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programs. ♪ [music] ♪ We are the paradoxical ape: bipedal, naked, large-brained. Long the master of
fire, tools and language, but still trying to understand ourselves. Aware that death
is inevitable, yet filled with optimism. We grow up slowly. We hand down knowledge.
We empathize and deceive. We shape the future from our shared understanding of
the past. CARTA brings together experts from diverse disciplines to exchange
insights on who we are and how we got here, an exploration made possible by the
generosity of humans like you. ♪ [music] ♪ - [Jean-Jacques] Today I'd like to present
you with some thoughts about Neanderthal evolution and to what extent climate might
have influenced their evolution. But before I start with the climate and
Neanderthals, I wanted to show you this slide presenting, on the left side, a
Neanderthal, and a modern human about the same age. And in front, you have two
skulls of extant apes, a bonobo and a common chimpanzee. And this is just to
show you how Neanderthals are different from us in terms of anatomy, in terms
phenotype in general. Modern humans and Neanderthal's ancestors diverged probably
about half a billion years ago, whereas bonobos and chimpanzees diverge much
earlier, probably somewhere between two and one million years ago. And one of the
mysteries regarding Neanderthals is, what kind of evolutionary process was driving
this very rapid divergence? The way we like to think of the Neanderthals- the way
they are presented in the literature- is this way: humans adapted to a glacial
environment, a peri-artic environment. As a matter of fact, if we have a look on
this very jerky climactic curve, you're going to see quite a few times today I
imagine, you can see that for the last half million years, 95% of the time the
climate was colder than it is today in the area where Neanderthals live, mainly
western Eurasia. This being said, the climate was not
always glacial, the glacial episodes were rather brief, actually, the most extreme
parts of these glacial episodes. And if you look on this map, this is a map
showing the distribution of places where Neanderthals have been found, you see that
actually they are not documented very highly in latitude. The northernmost
Neanderthal ever found was found 52 degrees of northern latitude, which is not
so high. Neanderthals lived also in places like Spain, and southern Italy, or the
Levant, that never really witnessed really glacial episodes. So the question is what
in their environment, first of all, drove their distribution, and also drove their
evolution? The distribution that you see now is, I
would say, misleading somehow because it is a palimpsest of the distribution of
Neanderthals through a very long period of time. In other words, at a given point in
the past, they never had this extension, so it's a sort of addition of many years
of distribution. It is very likely they reach their easternmost extension in
southern Siberia and southwest Asia only at some point in their late evolution. So
speaking about climate and the influence of climate on evolution, we have quite a
number of studies showing how climate can influence the biology and morphology of
modern humans. One of the most spectacular features that
relate to climate in extant humans is the body shape, in general. There are a number
of studies showing that the proportions of the limbs, the shape of the trunk, varies
with climate. Basically, people exposed to very hot climates need to cool their
bodies, so they tend to be slimmer, have narrow trunks, and longer limbs. People
exposed to very cold environments tend to be huskier, to have shorter limbs, wider
trunks. This kind of study is a multi-variant study taking into account
many populations. It shows how you can basically rank populations from the
tropics- on the right side you have people from East and West Africa- up to the high
altitudes, so in green you have European populations.
So if you take measurements of the body shape of a Neanderthal and you plot it in
this kind of chart-- this is the case for one Neanderthal very known called
[inaudible 00:07:34]. It falls beyond any modern European and even beyond modern
Inuits, so it is said to have a hyper-arctic body proportion.
