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The Behavioral and Biological Origins of Modern Humans

Origin of Modern Humans

Theory of Recent African Origin
  • humans disperse from Africa to Eurasia by 1 million years ago

  • populations then follow different evolutionary trajectories on different continents

  • modern humans evolve only in Africa and disperse to Eurasia roughly 50,000-40,000 years ago, replacing Neanderthals and other non-modern Eurasians
Here's the period of very recent African origin. The first two points are basically the ones that you saw already in the theory of multiregional evolution. First, a human dispersal from Africa between two million and one million years ago, probably closer to one, and second, a tendency then for populations on different continents to follow different evolutionary trajectories, to diverge morphologically. But the third point is where this particular position differs from the multiregional one. Only Africans then become modern humans. Populations elsewhere and in Eurasia, in Europe and Asia, don't survive to the present. They are in fact replaced by Africans spreading from Africa 50,000 years ago. This is the Recent African Origin theory.

What's the evidence for this? The fossil record above all else. We actually have people in Africa 100,000 years ago or more, 100,000 to 130,000 years ago who are, for all effects and purposes, indistinguishable physically from us. That's the same time period from which we have people in Europe, the Neanderthals, who are very different from us. Essentially, the Far East is a bit of a problem in this picture because the Far Eastern fossil record between a half a million and say 20,000 years ago is not so awfully wonderful. I can't say for sure exactly who was living in the Far East 100,000 years ago and exactly what they looked like. I can say it for Europe; the Neanderthals, without question. And I can say it for Africa, people who don't look very different from ourselves. But for the Far East, that's a bit of a problem. So there's still something to be learned. If you want to pursue this question of modern human origins, filling out the Far Eastern fossil record would be very interesting. My prediction would be that it will turn out that there were people living in the Far East 100,000 years ago who were as different from us as the Neanderthals are but also different from the Neanderthals. I think we have some fossils that already suggest that. And that these Far Easterners were replaced by Africans 50,000 or 40,000 years ago in the same way the Neanderthals were.

PhyloftheHomS.jpg If you try to put this into some broader evolutionary context, I hope this slide might help a little bit. This again, this shows humans going down, humans in the broad sense going down to five million years ago. The evolutionary novelty which distinguished the first humans was bipedalism, habitual walking around on two legs. The earliest human species we know of is called Australopithecus ramidus or Ardipithecus ramidus, genus doesn't matter so much, discovered by Tim White at Berkeley. He's out right now, in fact, in the area where he found this in the middle Awash Valley of Ethiopia, looking for more. A. ramidus existed between let's say five and four million years. At about four million or maybe a little afterwards we get the appearance of Australopithecus afarensis. Some people would stick something else called Australopithecus anamensus in between A. ramidus and A. afarensis. Do that or not, I don't think it matters a great deal.

A. afarensis is a very interesting thing, very well known human species, if I can use the term human to refer to it. There's lots and lots of fossils. The most famous one is the Lucy fossil also from Ethiopia found by Dan Johannsen in 1973, a forty percent complete skeleton and we have a lot of other fossils. So we know A. afarensis fairly well. What A. afarensis tells us is that even between, as you can see here, between roughly four million and three million years, people still retained a lot of ape like features. This is a true missing link, in my mind. These creatures, these A. afarensis creatures, were basically bipedal apes. They walked around as we do, no question about that, they were bipedal and this is very obvious in all the bones of the lower limb, but they had tiny little brains hardly larger than those of the chimpanzee. They had many features of apes in their teeth and in other aspects of their skulls and even some features of apes still in their bodies including very long arms and finger bones that tended to be relatively curved, which would mean that the hand or grasp was curved. It probably means--curled is perhaps a better term--it probably means they were still spending a lot of time in trees. In fact, the environmental information that we have suggests that in fact they preferred areas where there was grassland which they were foraging in but where they could also either find food or refuge in trees. So they were combining bipedalism with the continued ape-like dependence on tree climbing.

I don't want to get into too much more detail here because it will take away from the main subject, but then you can see, which is my view, what happens afterwards. There's a branch leading off here to something called Australopithecus ethiopicus. But I'd rather pay more attention to it. Normally it sort of the center line here. I have A. afarensis evolving into something we call Australopithecus africanus. Australopithecus africanus, in turn, is ancestral to the genus Homo . There's some nomenclatural problems in here. Whether you can use the terms exactly the way I have is a debatable issue. We won't worry about that for the moment, although I'd be happy to try and answer questions like that whether I'm using these species name in a legal sense. The more important point here is that at about two and a half million years there was a kind of branching that occurred out of Australopithecus africanus. It lead over on the right side here to things which are called the Robustus australopithecines. I don't have time to talk about them today but they're very interesting creatures. Maybe there's a time in the question period I can say something about them. And more importantly, from our perspective, we get the emergence of the genus Homo .

What marks the genus Homo ? What's its evolutionary novelty, it's derived feature? Brain expansion. So at about two and a half million years we get the first evidence for true brain expansion of the sort, of course, that marks us today and distinguishes us so clearly from the apes. Before two and a half million years, everybody had pretty much ape sized brains and Robustus australopithecines up here also had ape sized brains. But after two and a half million years we get the first creatures that have much larger brains, eventually of course approaching our own. If you want some numbers to put on this, these creatures that we call early Homo at say, two and a half million years had the internal capacity for the skull of between 600 and 900 cubic centimeters. The Australopithecines are all under 550 and usually well under. And if we were to take an average in this room, we would be about 1350 cubic centimeters for the internal volumes of our skulls.

The picture of early Homo evolution here is a little cloudy at the moment. I actually have two species on the chart here appearing at about two and a half million years. Homo habilis and Homo rudolfensis . Some people, many people would only have one which they would call Homo habilis. They would sink H. rudolfensis into H. habilis. I don't really have time to talk about the controversy there. It's a very interesting one; I think it can only be resolved with more fossils.

Something else that I do want to mention here though is that when we get the first of these bigger brained creatures we call Homo at two and a half million years, we also get the first stone tool and I'm sure those two events are connected. Homo habilis, according to the picture I'm presenting here evolves into a yet bigger brained creature called Homo ergaster which arguably was the first true human. Homo ergaster has a bigger brain and it makes it more like us than Homo habilis but it was also the first creature to live in environments that are strictly more like those of many historic hunter/gatherers. That is, there's hardly any trees. Very arid and highly seasonal environments where it probably was making its living largely by hunting and gathering in more or less the way that living hunter gatherers do and was not at all dependent upon trees for food or for refuge in the way that Australopithecines were and that Homo habilis may have been, as well.


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