(logo whooshing)
(computer beeping)
(uptempo piano music)
- [Narrator] 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.
(upbeat music)
- Okay, before I get started,
I wanted to thank CARTA,
And I want to thank the
organizers for inviting me to,
what so far has been, a
really interesting symposium.
And as Mark said,
I'm gonna talk about the
evolution of the human skull,
and it's gonna kind of
follow up a little bit
on some of the materials that presented
in the first two talks.
And so when we look at the human skull,
the skull of today's humans,
we can see that it's pretty different
from that of the Neanderthal
that you see on the slide,
you can see that the today's human skull
has a kind of a smaller face
that's kind of tucked
underneath the braincase,
and then we have a kind of a cranial vault
that is kind of much
shorter and sort of taller
than the elongated one that
you see for the Neanderthal.
And so there's this differences
that have been talked about
in the first two talks.
And if I put up another member,
other member of our own
genus, the genus Homo,
other than a Neanderthal,
you'd see that there are
also these differences.
Some of them are the same
as the ones you see with Neanderthal,
and some of them are different.
So there's some kind of unique
sort of features to the Neanderthals.
But in any case, the skull
of present day humans
is quite distinctive.
It looks very different from what we see
in earlier members of the genus Homo.
So as you heard about
in the first two talks,
we can use this distinctiveness
as kind of a way
to trace the emergence of our own lineage,
and the migrations of
members of our lineage
throughout the planet.
And so by looking at the kind of anatomy,
we can locate the emergence
of our lineage to Africa,
we can also couple this
with genetic evidence
which locates the origin
of our lineage in Africa.
And then genetic evidence
coupled with evidence
from the anatomy can allow us
to kind of trace the movement
of our lineage outside of
Africa, and around the planet.
And so this, this fossil
from Herto in Ethiopia
is kind of a good example of this, right?
We can use these features
as kind of a marker
to trace the origins,
to trace the migrations
of our lineage throughout the planet.
So this is of course, very
fascinating, very interesting,
we wanna sort of understand
how our lineage emerge,
and how it moved around the planet.
But there are other kind of
questions that we can ask
looking at this anatomy.
So for example, we could
ask questions like,
why don't our skulls look like those
of other members of the genus Homo?
Why do today's humans have
such distinctive anatomy
of the skull?
And we can ask questions about how rapidly
did our distinctive anatomy appear?
How rapidly did it appear?
And did it come in sort of all at once,
or did it come in over
a longer period of time.
And so these are kind of the questions
that I would like to focus on today.
Focusing on these questions,
not so much to using
this anatomy as a marker
for tracing the emergence
of our lineage and the migrations
of us around the planet,
but actually try and understand
why we look the way we do,
and how rapidly this came about.
And so in particularly,
my goal is to try to dispel
what I think are two
sort of misconceptions
about the evolution of the human skull.
So this first one, is that
misconception number one,
is that all differences between the skulls
of today's humans, and Neanderthals,
or all differences between
the skulls of today's humans
and other members of the genus Homo,
so not the Neanderthals, are adaptive.
And so adaptive is a term
that is used by evolutionary biologists.
But what I mean by this in this context
is that there were some sort
of functional differences
between, say our skull, and
the skull of the Neanderthals.
So they functioned in a
kind of a different way.
And so these differences
indicate something about differences
in function between us,
and these other members of the genus Homo.
So over the years,
there have been a number
of kind of adaptive
sort of explanations for these features.
And so on the one hand are
these adaptive explanations
for the Neanderthals.
So one sort of common one is
thinking about Neanderthals,
skull anatomy is somehow related
to the kind of cold environments
that they were living in and evolving in.
Another explanation
has to do with the fact
that there's evidence that
maybe they were using their jaw
as kind of as a third hand,
and so that there was kind
of this mechanical loading
that was associated with that,
and that might have had
some kind of consequences
on just the overall anatomy of the skull.
On the other hand, there are
kind of adaptive explanations
for sort of thinking about, well,
why do we actually look different?
