William H. Calvin, A Brain for All Seasons: Human Evolution and Abrupt Climate Change (University of Chicago Press, 2002). See also http://WilliamCalvin.com/BrainForAllSeasons/Mara.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
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William H. Calvin
University of Washington
July 2002 agricultural fires (NASA)
nothing like the view from the edge of a watering hole.
Our tents are on a slight rise, and the view consists of several
elephants wading knee-deep, hosing
down and flapping their ears in the afternoon heat.
Two wart hogs wander among our tents in an accustomed manner,
showing no interest in anything except grass – which they eat while
kneeling on their front legs. This
shows a proper reverence for grass.
The baboons are not reverent, not about anything.
Should they prove obnoxious, the Maasai askari is armed with two
short bare sticks looking like thick drumsticks, which he bangs
together to threaten them. To
the baboons, the guard’s sticks must sound like giant canine teeth,
gnashing in threat. Perhaps
after a second beer, I might speculate about how rattles and drumsticks
came to be invented by our ancestors, but not so early in the day.
Rivers tend to meander if they aren’t hurrying rapidly
downhill. Since meanders
are often eliminated when a flood erodes through a bank to create a
more direct route, there are lots of dead-end channels (“abandoned
meanders”) which make excellent watering spots.
And they tend to fill up with delicious vegetation, something
our upright ancestors might have noticed as well.
I can even imagine them as refuges from predators.
If you splash water toward an animal’s face, it stirs up the
blink reflex and tends to make them cautious about proceeding.
Offshore islands with trees are a good refuge.
Gallery woods may form along the river, trees extending as far
from the water as roots will reach, and our ancestors might have found
them comforting. Could we
somehow test australopithecines on Gordon Orians’ landscape
esthetics, they might well prefer the combination of shoreline features
and forest features – not isolated trees in a grassland with a pond,
what we seem to prefer.
The Maasai Mara is the northern end of the Serengeti Plain,
grasslands interrupted by the occasional acacia tree and the even more
occasional waterhole or stream. With
the aid of some DNA-coded recipes, the grass is readily turned into
herds of elephant, zebra, bovids of various types, and antelope of even
more types. The various
seeds feed the baboons. The
trees and bushes feed the giraffes and other browsers.
The grazers in particular feed the lions, cheetah, and leopards.
The leftovers feed the jackals, hyenas, and vultures (though
they too will hunt on occasions).
As Phillip Tobias said, “From 1925 to 1995 almost everyone
grew up on the ‘received wisdom’ that the Hominidae (the
family of mankind) was born on the savannah, believed to have been the
ideal crucible in which the strange form of locomotion known as
bipedalism came into being.” But since Serengeti-scale savanna scenes
are only one or two million years old, our earliest after-the-apes
ancestors didn’t move into this scene so much as they evolved with
it, as the slower climate changes and uplift produced more grass and
less forest. Even worse
for the savanna bipedalism hypothesis, the earliest bipedal hominid
fossils are found with flora and fauna that are associated with forest.
So before Homo erectus days (they are found in much more
arid areas), hominid habitat may have been forests – but perhaps ones
near a river or water hole.
For all three of the major steps up to becoming human –
upright posture, bigger brains, and the modern mind (a separate
problem, more in a minute) – we are back to square one, having
eliminated the favorite ideas of prior decades through a lot of hard
work by the primatologists, paleoanthropologists, and archaeologists.
Not one of those steps now has a fleshed out, widely agreed upon
theory for why it happened, though there are some interesting
candidates. This is
progress, but we badly need fresh ideas.
are particularly vulnerable to fire, because they can spread it
from tree to tree so easily. And
it can take centuries for a forest to come back, usually through a
succession of grass to brush to small trees and eventually to
climax-forest trees. When
brush burns, you may get grass for years afterward.
(No one knows how far back the practice goes of setting fires to
clear the underbrush, improving the forage for antelope the next year,
and favoring the various plants that produce berries, but such fires
are a common practice of preagricultural peoples around the world.
When flying over agricultural parts of Kansas, one can still see
mile-long fires as fields are burned on a windless day, with a vertical
curtain of smoke hanging in the still air.
There is a great deal of forest and field burning in India and
southeast Asia today.)