Interestingly, if you plot on the same chart early modern humans who came into
Europe about 50,000 years ago to replace Neanderthals, they plot very close to
populations from modern Sudan- which, by the way, it's certainly one of the best
arguments to make that come out of Africa, besides genetical arguments. However, we
should be very cautious with these features, because climactic adaptation is
not just a biological adaptation in humans, it is also a cultural adaptation,
a technical adaptation. In other words, we suspect that even if Neanderthals were not
exposed always to very cold climates, because of the limitations of their
technology, the biological response might have been higher than what we have in
extant humans. And as a matter of fact, if we look at the archeological record, we
find very few archeological sites left by Neanderthals in truly very arctic
environments. It looks like during the coldest phases of the glacial episodes,
large portions of Europe had been abandoned by the Neanderthals. There are
other features that have been said to be related to climate in Neanderthals,
especially their very peculiar facial morphology. They have a very strong nasal
facial prominence, this very big nose projecting, and on the side of the nasal
aperture, inside the face, you have these volumes which are sinuses, which.are said
to be very developed in Neanderthals in general. In a sort of naive way, people
have thought for a long time that the development of sinuses in Neanderthals
were a kind of insulation against cold. This idea has been very criticized.
Actually, it's completely abandoned today because we see more these sinuses as
filling an empty space between other structures that are adapting to different
functions. It is more interesting to look at another aspect of the face, which is
the nose. Actually, if you look at extant humans, you will see that one of the most
varying parts of the face is the nose and the shape of the nasal aperture. One of
these skulls comes from Germany, and the other one from Zaire in Africa. And
immediately you can see that the shape of the nasal aperture is very different in
these two individuals. There is quite a number of studies showing that actually in
humans, the nose, and especially the inner nose, is adapted to the climate conditions
in different regions. Primarily what we have is a problem with cold and dry air.
Individual populations that are exposed to cold and dry environments tend to have
nasal cavities that are higher and narrower, in order to increase the
turbulence of the air that is inspired, and to increase the contacts between the
mucus tissues and this air to warm it and to moisten it.
The nasal pharynx seems to be more depending on moisture, the nasal cavity
itself with cold. So what about Neanderthals? Well, at first look, the
Neanderthal seems to match very much this prediction, because they have this huge
nasal aperture that is somehow unexpected if it was exposed to cold environments.
Actually this nasal aperture is especially broad in its upper part, which is not what
we find in modern tropical populations. But if we look inside the nasal aperture,
we see that there are a number of structures that inflate the walls of the
nasal aperture in order to narrow this nasal aperture. Although the nasal
aperture is very broad outside, the cavity inside is much narrower and much more the
prediction we would make about a cold-adapted population. Last but not
least we have now, a number of information coming from paleogenetical studies, and
I'm sure there are a much more to come in the future. But we know already that there
are a couple of features of Neanderthals that we can relate to: the climate, the
environment. And I would like at least to mention this gene called MC1R which is a
receptor involved in the red hair and fair complexions. And very likely, at least on
the Neanderthals on which this gene has been detected, we deal with populations
that have light skin color and red hair. So we share some adaptations to the cold
environment in Neanderthals, but the question is are there other effects of the
climate on their evolution? I would like now to deal with something else, that I
found probably more important than adaptation itself. One of the questions
about Neanderthal evolution is, why do we have this divergence between an African
lineage leading to us and this Eurasian line about half a million years ago? What
happened at this moment? Why then, why not before? It raised the question of when
exactly we have the first Neanderthals. And the first Neanderthals we have in the
fossil recall are about 400,000 years old. They are found in England, they are found
in the UK-- in Spain. And this age, a little bit above 400,000, has been for
some time in conflict with dates that were provided by geneticists. Geneticists,
using a molecular clock based on computations using the assumed time of
divergence of fossil groups, came to much earlier younger ages for the divergence of
Neanderthals and modern humans, something around 300,000. That was a bit problematic
for paleontologists. But recently, because it's now easy to
sequence the complete genomes of parents and children, we can compare this genome.
