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Video transcript

Well, welcome to week four of the evolution course from the American Museum of Natural History. This week's lectures we're going to talk about human evutltion, and we're dividing it up into two lectures. So in first lecture, we're going to talk about what does it mean to be human, and I'm talking in terms of a scientific paleoanthropological way of looking at what is a human. We're going to be talking about how apes and humans are related to one another. And then we'll be talking about the diversity of humans, how there are many different kinds of humans in the past. And then we'll talk about the phylogenetic history of humans and their biology. The story begins in Africa and mostly in east Africa in the Afar triangle here and down through the Rift Valley. But there are very important fossils over in the Sahel area in the Chad area, and of course down in South Africa. And the time where we're going to be talking about these fossils was very very different environmentally. It wasn't quite as dry as it is now. And there were a lot a lot more forests around. Here, here are chimpanzees in the bonobo. And then everything from this point forward are called humans. So when the split took place between the human line and the chimpanzee line, this group is called humans. And it starts around 7 million years ago, 6 million years ago, with a number of key fossils in this area. And we have a huge number of species along this, this lineage. But as you can see we were all great apes, we are embedded in the homioidea. A hominoidea includes also the orangatang and the, and the gorilla. And then like I said, the chimpanzees and then all these groups that are called humans. And it's not up until around 200,000 years that we got anatomically modern humans. So we will be dealing with all of these organisms. We're not going to be talking about each and every one of them. But it is this line. So here's, here's the chimpanzee line. And this is the hominin line. And it includes all of these, including a, a host of species that are also in the genus homo. We're homosapiens, but everybody has heard of the nethanderals, and probably homo erectus. And then there's a whole series of other predecessors to humans that we will also mention. Now we tend to think that there were major gaps, so evolutionary breaks, so above homo erectus and to the present, that's when brain size really increased. From Homo ergaster in that in that region, that's when body size greatly increased. So body size and brain size are two very important characteristics. Let's start with the earliest fossil that is along the human line. And this is Sahelanthropus tchadensis and as it say's it comes from the Sahel, which is that region between the rain forest and the Sahara desert. And Tchadensis because it's found in Tchad. This is a really interesting fossil. It's really only known from the skull. But it's a very nice skull. It's crushed, but it tells us an awful lot. He had the brain size of a chimpanzee. And you can see that it has this very large brow ridge, and that will keep coming back in a whole host of, of slides on these various hominids, because brow ridges in humans are very reduced. But in a lot of more primitive hominids, they're, it's still very large. So he had a very large brow ridge. The other thing that he had that was very interesting was that the face is short. So you can see a modern chimp. And and they, their face projects out, it's called progn, prognathous. And, and, so, this creature was much more like the human, which is kind of like a flat face. But the really interesting thing about this fossil and the, one of the things that got people excited was that the foramen magnum is located down below. And the foramen magnum is where the brain chord and the vertebra attaches. And that meant that this creature walked upright, at least to some degree. Now, a little bit later, a couple million years later, one of the more famous fossils was found, called Ardi, for ardipithecus ramidus. Now, Ardi is also upright as you can see. It had a flat foot. And it had a long, fairly thin, post-cranial skeleton. And it has these long arms, and a hand that looks like ours except the fingers are a lot longer. And you can see from these that the difference in the feet of us and Ardi was that the big toe pointed out to the side. So if, while it walked, it also climbed. And, this is a picture of a chimpanzee climbing, and here is the chimpanzee foot. And here's our foot. And you can see our big tow points forward, because we're fully obligatory walkers. But pan can walk on its hind legs, and ardipithecus also had a sh, much more similar to, to the chimpanzee. So we kind of know that the big toe was not used in propulsion, and its grasping and it in and, and it facilitated climbing. But also, this thing was an upright walker at times. And, and it too had a very prognathic chin just like the chimpanzees do. And you can see the brow ridges here are very thick and heavy. This is the picture of a hominid tree. Now not all the fossils are on this. And the relationships amongst a lot of these species are very poorly