So what does today's
humans look so distinctive,
and probably the most fascinating,
and maybe kind of interesting,
and exciting one is to sort of think about
that it may have something
to do with speech,
because of course, speech and language
are so important about what,
characteristics about what makes us human.
And so there could be
something about our skull
that has to do with the
ability to produce the sounds
that allow us to create languages.
But there's an alternative.
There's another kind of possibility
that I think we should
sort of seriously consider.
And so there's this evolutionary process
that we call a genetic drift.
And genetic drift is this process
where there are these chance changes
that happen in populations
just because any population
is finite in size.
And so if you think about
different regions of the genome,
so different genetic loci,
and there's going to be different alleles
at those different loci.
And those alleles are gonna
be in certain frequencies,
and kind of any kind of human population.
But by this process of genetic drift,
you can have shifts in the
frequencies of those alleles,
these kind of chance changes,
and just the fact of the
population is finite in size
means just because of this
kind of sampling process
that happens, as parents
give rise to offspring,
and offspring give rise
to further offspring,
you have this process
of genetic drift acting.
And so if some of these alleles,
some of these loci underlie
the anatomical differences
that we see in the human skull,
then you're also gonna get
these these changes that you see
in the skeletal anatomy as well.
And so you can imagine kind of a situation
where you had some sort
of an ancestral population
to Neanderthals, an ancestral
population to Neanderthals,
and kind of us, but then
they diverge from each other
over a period of hundreds
of thousands of years.
And so there would have been lots of time
for this process of genetic drift to act,
and produce at least
some of the differences
that we see in today's skulls.
And so my colleagues and I have
tried to address this idea,
the idea that maybe a
lot of these differences
could actually be non functional,
they could actually be due to
this process of genetic drift.
And we try to address it
over a kind of a number of different ways,
a number of different directions,
but I wanna present today
is kind of a new kind of
approach that we've taken,
and some new results on this.
So in order to kind of address this,
we have to have some way of
kind of quantifying the anatomy
and you've seen this already
in the first two presentations.
So we use these kind of methods
of geometric morphometrics
where we get to take these
kind of anatomical locations
or landmarks, or semi landmarks,
and kind of use them to characterize
the kind of size and
shape of the of the skulls
of today's humans, but also Neanderthals,
to really kind of document
the variation that we see.
And then we take this data,
and we analyze it in a particular way.
And the approach we've taken
is we use kind of models
from quantitative evolutionary theory
to kind of make predictions
about what we would
expect if the divergence
was entirely due to this
process of genetic drift.
And then we compare that
with the observed data,
and see if it looks like
it's consistent or not.
And so here's come some of the results,
and you're looking at these
two dimensional plots,
which are kind of similar
to what you've seen in
the first two talks.
Kind of what you're looking at
is actually a little bit different,
but you have the same kind of axis
of variation, these
principal component axis,
and the black arrows
are kind of observed
patterns of variation,
and this is for a comparison
between Neanderthals,
and different populations
of today's humans,
and also between a little
more ancient kind of humans,
we call Upper Paleolithic individuals
from the Upper Paleolithic from Eurasia.
And so that's what the
black arrows are showing,
but what the red ellipses are showing
are the expectations if
everything was entirely
due to genetic drift,
and what you can see here
is that the black arrows
are actually within the red ellipses,
which suggests that these
patterns of variation
we see in the actual
empirical data are consistent
with this process of genetic drift.
So this was an interesting result,
it fits with some of the other analysis
that my colleagues, and I
have done over the years,
but one thing you might
be asking is, well,
maybe it's consistent with genetic drift,
but do we actually have
kind of the ability,
or the power to kind of detect deviations
from this model of genetic drift?
Could the data sort of
look like it's consistent
because we don't really
have an ability to detect
deviations from that.
And so one way we took
to sort of address this
is we did another comparison.
And in this case, we're
comparing us, Homo,
Homo sapiens,
to a number of different
species of great apes.
So we're comparing with
common chimpanzees,
we're comparing with bonobos,
and we're comparing with gorillas.
And what you can see is
in the same analysis,
the same kind of analysis,
you can see that the
situation is very different.
