a more primitive version of humans that they're somehow cavemen and we're modern men that's not the case the belief is that Neanderthals are either cousin species we have a common ancestor or that they're actually a subspecies of human beings and there's some belief that they might have interbred with homo sapiens and maybe some or a good number of us have Neanderthal Neanderthal genes if there's nothing to be nothing to be ashamed of it's just something you know unfortunately that Neanderthals just just get a bad name because of uh I guess our our popular culture if anything so this is a a drawing of a Neanderthal brain they actually had a fairly large cranial capacity although scientists say for you know they kind of make one reason or another why we think that they were might have been more primitive than Homo Sapien but who knows we don't know we're constantly we're constantly learning things every day but of course the whole point of this is to talk about how humans showed up on this planet and the first really human fossils we find about 200,000 years ago and this remember we're in the genus Homo and now we finally found something that looks just like us anatomically at least we can't study its behavior in all the rest and now we get to Homo sapiens the homo part once again means man and the sapiens the sapiens means thinking so we can debate whether it's an appropriate title for our species but it's thinking man and so once again the Neanderthals they were they were either cousin species for a lot of this time especially once Homo sapiens showed up and maybe Homo sapiens showed up before this we just haven't found the fossils yet they were maybe both inhabiting the same planet maybe there was some interbreeding but the Neanderthals disappeared about thirty thousand years ago thirty thousand years ago these guys disappeared maybe some of them kind of got mixed in with the Homo Sapien started to interbreed with them or they might have just been killed off because they were fighting over the same the same the same ecosystems and I have I've made a little sample here of Homo sapiens just in case just in case well I'm assuming most of you watching this video are one but just in case here's my little sample we can we can debate we can debate how representative of a sample of of our species this really is