It has been possible to find that the rate of mutation assumed by this molecular
clock was not quite right. And new estimates came with a rate of
mutation much more reduced, [inaudible 00:15:34] was initially thought. And this
new rate of mutation is confirmed by the study of some fossil material- this is a
femur of an early modern human found in western Siberia, for which we have the
complete genome. We have the dating, it's 45,000 years old. So it's easy to compare
the genome of this early modern human with extant Europeans and to have a notion of
the rate of mutation along this lineage. It confirms this reduced rate of mutation
that has been recently proposed. So it means that the time coalescence time
required for Neanderthals and modern humans fits rather well this emergence in
the phenotype, in the morphology, around 400 or 450,000. What has been going on in
this time period? We have a list of features proper to Neanderthals that we
see emerging through time by a process of accretion, and it's basically a shift in
frequency of these features that we see more and more along time. About 200,000
years ago, in the isotopic stage seven, we have basically reached the Neanderthal
morphology completely. So the story unfolds between, say, 450-- I'm talking
about morphology here, and a little bit less than 200,000. And it goes at
different speed, depending on the different anatomical areas, and we suspect
that one of the mechanisms driving this evolution is not adaptation, it is not
selection, it is something that geneticists call drift. And this drift is
mostly depending on demography. So what is it about?
It's simple. You have variability of a population in terms of genes and in terms
of morphological features, and if you reduce the size of this population-- if
you reduce it dramatically, and then re-expand this population, you are going
to have, again, a large population, but with reduced variability just by chance.
Just because only some of these features went through this bottleneck. And we have
something like that with Neanderthals along the Neanderthal lineage- I could go
through several features, cranial features, facial features. I'll just pick
one example which what we call non-metrical dental features. And these
non-metrical dental features have a frequency that increases along the
Neanderthal lineage. We know they were part of the variability of Neanderthals of
the middle Pleistocine hominids before the Neanderthal emerges, and they seem to be
fixed a little bit by chance in Neanderthals and reach a very high
frequency in later forms. So what could drive this evolution? We think this jerky
curve that you saw several times already, in this period of time, say around 800 to
400,000, becomes even more jerky. And we have 600,000 years ago for the first time,
first major glacial episode in western Eurasia. We think that this first major
glacial episode resulted in, for the first time, an isolation of western Eurasia, and
a dramatic reduction of the population living there.
This is confirmed also, now, by paleogenetics using the high-resolution
sequencing of Neanderthal genomes. It is possible to make assumptions on the
evolution of the population size through time, and we see that contrary to what we
have in the ancestry of modern humans, we have around five to 600,000-- dramatic
reduction of the population size of this group. So the story unfolds this way: we
have in the early Pleistocene western Eurasian hominids, we still see a lot of
changes between southwest Asia, Africa, central Asia, western Eurasia. And with
the isotopic stage 16, about 600,000 years, we have probably for the first
time, this separation that matched the genetic data. We have another major
bottleneck with isotopic stage 12, and soon after, this is when we have for the
first time, Neanderthal features emerging in the phenotype of European hominids. And
let's say 200,000 years later, after a number of bottlenecks, we have basically
fixed this Neanderthal morphology. So to finish I would just like to say one word
about Neanderthal extinction, and I think I-- I hope I convinced you that climate
played a major role in the rise of the Neanderthals. The question is, did the
climate play a role in the fall of the Neanderthals? There's a number of theories
about that, that Neanderthals got extinct naturally before modern humans moved into
Europe. What we think about the emergence of
modern humans into Europe is a scenario that is a bit more complicated today than
it was a few years ago. We think we have two major episodes of colonization of
western Eurasia, one corresponding to what we call initial [inaudible 00:21:42]
sometime around 48,000. And a later one for western Eurasia around 42 to 43,000.
It has been argued that in this time period, the climactic curve is especially
jerky, and that that would have driven the Neanderthal to extinction before-- or
facilitate the replacement by modern humans. Well, I think when you look at
this curve it is very difficult to see more jerkiness in this period than before.
I think Neanderthals survived all sort of climactic changes before modern humans
arrived in western Eurasia, and I think the main disaster Neanderthals had to face
was not a climatic disaster: it was us. Thank you. ♪ [music] ♪
