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/Naivasha.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
Available from amazon.com or University of Chicago Press.
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William H. Calvin
University of Washington
in the Southern Hemisphere once again.
We watched, I confess, the little handheld GPS unit to mark the
exact (within three meters, or so it claims) crossing line, the place
where there is no Coriolis effect to deflect moving bodies.
No ceremony, not even toasting with water bottles, as I’m
traveling with people who live on the equator.
Didn’t even see the guy at the equatorial tourist trap, the
one who claims to show you whirlpools reversing direction as you step
across the equator. He has
a basin of water with a hole in the bottom, the whirlpool spinning
clockwise. When he steps
across the line and refills the basin, it spins counterclockwise.
This isn’t, alas, a demonstration of the Coriolis effect as it
is intended to be, but only an example of how people fool themselves.
The Coriolis effect is vanishingly small at the equator, and for
hundreds of miles around.
Furthermore, whirlpools in washbasins and
bathtubs do not obey the Coriolis effect rules, even at higher
latitudes. Hurricanes do, but there the Coriolis effects act for long
times and over long distances. What
direction a small whirlpool turns is just happenstance, as any science
class would know if the teacher sent them home to survey their sinks
and toilet bowls, tabulating the results from a dozen homes.
It’s likely that this guy doesn’t know all this; indeed, he
probably isn’t a knowing charlatan at all, but has just empirically
learned what works to initiate a whirlpool in the right direction (how
he rotates the bowl to show visitors, how he pours the water when
refilling the bowl). Such
things can operate at a subconscious level, and any number of
scientists have gotten fooled in a similar way. The difference is that science is pretty systematic about
discovering such errors and moving on.
at Lake Naivasha, there are also hippos in the night (mama,
papa, and junior, I was told at breakfast by the cousin, who asked the
watchman to wake her up when they appeared).
The watchmen here usually chase the hippos away, at least when
tourists aren’t awake to see them.
It seems they pull up the grass by the roots, leaving unsightly
spots. The hotel planted
the wrong kind of grass, probably back in the 1930s when this was an
overnight stop for the “flying boats” that provided passenger air
service between London and South Africa.
Thus I see two grown men bent over, swinging long knives to cut
the exotic grass.
Only 26 km west of here, up the Mau Escarpment, is Enkapune Ya
Muto rock shelter (“Twilight Cave”), currently a hot topic because
it contains the earliest evidence of beads -
such decorative art is evidence of the modern mind.
About 50,000 years old, it is earlier than in Europe (where cave
art is the more spectacular evidence).
All of those millions of years of bigger brains, and finally
evidence of thinking somewhat like us.
A short boat ride away is Crescent Island, where one can walk,
in the company of a guide with whom the herds are familiar, among the
giraffes and waterbuck and gazelles.
Obsidian flakes are everywhere, some of which are just the sort
that hominid toolmakers would have prized.
Some microliths can be found here, even hafted ones from a few
thousand years ago. The
reason that volcanic glass is so prevalent is that, just offshore,
there’s an old volcano lurking in the depths.
My cousin kept exclaiming over the obsidian, passing me one
flake after another. I
kept saying, after a brief inspection, that the proffered flake was
probably not archaeological, but merely happenstance.
Still, if you were in the objet trouvé stage of tool
use, this island would have been heaven, what with such single-edged
razorblades everywhere. Such
a place could have been where hominids discovered the virtues of sharp
edges and, when they exhausted the local supply, made the transition
from found-object tool use to Glynn Isaac’s shatter-and-search
The giraffes and the archaeologically-suggestive obsidian flakes
are surely the reason why most people visit Crescent Island, but I
actually came because of reading about the climate cores recently
drilled offshore. Crescent
Island Crater is underwater, just offshore.
Old volcanic craters are not uncommon hereabouts, but the
significance of this one is that it provided a protected underwater
basin from which comes a lake-bottom core, one with a nice
1,100-year-long record of local climate, showing all its ups and downs
via the inferred salinity of the old lake bottom layers.
story told by the Crescent Island crater sediments is that the
Medieval Warm Period (from about 500 to 1315) was a bad time for
Africa. It is known from
other sources that these were years of drought-induced famine,
political unrest, and large-scale migration of tribes.
What the cores say is that the lake shrank dramatically, and for
many decades at a time.