So in this case, all of the black arrows
or most cases in the
different projections,
the black arrows are
outside the red ellipses,
which suggests that it's inconsistent
with a divergence by genetic drift.
So well, this gives us confidence
that we actually have an
ability to detect deviations
from this process of genetic drift,
and so the situation that we see in Homo
within our own genus actually
looks kind of different
from the situation we see
when we compare Homo sapiens
to other great apes.
So the next misconception
that I wanted to talk about
is that the modern human
skull appeared rapidly
about 200,000 years ago in Africa,
and the first presentation
by Professor Hublin
talked about this some,
and so I'm gonna cover
some of the same ground.
So when we actually look
at the fossil record,
what we see is we don't see a
kind of an abrupt appearance
of this kind of modern anatomy,
this anatomy that kind of links us,
these fossils with today's humans,
we actually see kind of as
old as maybe 300,000 years ago
from Jebel Irhoud,
we see faces that look
similar to today's humans,
but the brain cases
don't look very similar
to today's humans.
And then kind of more recently in time,
we see kind of more of these
features like today's humans,
and so it seems like it's
kind of this gradual,
and sort of lengthy process
where you get this accumulation
of these features through time.
So it doesn't seem to
be a very kind of abrupt
or sort of punctuated appearance
of modern human anatomy
at 200,000 years ago.
So all of this, what I'm talking about,
is kind of a bigger sort of
sweep of human evolution.
So we're looking back
kind of many hundreds
of thousands of years,
and sort of tracing this
very longer kind of period
of the emergence of our lineage,
but there's actually some
events that happened,
actually quite recently in time,
which I think were very important
in sort of determining, you know,
what the skull anatomy of
today's humans looks like.
And what I'm talking about is agriculture.
So in the last 10,000 years,
we have the emergence of agriculture.
and sort of step back a minute,
and sort of think that, so
before 10,000 years ago,
every single human on the planet,
all the foods that they were eating
were coming from exclusively hunted
or wild gathered resources.
And then after 10,000 years ago,
you have the emergence of agriculture
in many different parts of the world,
and agriculture spreads very
widely, and basically today,
almost everyone gets their
food from kind of agriculture,
so from domesticated animals,
and domesticated crops.
And so there's this massive
transition in our diet,
and our subsistence that's happened
just within the last 10,000 years.
So this is a very recent event,
but it's a kind of a huge
shift in our life ways.
And so for a number of years,
researchers have kind of speculated,
and also collected data to suggest
that this transition to agriculture
actually had a pretty
profound effect on our skulls.
And so the basic idea was is
you go from very kind of hard
sort of foods to a much
kind of softer foods.
And because of these softer foods,
you had a much kind of
lower mechanical loading
of your jaws,
and because of the lower
mechanical loading of your jaws,
you have these changes in
the anatomy of the skull.
And so this figure right here
is kind of a reconstruction
where you go from the black outline
of a kind of a hunter gathere,
to the blue outline of an agriculturalist.
And so this basic idea is
that there's this big shift
that happens with agriculture,
that explains some of the
distinctiveness of the skulls
of people living today.
And so this has been tested actually
on a number of kind of samples,
but mostly at a regional scale.
And so a number of years ago,
a former graduate student of mine,
and another colleague
wanted to test this at a
much kind of larger scale,
at a global scale.
And so we collected
samples of skull anatomy,
documented skull anatomy
at a global scale.
So we wanted to kind of really understand
kind of the geographic distribution,
but we collected our
samples in a particular way.
So within most geographic regions,
we actually had a matched sample
of a hunter-gatherer group,
and with an agriculturalists group.
So for example, in France,
we have Mesolithic sample,
so Mesolithic hunter-gatherers,
matched with a Neolithic sample,
so Neolithic agriculturalists.
And so this really gave us a sample
that really allowed us
to really sort of think
about how these shifts and diet
would have affected the
anatomy of the skull.
