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- [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.
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- So one of the main events of the last 10 years,
is the publication of my longtime collaborator,
Nick Blurton-Jones book, 2016.
We've waited for decades.
Takes a long time to do what he did in that book.
Theoretically, very thoughtful,
well-warranted hypotheses he's testing
with demographic data.
I recommend it very highly and one positive review said
was positive about the book and also mentioned
"Enlivened by the photographs of Jim O'Connell"
who is also a part of this
tripartite long-term collaboration.
I'm taking advantage of some of Jim's photographs
in what I'm gonna try to do in 14 minutes, let's see.
So, wow, what are we certain about?
You know this is science, so my ideas have changed
about so many things
and I expect they'll continue to,
but I think we all, mostly, are fairly certain
that we got to be human long before the Holocene
which is the last 10,000 years,
long before agriculture, and that makes the places
where we find people living on wild food
and solving foraging problems every day,
an opportunity to see what those are and how people do it.
And they vary remarkably by sex, age and local ecology.
Both the problems and the solutions.
But I can make these generalizations,
maybe a few exceptions here and there,
but really not very many, globally,
where people are foraging.
Men tend to prioritize
things that come in really big packages,
produce really bonanzas but they're very unpredictable.
Women tend to specialize in things that are very reliable,
they come in smaller packages.
And kids, they try in a lot of places to be, and are,
very active foragers, but they're not big enough
or strong enough to fully cover
their nutritional requirements.
It's also true, across all the examples that we've got,
that in a foraging population,
actually this is true in all human populations,
that nobody eats all they acquire,
or acquires all they eat.
The things that men tend to target go around to everybody,
everybody claims a share.
They do not go especially to their own mates and offspring.
Women on the other hand, their daily take
is what makes sure the kids get fed every day.
And a fourth thing that's true, generally,
in well every,
all of us, here in this room
and people everywhere on the planet rely on fire
and cooking.
So, what's likely?
Well we are here in a kind of political economy,
an ecology that's different from what's been going on
over the history of human evolution.
And the Holocene especially is really different
from the Pleistocene.
So the Holocene is this last 10,000 years,
when we get this real equilibration in climate.
But even where we're looking at
It's only us moderns that are left on the planet.
But even where people are part time foragers,
or where they're using motor vehicles or firearms
or metal pots and pans, all things that are very recent.
Even there, we can learn enormous things by paying attention
to the trade offs they face.
So these illustrations show those Hadza guys
who have posed for their picture on a giraffe,
it may look like this was group foraging.
But the way these guys do it is one hunter at a time,
but if it's an animal that's well struck,
others then join in the tracking
and everybody comes to the kill,
and these guys have posed on this giraffe.
But there they are using these projectile weapons
which archeologists tell us weren't around
until the Upper Paleolithic.
So this is a kind of technology we can't take deeply
into the past.
They put metal tips on their arrows.
That's a Hadza great-grandmother there,
she's using a metal pot for the thing that her grandchild
is waiting for dinner.
And then so these middle photographs are from Australia.
The Alyawarra in the center part and the Martu
in the western desert,
and these guys are using firearms and motor vehicles
and yet we can learn from the trade offs they face.
And then finally this Ache woman,
so these are foragers in eastern Paraguay,
there she is using a steel-headed ax.
Nevertheless, we can,
by paying attention to the problems people face
and how they solve them
What we see is that the trade offs that they have to solve,
social trade offs, gastric trade offs,
that are ultimately reproductive,
can account for the kind of variation that we see.
So what would we like to know?
Well we know something about the trade offs that we can see
where we can actually watch people facing them.
What were the trade offs for our ancestors
and our collaterals?
Not enough time to go into these,
well maybe I have to a little bit.
So this, we've seen figures that show some of this,
what the map is showing there.
The yellow is showing Homo erectus getting out,
right after our genus appears, it gets out of Africa,
into the temperate and tropical old world
where there had not been hominines before.
Then the neanderthals, they're kind of covering up
some of the Homo erectus space,
and they are, outside of Africa.
And then the lineage that gave us almost all of our genes,
gets out of Africa maybe only 50,000 years ago,
whoo to Australia, to Europe,
and then populations stay very low.
So one of the things that Nick spends a lot of time on
in this book is what he calls
The Forager Population Paradox.
Wherever we see people hunting and gathering,
where people do good demography, and that ain't easy
to really create believable life tables,
populations are always growing.
And they can't be always growing,
otherwise we'd be up to the moon in elephants,
as Darwin reminded us.
Even at the lowest level of population growth,
we see if you had
Starting with 100 people and populations were growing
at a quarter of a percent a year, in 10,000 years,
there would be 7 trillion people.
That's three orders of magnitude more
than we've already got.
So, why did populations stay so low so long?