understood. Because, one of the key things overriding a lot of knowledge about the human record, is that the fossil record is scattered. We have these really, really early hominids down here, and then there are all of the homo up at the top of this tree. In between are groups called australopithecines. Very famous hominid fossils. It is in this time period, right here in the middle, that hominids started to walk upright more and they also started to develop tools. So tools go back several million years. We don't believe language for instance began until up with homo sapiens. This is one of the most famous fossils of all time. It's Lucy, australopithecus afarensis, and this couple was reconstructed because in East Africa, there is a volcanic ash that shows a bigger individual and a smaller individual. The bigger being the male, the smaller probably be the female, walking. The footprints are in there. And so, this is actually a reconstruction of how they must have been. They could not have separate from one another. They were walking very close. So we think the male had his arm around the female. Within those astralopithecus, there's two different kinds of morphotypes. One of these is called the gracile type, and the other is called the robust type. And it has to do with the cranial morphology. These look a lot more like a modern humans, because the brow ridge is reduced some. Whereas in the Robust, it's a very strong brow ridge the, the muscles and the bones are very robust in these australopithecines. And some scientists divide them up between the genus Australopithecus and Paranthropus. But both of these are groups that lived 4 to 3 to 2 million years ago. Here's a good example of the two examples of the Robust early hominids. And you can see from their faces, the brow ridges are really big. The zygomatic arches on the side which hold some of the jaw muscles are very broad and very aeep, and, and very strong. These were formidable organisms. And this is what they would look like. This is parathropus robustus. They hunted animals if they could, small animals. And they ate a lot of vegetation. And they walked up right most of the time. And they developed certain kinds of tools to help them dig tubers or to cut off meat from bones. We now move into the groups of species that are early Homo or closely related. one of these is Homo habilis. And it was at one time put in Australopithecus, and, by some anthropol, paleo-anthropologists still is. But they invented these hand axes. They were the first to expand the brain. And started becoming the size of, of, of homo sapiens, or small homo sapiens. Now, homo erectus is a, as we will talk about more, it's a mirage really of different kinds of early, early homo that spread over much of the world. And one of those is this Nariokotome Boy which had a height of about five foot nine. So, it clearly we started getting larger body size, a flatter face, and in an, an expanded cranial capacity. Homo erectus, as I said, moved in to middle, the Middle East and across Asia. And one of the populations they found is in Dmanisi, Georgia, the Republic of Georgia at around 1.77 million years ago. So there were populations going out of Africa. And this one is, you can see, it's very similar to a human skeleton with these long femur and tibia and, and a fairly long humerus. But the humerus is not twisted like it wa, is in humans. So we, we pronate our thumbs, and, and it calls for not only the ulna, but also the humerus moving, moving around the joint. So, this didn't have that twist in it, but they had long proportions, they had modern, kind of modern bone proportions. Now we get up to what we, what we would call the first cosmopolitan Homo. It was around Ethiopia between a million years, 600,000 to 200,000 and it got up into Eurasia And it is sometimes considered to be the kind of common ancestor of the neanderthals and homo sapiens. And here are just a series of skulls that are found with Heidelberg ancestors. And it was probably the most advanced in using fire up to that point and they had spears. And we think they were cognitively cognitively complex simply because of their brain size. But we, there's no evidence right now for any kind of symbolism. Any art, any decoration of tools or things like that. Now, we've covered up almost up to our own species Homo Sapiens. And, we can see that humans split from the chimpanzee lineage 7 to 7 and a half million years ago. And there are many species of humans. We've talked about all the australopithecines, all of those critters that came before australopithecines. All of these are considered by paleoanthropologists to be humans, part of the human lineage. And there's fine gradations in morphology, and fine gradations in their behaviors as they became more and more, and more like us. But we still don't know a lot about, about, about them. There are probably way more species than what had been discovered so far, simply because the fossil record is incomplete in various parts of of Africa. And again, modern, anatomically modern humans didn't begin until around 160 to 100, 200,000. And we'll talk about how these estimates are made in, in our next lecture.