The living creatures we're most closely related to are chimpanzees and bonobos
and we share 99% of our DNA with them. They are more closely related to us than
they are to gorillas. So the animals that you go and see in a zoo, chimpanzees and
bonobos and gorillas, two of them are more closely related to the people who
drive cars and buy ice creams at the zoo, than they are to the other sort of
animal in the zoo.
We are most closely related, among the living animals, to
chimpanzees and bonobos. If we compare their DNA with our DNA, molecular
biologists tell us that the differences probably took between six and eight
million years to accumulate. That leads to the hypothesis that we shared a
common ancestor with chimpanzees and bonobos. We have changed a lot in the six
to eight million years. I mean, that seems a long time, but in terms of
evolutionary history it's a very short time. We have changed our posture, we have
changed our diet, we have changed the extent to which we rely on cognition to
be successful. So a whole lot of things have happened in those six to eight
million years. Human evolution researchers like me want to know what
happened in that six to eight million years and the only way we can do that is
to find evidence of the creatures that were between that common ancestor and us.
And this evidence is either in the form of fossils and what those fossils look
like. In the relatively recent past, we also
have evidence from the genome. So
that's a new thing. And so we look for evidence of creatures that are more
closely related to us than to chimps and bonobos and gorillas and that forms the
basis - that's the evidence that we work with. But this is a little like trying to
reconstruct a traffic accident when you have no witnesses, no skid marks and the
only parts of the damaged cars are tiny bits of scraps. That's unfortunately the
business we're in. So we work on pretty slim pickings in terms of evidence and
we have to guard against the temptation to over-interpret it. There are times
when there is more than one sort of animal alive that we think is either our
ancestor or our close relative. Now, if you're alive today, the only
thing you have to have is ancestors. You don't need close relatives. And I
have a picture of my great-grandparents golden wedding and everybody in that
picture apart from my aunt Kitty is an ancestor of mine. My aunt Kitty was a
lovely lady but she wasn't necessary for my existence. And so we try and sort the
fossil species that we find - which ones are the equivalent of our parents and
grandparents and which ones are the equivalent of my aunt Kitty. We assume
that the more morphology you share, the more likely you are to be an ancestor.
But it turns out that sometimes because organisms are living in the same habitat,
or similar habitats, they develop independently the same solutions to
living in those habitats. So therefore, shared morphology is not
always equivalent to shared ancestry. Modern humans is just one species. There
are hardly any groups where there's just one and the problem is if you just have
one living animal, you have nothing living that you can usefully compare it
to until you get to a chimpanzee and a bonobo. If you go back fifty thousand
years, modern humans were sharing the planet with Neanderthals with Homo floresiensis,
with the Denisovans and probably with at least one other species
which we have evidence from DNA but we're not sure what it looks like. There
are some differences between us and chimps and bonobos that are sort of
counterintuitive. We are very dexterous. We knit, or if you're lucky you can
play the piano and the violin. You would have thought that means we have more
muscles in our hand than chimpanzees and bonobos, but we don't. We have many
less. So actually what makes us dexterous is losing muscles, not gaining
muscles. The exception is that we have an extra muscle in the thumb and that seems
to have made all the difference. Human brains are about three times the
size of the brains of our closest living relatives the chimpanzees and bonobos,
but they are not only larger but weight for weight, they're more expensive to run.
So our large brains are also expensive to develop. So our large brains are like
a sort of luxury car - they cost more to buy and once you've bought it, they cost
more to insure and to run. And so there has to be a really good
reason for us to have a large brain. These extremely costly brains, you would
have thought that by now we would have come up with a really good reason why
they are large. But we don't have a, sort of, one compelling reason. Some people
think that once we changed our diet to include meat, this allowed the brain
to enlarge. Some people think that complex, increasingly complex social
organisations require large brains for you to remember all the people that you
have some interaction with. Some people think that the development of complex
objects that requires you to understand the various stages in the process, they
think that would have been facilitated by a large brain. But my
sense is that there is no one explanation. The estimated genetic
variation in the whole of modern humans is less than the observed genetic
variation in a relatively small group of wild chimpanzees. How can that be? Well,
the only way it can be, because we shared a common ancestor and we have been
accumulating genetic variation in a clock-like manner ever since, is that for
a period of our evolutionary history there were very few of our ancestors and
that reduction, it's called a genetic bottleneck, reduces the accumulated
genetic variation and so the only genetic variation that makes it through
the narrow neck of the bottle is a relatively small amount.
And it's been estimated that within the last 100,000 years, modern humans were
down to maybe an effective breeding population
of 10,000 individuals. We nearly didn't make it.