When a drought hits, the lake margins become refugia for various
. Many forests then
burn down, once lightning discovers dry trees.
A year later, the former forests become temporary savannas.
This creates a boom time for the grazing animals, with their
short generation times (no “childhood“ at all – many females
become pregnant shortly after they themselves are weaned), allowing
population doubling and redoubling in just a few years.
So the aftermath of a drought, one severe enough to burn off a
lot of forest, is potentially a boom time for experienced predators on
grazing animals. Which our
ancestors likely were, and probably well before two million years ago.
at the waterhole features birds that sound like oboes.
Now there is a family of four wart hogs mowing the grass, not
the same ones as yesterday. I see a fish eagle perched, eaglelike, in a crooked treetop
with a good view. There is
a special quality of low-angle morning sunlight, casting features in
high relief compared to the flat lighting of midday.
Perhaps the eagle has also noticed.
What I need is some equivalent of early morning sunlight, for
throwing into sharp relief the mental traits that evolved alongside, or
ahead of, the stones and bones we can actually recover.
To appreciate the importance of our abilities to reflect and
speculate, you have to have a foundation in the mindless stuff like
feelings and emotions – among which are landscape esthetics.
To appreciate the occasional emergent properties, and the new
uses for old things, you need to get behind the names we give things. (Is there really “language cortex” or have
useful-for-language areas kept their original functions for the most
part? A “gene for
symbolic behavior,” or just another application of a more general
The physiologist Albert Szent-Gyorgyi liked to say that, for
every complex problem, there is a simple, easy to understand, incorrect
answer. We humans are very
much into storytelling, and it is likely that there are some gut-level
mindless criteria which make us like one story better than another
(though our rational facilities may sometimes allow us to supercede the
gut-level guides). We can
be too readily satisfied when selected parts all seem to hang together
nicely. We may, for example, argue in a circle without realizing it,
making no progress at all except to satisfy our esthetic sense about
what constitutes a good story. Scientists,
in the course of our education and researches, encounter a lot of
stories that no longer satisfy because we have learned that we (or our
predecessors) were earlier fooled.
With this caution, let me talk about acquisitiveness.
Birds are acquisitive of nesting materials. We tend to connect that with rearing offspring and give it a
purposeful label. Packrats
collect all sorts of things for building “houses.”
They cut branches, cactus pads, and leaves.
Then they pick up anything loose to add to the structure –
bottles, cans, mule droppings, bones, papers, or even mouse-traps –
just to fill space. We can
still ascribe purpose to this, but, because the class of objects is
becoming more general, we have to be careful of our labels.
From this, we move to the acquisitiveness of the local baboons,
the ones that conduct raids on our tents near the waterhole.
They are reported to have a particular fondness for toothpaste,
but their tastes are nonspecific enough that they are just as likely to
make off with your camera bag, without checking first for anything
edible. Back home in
Seattle, my neighborhood is plagued by someone who is so acquisitive of
battered old cars that he has left dozens of them parked on our
streets, coming around occasionally to push them a little farther down
Acquisitiveness is also the concept which I have applied (it’s
somewhere in Lingua ex Machina) to how the human infant builds a
language machine by listening for patterns. As
in nest building, there may be some general principles behind it –
yet ones so loose that they don’t specify too many of the
particulars. First, infants begin forming up categories for the common
speech sounds they hear, not whole words so much as the little units we
call phonemes, less than a tenth of a second in duration.
Categories allow them to generalize across speakers, so that the
sound and the father’s somewhat deeper /ba/
sound are treated the same despite their differences.
By about a year of age, babies stop hearing many of these
differences, having standardized them: a /ba/
is a /ba/.
The bird that sounds like a musical telephone bell is back,
causing reflex jerks of my shoulders now and then.
I am trying to persuade myself that, because the call doesn’t
repeat at an exact interval like a true telephone bell, it isn’t
civilization intruding. But
categorical perception keeps winning, conforming the sound to my
telephone-ring standard and grabbing my attention away from other
By a year of age, babies are discovering patterns in the strings
of phonemes and acquiring a few new words every day, just from the
examples they hear (long before they begin speaking them).
You can say kids are like sponges soaking up words but that’s
too passive a notion, one of the reasons I prefer the more active
“acquisitive” as the characterization.
