Kristen Hawkes: Hunter-Gatherers/Life History and Reproduction


(digital beeps) (lively 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. (intriguing keyboard music) (upbeat music) – 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) (upbeat music)

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Reader Comments

  1. Rockin BoBokkin

    Can this model be applied to the Dorset people? Is their disappearance related to the introduction of Northern Siberians into North America?
    Also, why does she present such a late date for harvesting activities, when plant DNA is showing cannabis and cereals as being harvested well before 10,000 years ago?

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