Paradoxically, the Little Ice Age (roughly 1315-1865, when most
of the world was generally about 1°C cooler, thanks to one
of those minor 1,500-year-long climate rhythms) was a good time in East
Africa, thanks to how it affected East African rainfall.
The good times were relatively uneventful periods of political
stability, consolidation of kingdoms, and agricultural success.
And thus growth of populations.
But what interrupted even the five-century-long good times in
East Africa were serious episodes of bad times.
Such were concentrated in three periods:
around 1390 to 1420, again from 1560 to 1625, and then from 1760
to 1840, periods when Lake Naivasha (and many a big lake in East
Africa) was shrunken and salty.
So, even in the absence of human modification of climate via fossil fuels and cutting down forests, it looks as if Africa is subject to episodes of prolonged (30, 65, and 80 years) drought even in otherwise good times. And if you live elsewhere, don’t feel smug about your ancestors having had the good sense to emigrate from Africa (everyone’s ancestors used to live here 50,000 years ago). There have been big, prolonged droughts in North America as well, and the evidence is accumulating elsewhere. There’s a lovely set of fossil tree rings from Chile, a 1229-year-long period from sometime about 50,000 years ago, which also shows droughts lasting a century, with abrupt onsets and ends.
Droughts are often regional, such as the Dust Bowl of American
Midwest and Great Plains from 1931-1939.
Numerous farms had to be abandoned, and overall agricultural
productivity dropped sharply; my mother tells me that when it rained in
Kansas City, it rained mud.
Such droughts may be simply initiated by chance events, such as
random fluctuations in storm tracks over the years.
But they are sustained by feedbacks that make them worse and
delay recovery. A few
weeks of abnormally hot weather may dry out the topsoil, reducing what
plants absorb through their roots.
As their leaves wilt, the ground beneath the plants becomes even
hotter. As plants cease
evaporating groundwater, the air becomes even less humid – and since
this near-ground humidity is about half of what condenses into summer
rainstorms, there is even less rain.
Stressed plants may die. What
rain does fall may then run off quickly, since dead plants no longer
extract water from the topsoil – and so the water table drops.
Once the water table drops significantly, no plants grow the
So there is a self-perpetuating aspect to a drought, once it
gets started. Only another
chance event – perhaps decades later – that happens to bring a lot
of rain for several years in a row will manage to restart the
Sometimes there are seesaws operating, where one region improves
at the expense of another. But
there are also some droughts that are worldwide, everywhere getting hit
at about the same time. Everyone
loses (except maybe for the waterhole predators), almost everywhere
(except maybe Antarctica and the South Atlantic Ocean, where few people
can live). They are the aforementioned “abrupt cooling episodes” but
they could equally well be called “severe drought episodes” or
“dust storm centuries.” Temperature
is often the easiest thing to measure, thanks to the oxygen isotope
ratio correlating with air temperature, but it is not necessarily the
We tend to concentrate on the downside of droughts because of all the human misery they cause. But an evolutionary biologist also looks at the recovery, because the transition is often a boom time. Things become possible in boom times that are difficult in the more static periods before and after the transition period.
waves of immigrants, who then compete and interbreed with one
another, is what has happened all over the globe.
It probably happened as Homo erectus spread out of Africa
almost two million years ago. It
may have happened again a half million years ago as Homo
heidelbergensis spread out of Africa.
And it surely happened again as Homo sapiens spread out
of Africa during this last ice age.
We’re accustomed to thinking about this as gradual spread,
associated with gradual climate change and gradual retreats of ice
sheets. Migrations are, however, stimulated by bad times, as in those
three East African droughts during otherwise good times.
Further, we recognize that migrations from, say, Africa to China
are not Lewis-and-Clark-style expeditions with a goal in mind.
Rather, they are likely a series of successive occupations along
the way, with populations following their favored foods up and down in
elevation over the years as things shift from warm and wet to cool and
For humans, this likely meant following the grazing animals up
and down, depending on the elevation that the grass grew without
turning into bushlands. The
temporary grass after a fire might lead them in new directions.
This occasionally led them within hiking distance of a mountain
pass, whereupon they discovered more prey on the other side – and so
they were pumped over the pass. There
is even a suggestion that such slow climatic pumping could have carried
Out of Africa populations from one river valley to another along a
northern “Silk Road” eastward to China.
But now that we see the role of droughts more clearly, and now
that we see the Ice Ages revealed as the Chattering Ages, it is worth
rethinking the pumping aspect. What
additional factors might be coming into play as human evolution was
given abrupt opportunities and challenges by the abrupt warmings and
coolings? Slow strokes on most pumps have different yields than fast
strokes, leakiness being what it is.