I suspect they’re searching for patterns in their environment.
Eventually the kids incorporate the more reliable guesses about
underlying pattern as new mental categories.
So kids have pyramided words atop the phonemes, and now have
compound structures made from building blocks.
But then they do it again, discovering patterns in the strings
of words they hear and inferring the grammar of that particular
language: ways of making
plurals and past tenses and nested phrases.
This happens between the ages of 18 and 36 months.
Then they’re off detecting patterns on even longer time
scales, that of the collection of sentences we call a story.
They infer that a proper story has a beginning, middle, and a
Now you can talk about this as separate instincts for phonemes,
words, syntax, and narrative – or you can try out an overarching
principle. Mine is that it
is just repeated instances of pattern-finding acquisitiveness, on
longer and longer time scales. It
may, of course, turn out to be something of both the general and the
specific, but what’s interesting is how much you can build by just
pyramiding ‘acquisitiveness’ concepts four times over.
It’s a pyramid, about like compounding atoms into molecules,
molecules into crystals, and then making sharp tools out of the
by kids is why there might be more than just cultural spread
involved when converting to widespread syntax, sometime before 50,000
years ago (more later). Yes,
I can see acquisitiveness for words per se having been around for a
million years or more. But
pyramiding to syntax and then narratives in the preschool years, well
before much plan-ahead or accurate throwing develops, happens so
reliably in most modern kids that it makes me wonder if that
higher-order acquisitiveness is an additional adaptation, backstopped
by a speciation-like event.
Of course, it need not be true speciation but simply assortative
mating. Though exceptions
abound, tall people tend to select other tall people for mates, blacks
tend to marry blacks, and so on. It
takes little imagination to suppose that articulate women, operating in
a system that allows female mate choice, would prefer equally
articulate men. And so the
rich get richer and the poor get poorer, once again.
Unlike true speciation, assortative mating isn’t backstopped,
with a ratchet to prevent backsliding.
Like dog breeds backsliding to mongrels, things can still become
average. Could syntax be
lost if something like prolonged suspension of female mate selection
(say, via a return to harems) removed this tendency towards divergent
selection? Perhaps not, because kids now have such strong multistage
acquisitiveness for language that they can reinvent syntax, if no
longer exposed to structured speech.
Those who acquire words early enough and fast enough (nine new
words every day) might, like the deaf playmates of hearing parents,
invent their own structuring.
early-morning light has gone, the sun is hidden behind a cloud,
and I can smell a proper English breakfast, not one of your
out-of-the-box breakfasts. “Out
of the box” thinking is another matter, deriving from “boxed in”
and the escape there from. Escaping
isn’t easy, and for some very fundamental reasons such as the simple
categories we fall into, ones where their efficiencies for some uses
serve to blind us in other ways.
After a year or so of exposure to the Japanese phoneme that lies
between the English L and R sounds, for example, an infant will no
longer hear the difference between L and R if not early exposed to
English speakers. The
infant has created an efficient box that, by standardizing variants,
enables rapid processing of speech sounds.
But it does mean the English words, rice and lice, now sound the
Well, we do exactly the same thing at higher levels, too – you see the problem in medicine all the time, where putting things in boxes serves to promote speed in decision making but also blinds the practitioners to the subtleties. One expects that it even applies to those who learn some genetics, but not enough to be able to “think out of the box” on the occasions when mutations and chromosomal rearrangements just aren’t the name of the game.
Certainly most of us who contemplate climate and human evolution
have gotten trapped, at one time or another, in the Ice Age Box.
That name, “ice age,” traps us into thinking that the last
2.5 million years were all about giant ice mountains coming and going.
It traps us into thinking that such periods were icy cold when,
here in the tropics, they were merely 3°C to 5°C
cooler, not exactly fur coat weather.
It distracts us from characterizing periods as “dry” by
focusing our attention on temperature per se.
And the “glacial” pace of the ice ages, where it takes major
ice sheets 100,000 years to come and go, may well focus us on the wrong
time scale. It’s the
things that happen in a few years, like droughts and fires, that might
be the relevant aspect of the “Ice Ages” for big brains and
toolmaking. Indeed it may
be the decade-long transitions between states, not the steady states
(warm-wet and cool-dry) themselves, that create the most relevant
challenges and the most relevant new niche opportunities.
of the explanations offered for prehuman evolution, as I
mentioned earlier, suffer from the “Why us?” problem.
