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.
Theory of Recent African Origin
- humans disperse from Africa to Eurasia by 1 million years
- 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
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.
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.
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.