And then there were places where they didn't,
and there was spreading,
boy would we like to know how to account for that.
So what do we do?
Now well first of all, one of the important things
about our Hadza project was,
O'Connell being an archeologist
was really paying attention to the archeological reflection
of the behavior that we're looking at.
And more attention to that.
How the sort of behavior we see is reflected
in the archeological record.
You can't just dig it up and let it talk to you,
you need to have a way to go at that relationship
and especially for questions about fire and cooking.
And of course more paleoecology so we can really understand
what the opportunities and constraints are,
including fire,
spreading grasslands meant more landscape fires and so on.
And maybe I should skip this,
I was asking Anne if this is possible, (laughs)
can the population geneticist maybe,
as we get more data and more into DNA, can they
The favored hypothesis for people
who confront this forager population paradox
and who've tried to model it,
is that populations grow, but then they crash.
They crash and they go locally extinct.
Well can the
Is there any way that we can get something
out of the population genetics about that?
Maybe not.
So now I'm turning to life history and reproduction.
Very related, here, that we've talked about
how actually chimpanzees are closer to us
than they are to gorillas.
So our closest lineage, living, is chimpanzees and bonobos.
But our life histories are different
in these fascinating ways which,
my favorite hypothesis is what underlies
so many other things about us.
Much greater longevity.
In hunting and gathering, mortality regimes,
a third of the adult female years lived are post-fertile.
So what is that about?
Maturation takes longer.
But weaning is earlier.
So both birth intervals are way shorter,
and if we do all the allometries properly,
they're really short.
I'm gonna try to get at that.
But here are three figures to illustrate this life history.
We're just looking at the female part
of the population pyramid
for three different hunter gatherer populations.
I can't say very much about them, there isn't time.
But the orange bars are the girls who are not
haven't had their first kid yet,
the green bars are the women in their childbearing years,
and then the golden years above, right.
And what the length of the bars, the width of the bars,
is the fraction of the female population in those ages.
But what I wanna underline is that life expectancy
in all these cases is well less than 40,
but the reason for that is because of all the infant
and juvenile mortality.
All the little short lives that go into that average.
And if, in any of these populations, you made it through
to adulthood, chances are so good
that you will live well beyond your fertility.
So, what is likely?
Well, since I'm the one who's talkin',
I'm gonna tell you what I think is most likely. (laughs)
And actually I think some people would be prepared
to agree with this.
Genus Homo evolved in Africa as these climate cycles
were reducing the forest and spreading savannas,
much more seasonal environments, landscape fires
in these savannas.
But the kinds of plants that do well in savannas
are really different than the things that do well
in, my heavens, in forests.
And to take these resources,
size and strength really matter.
So little kids, they try, so these are Hadza photographs.
Little kids try but they're just not strong enough.
Versus great ape foods, where if you're a chimpanzee,
wow, you are nursing and your mom is carrying you along.
You are also acquiring your own food.
Within the first year, we know from the isotopes
that those kids are feeding themselves a part of their diet
before they're even weaned.
And see how the life history looks different?
In the case of humans,
what we've got is this extended slower aging,
extended longevity, it goes with that story.
Hence this hypothesis that what happened
was the subsidies that came from the older females
shortened the birth spacing for their fertile daughters,
more descendants, that kind of longevity increased
in future generations.
And we can't go back and look!
So mathematical modeling is a way to get at this.
And this is a recent version of modeling,
Peter Kim at Sydney University.
What you see on the panel on the left is showing,
with this age and base model,
the equilibrium if you have chimpanzee-like,
great ape-like, life history and longevity
stays essentially the same,
as does the end of female fertility.
Once the few older females,
that actually are coming to the end of their fertility,
once we allow in the model,
allow them to subsidize the dependent juveniles,
then what happens is
If the simulations escape the basin of attraction,
they move to the human-like longevity,
but they keep female fertility
stuck there at the same place
because it's the grandmother effect that does it.
And what happens is, this has a huge effect on the boys.
So I'm not gonna have time to talk about all these things,
but I was showing just the female side of the life history,
now using the same sources, I've included the males.
And what happens when this life history change,
with female fertility ending at the same place happens
and longevity increases in our lineage,
is we get all these old fertile males.
All these old guys, way more fertile males than females.
Across all kinds of animals, including even invertebrates.
When you have male bias sex ratios in the mating ages,
mate guarding comes to be the winning strategy.
And in our lineage, what's so especially important
is that, now we've got all those old males,
what the other guys think of you is really important.
Whether you can claim a mate and hang onto her,
and their respect really matters, hence these bonanzas,
so goes the hypothesis.
Well, there is more to say about brains,
but the set of things that fit together here is astonishing.
And I pass on to Alyssa to add some stuff about kids
that I would have talked about if I'd been more organized.
(audience laughs and applauds)
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