An explanation needs to explain why all the changes happened to
our ancestors and not to other omnivores such as the chimpanzees and
the bears. My explanation,
whatever its faults, at least handles that one.
The opportunities associated with bust-then-boom episodes were
for a savanna-adapted ape. As
apes rather than big cats, they had the brains and hands suitable for
using tools. As waterhole
predators they would survive the drought aspect better, with the larger
lakes serving as refugia. As
savanna specialists, they would have a population boom in the next
generation as the grazers expanded their range into the former forests.
Then dozens of generations later, when the sudden switch to
warmer-and-wetter allowed a quick grasslands expansion into former
deserts, there would have been a second boom time.
So – at least if you eat grass or grazing animals – the
cycle is two ups for each downer:
A boom time gets rare variants surviving long enough to
reproduce. Nothing like
this one-two opportunity happened to the chimps and bonobos, whose
populations probably stayed small during the cool-and-dry phase, only
slowly expanding when warm-and-wet finally came back.
Note that the contract-and-boom scenario repeats for hundreds of
major episodes and thousands of minor ones, thanks to the climate
instabilities and their rapid transitions.
Gradual changes would accumulate from repeated catastrophes.
“Catastrophic gradualism,” pumped by climate instability,
affords us one candidate for how we ascended.
Alas, boom-and-bust also illustrates how we could lose it. I’ll save that for the long flight home.
at the waterhole seems promising, with its long shadows and
renewed breezes, but we have been sobered by the prospect of leaving
this Serengeti paradise tomorrow.
I learned a lot yesterday about the differences between lakes
and the more ephemeral waterholes which dot the landscape here.
And it provides one way of summarizing what happened in the last
five million years since hominids were still close to the apes.
The big lakes that we saw up north in the Rift Valley, like
Baringo and Nakuru and Naivasha, are clearly great magnets for
wildlife. They vary in size over the decades but seldom actually turn
salty and then dry up. One
can imagine our early post-chimp ancestors hanging out around a lake,
profiting from the way it focuses resources for predators.
Lakes bordered by forest, with a zone of former mud flat for
grass, would have made for a nice transition, the comforting landscape
esthetics of the chimplike forest still close at hand.
Rivers too are always there, though some seasonally dry up
enough so that you have to dig a little, to find the water flowing
under the surface. Rivers
often have gallery woods bordering them, again a comfort and a refuge.
Australopithecines likely nested in trees at night, just to get
away from the big cats. Clearly
rivers are prime expansion territory in good times, when lots of kids
manage to grow up at the lake edge and need territory of their own.
When a drought hits later, the hominid population might have
shrunken to what the lake margins could support.
Rivers have waterholes, too, in those abandoned meanders that
begin to silt up. But the
Serengeti also has a different type of waterhole, far from any obvious
stream. It is as if there
were underground streams that had had a section of their roof cave in,
probably with a little help from the elephants who like to dig for mud,
perhaps because their low-frequency hearing enables them to hear
flowing water beneath their feet.
(The Maasai cattle herders outside the park like to have
elephants around, because they dig waterholes that the cattle can
share, but their wildlife enthusiasm doesn’t extend to competitors
for grass like zebras or to predators like lions.)
The Serengeti has a foundation of crystalline rock that the ground water doesn’t penetrate very well, topped by volcanics that make good soils. Indeed the elephant behavior provides many species with access to water at sites far from lakes and rivers. The savanna is dotted with lone acacia trees and lone termite mounds and, less often, with unexpected waterholes. Looking out across the savanna, you may not realize that a waterhole is there (many have no bushes or trees nearby) until several zebras climb out of it – the way horses climb up out of a folly, dug by an English landscape architect to hide a fence. The waterhole really is a hole.
Landscape esthetics suggest that our ancestors made the
transition to being comfortable in such settings, not merely tolerating
them. And this is in spite
of the fact that open savanna is prime predator territory, where there
are few places to hide. Savanna
away from permanent water was the great expansion opportunity for
hominids, tenfold greater numbers of grazing animals clustering around
those isolated waterholes. The
waterholes tend to be ephemeral: sometimes
watery, sometimes muddy, and sometimes really dry.