So we collected these kind
of anatomical landmarks
that you've seen a lot,
you know, earlier in my talk
and also in the other talks,
and actually, you know,
maybe a distinction to make
is actually so far everything
that I've been talking about
hasn't really technically been the skull,
it's been technically about
what we call the cranium, right?
So anatomically, the skull
is composed of the cranium,
and the mandible, the
mandible being the lower jaw.
So I've mostly just been
talking about the cranium,
but in this study, we actually
are talking about both
the cranium and the mandible.
And this is important
because the lower jaw
is really kind of
implicated in these ideas
about chewing and the
mechanical demands of chewing.
So let me walk you through
a few results here.
So the analysis that we did
really allowed us to sort of figure out
what are the different
factors that contribute
to the variation that
we see in our samples.
The variation in skull form
that we see across different individuals.
And what you're looking
at on these graphs here,
so the top is the cranium,
the bottom is the mandible,
and when you're looking
at is this distribution,
so how far out it is.
So the closer it is to, you know,
the 400 end of the X-axis,
or the closer it is,
or closer into the 50 end of
the X-axis that's further in.
So if it's further out closer to 400,
it means that it's kind of
a more important factor.
If it's closer to the 50 side,
it means that it's kind
of a smaller factor.
And what it turns out
actually is the most important
sort of source of variation
is actually individual level of variation
kind of within groups.
And so this tells us any human group,
or any human population,
is there's actually a lot
of individual variation.
So if you take a human group
from anywhere in the world,
and you kind of look at their skull form,
there's a lot of differences actually
between different individuals
just from within that group.
So everyone's kind of an individual,
everyone sort of looks kind of
different from everyone else.
And this is kind of an important
thing to kind of point out,
is actually when you look at most features
of kind of anatomy or morphology,
most of the differences are actually found
within any human group.
And if when you look at the the genome,
you actually mostly see the same picture.
So most of the variation in humans
is actually found within groups,
so we found this too.
But the next kind of most important factor
is what we call kind
of population history,
and so this is,
populations that are more
closely related to each other,
had a more kind of shared history,
are gonna look more similar
in their skull anatomy,
the populations that had
a more kind of distant,
shared, or a shorter shared history,
a more distant relationship.
But then finally, there
is an effective diet.
So this is kind of hard versus soft diets.
And so diet,
although it's kind of the
least important factor,
it is a kind of a significant effect,
and so it does seem to
have, at a global scale,
have actually shaped the
anatomy of today's skulls.
So we can see is that these
kind of different factors
that are sort of overlaid
on top of each other,
and this combination is
what allows us to understand
the skull of today's humans.
So just to now look at a little
bit of some of the details
a tiny bit,
so for some of the factors that we saw
that were some of the aspects of the skull
that were related to diet
were in the mandible,
this lower jaw.
But there's also parts of
the cranium that we saw this.
So what you're looking at here
is points that are documenting
the attachment site
of one of the major kind of
muscles involved in chewing.
And what you can see is in yellow
are the kind of hard diet individuals,
and in purple are the
softer diet individuals,
and so you can see that this aspect
is being shifted by the
shift in diet, okay?
So kind of in summary,
why do the skulls of today's
humans look the way they do?
So I think that many of the difference
between today's humans and earlier Homo,
may be due to this
process of genetic drift.
So these kind of chance
changes that you have
that happen in populations,
just because they're finite in size,
are probably explaining
a lot of variation we see
across kind of present
day human populations,
and their skulls, but also differences
between Neanderthals and us,
and other members of the genus Homo.
But it's also important to
remember, and this came out,
I think in the first presentation,
that human skulls didn't stop
evolving 200,000 years ago,
they continued to change by
this process of genetic drift,
but also in response
to kind of local
environmental circumstances,
local conditions, things
like the shifts and diet
that we had with agriculture.
And so you can see that there's
a bunch of different factors
that are kind of layered
on top of each other
that really allow us to sort of understand
why the skills of today's humans
look different from other
members of the genus Homo.
So with that, I'd like to
thank all of you for listening.
And I would like to thank my collaborators
and funding sources, and curators,
who gave access to collections,
and thank you very much
(audience applauding)
(upbeat music)