This change can be seasonal but even your better waterholes can
dry up for years.
Just as the lakes-to-rivers expansion would reverse in droughts
to center on lakes, so the further rivers-to-treeless-waterhole
expansion would have been even more vulnerable in droughts.
Surely there was a lot of friction between groups and, when the
drought crunch came, the peripheral savanna specialists probably tried
to displace the ones that had comfortably stayed nearer the permanent
has a bloody side to it.
Decades-long droughts, as I mentioned at Lake Naivasha, are
rather common events, even in the good-times centuries of the
historical record. And the
ice cores now show that worse droughts have happened worldwide
every few thousand years when warm-and-wet flips into the cool-and-dry
mode (and the Amazon flow is cut in half).
These droughts are far more severe and far longer, and sometimes
they come in a series, a century when the climate goes mad with
The modern human population may not be limited by predators, but
we’re still subject to population crashes from various causes.
The Four Horsemen of the Apocalypse supplemented death with
warfare, pestilence, and famine. The
plague years in Europe were bad enough, where more than a third of the
population died, but the natives of the Americas suffered far worse
from the smallpox and tuberculosis brought by the somewhat resistant
European settlers. Most of
the native population was lost simply from infectious disease, running
ahead of the settlers.
Famine from abrupt climate shifts has been equally dramatic and
even faster, with some isolated populations wiped out entirely on many
occasions. What the
population biologists call “die backs” were surely a common
occurrence in human evolution. The
whiplash climate changes would have been far more stressful than the
usual droughts, simply because of their widespread alterations in the
availability of plants and prey animals.
Everywhere that humans lived, climate would likely have changed
simultaneously – even the tropical regions would have had changes in
rainfall patterns, disrupting the food supply.
The tropics may not have cooled more than 5°C, but
the rainfall patterns there would have abruptly shifted within the time
scan of a single generation, creating patchy resource distributions,
and raising questions of refugia.
Many subpopulations would not have been able to relocate before
modern human population expanded from millions to billions after
agriculture revolutionized things.
In the United States and Canada, fewer than two percent of the
population can now feed the rest, given modern technology, credit, and
transportation (only a century or two ago, 80 percent were engaged in
just raising food).
But the population expansion does make us all very vulnerable.
Technology hasn’t done away with the more severe widespread
droughts for which stockpiling will prove inadequate.
In good times like today, populations expand to fill the space
available within only a few generations.
And it isn’t just physical space:
as farmers manage to feed more people from each acre of land,
population growth fills in that “excess capacity.”
Let the productivity slip for any reason, such as a worldwide
drought, and people starve if their society hasn’t devised good
backup food sources. Birth
control, by keeping the population levels to what can still be
supported in the least productive years, would be a humane alternative
to the boom-and-bust cycle.
Can we think our way out of this, produce yet another technofix?
Birth control might do the job if there were a long transition
into a drier climate, slowly downsizing the human population to track
the resources. Though I
doubt it, we might actually bypass the usual downsizing via the Four
Horsemen of the Apocalypse, might even bypass what African history
teaches us about the last thousand years of half-century-long droughts
in otherwise good times. Hothouse
agriculture might, were it widespread enough, even feed a larger
population than today’s. Given
enough time to make a gradual transition, all sorts of technofix might
be possible, much as in the story that some thinkers advance – the
notion that the Earth might actually be able to support twice as many
people as now.
Alas, they assume some form of stability that doesn’t exist,
as they would know if they bothered to learn the paleoclimate story.
The transition-time factor cannot be left out of the equation.
We are very vulnerable to sudden decade-scale change.
When it happens, the struggle is not pretty.
sun sets rather quickly in the tropics.
It’s dark already, with the distractions of the southern skies
beginning to appear. This
isn’t at all the slow transition that I’m accustomed to, living at
a latitude where twilight is prolonged.
Sunset in the Serengeti reminds me of the Eurostar train popping
into Paris, without the slow transition from farmland to city via
I hope that’s not a metaphor for our future climate, that it will arrive more suddenly than we expect. But the record of past climates certainly makes it likely. Until a dozen years ago, everyone lived in blissful ignorance about the severe climate flips that happen quickly, with just a few short years before hard times set in with a vengeance.
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