An abrupt drought can provide challenges (having to eat an
entirely different diet because the customary prey and plants
disappear) that may cause many subpopulations to fail altogether.
The European settlement in Greenland, established during the
medieval warming in the year 982, died out by 1540 during the Little
Ice Age (though the Greenland Inuit, with their better boat-building
and clothes-making technologies, survived).
The Little Ice Age temperature fluctuations were only a fraction
of a whiplash cooling.
Though economic and political competition has surely been with
us for a long time, and warfare and genocide have provided many
examples of group extinction, remember that groups need not compete
against other groups like sports teams in order to evolve by group
selection. Instead, the group’s characteristics may cause it to thrive
or fail, perhaps even unaware of other groups.
ape standards, we humans are quite versatile, a jack of all
trades compared to the rest of the animal kingdom.
We can eat vegetarian diets like the gorillas do, thanks to our
modern ability to prepare a wide variety of otherwise toxic plant foods
through soaking and cooking. This
allows us to live in many different climates, and without a
gorilla-length gut. We can
also survive on very few plants, as the carnivores do, thanks to
augmenting the rudimentary ape hunting skills with those of throwing
There are indeed other omnivores, such as the bears and
chimpanzees, but we’ve carried versatility to an extreme.
Some of this is cultural, aided greatly by our language
abilities, but much of it seems to have worked its way into the gene
pool. We see year-old
children hammering on their plates, a developmental program prompting a
behavior that was greatly augmented in the last six million years of
ape-to-human evolution. Older
children come with maddening predispositions to throw rocks downhill,
Even in a modern vegetarian society, those hunting
predispositions are there, facilitating a rediscovery of throwing
techniques and their augmentation via tools such as spears.
Though children growing up may lack role models for the
techniques, our genes have carried along many predispositions to
discover things, ones that helped our ancestors survive, once upon a
time, in some different climate.
Let the climate abruptly dry and some groups will survive better
than others, simply because they have the versatility to rediscover
some plant- or meat-acquiring technique that had fallen out of use
dozens of generations earlier. The
flickering climate has been worse than the “bait and switch”
schemes of the disreputable advertisers:
offer a “come on” in the form of a suddenly blossoming
environmental niche (those abrupt warmings from the cool-and-dry mode
of climate), and then, once the population has grown to fill the space
available, change the name of the game with a sudden reversion to the
cool-and-dry-and-windy-and-dusty mode – a switch so sudden and so
serious that it forces a scramble for survival in an increasingly
isolated subpopulation, and all within a single generation.
fragmentation of populations, and the rediscovery of empty
niches, are among the major accelerators of Darwinian evolution.
Many animal species were likely affected by such
expand-and-shrink cycles, not just our particular ancestors.
Some group selection via climate has probably affected other
social species, such as the dogs, using the same happenstance makeup of
small subpopulations and their survival via social cooperation.
So far as I know, bears and chimpanzees haven’t had a brain
boom during the last several million years.
Something must have been special about how our omnivorous
ancestors made their living. Something
engaged the gearshift which allowed climate-driven fragmentation to
help ratchet up our cherished beyond-the-apes attributes – altruism,
simple language, structured thought and language, and an ability to
anticipate the outcome of a proposed course of action, what it takes to
achieve ethical behavior (or plan a war).
What was this something?
As I mentioned earlier, the brain doesn’t evolve via a bump
here and a bump there, as different functions come under natural
selection. Except for
smell, it looks as if it’s something closer to “enlarge one,
enlarge them all” – not exactly the mosaic selection which
adaptationist reasoning tends to assume.
Furthermore, despite the names we tend to give cortical areas
like “visual association cortex,” many cortical areas are
means that you don’t need natural selection for music, for example,
to evolve musical abilities; they could easily be a spare-time use of
the language machinery of the brain, with its ability to handle
structure. Music could
have been bootstrapped by language.
And, as handy as structured language is, it could have gotten
its initial big boost up from ape-level utterances via a similar route:
a spare-time use of neural circuits that were being shaped by natural
selection for some other function with a more immediate payoff than
intellect – in other words, the payoff that augments the neural
circuits need not be payoffs from the higher intellectual functions
themselves. Indeed, the
language areas of the present-day human brain have a lot of overlap
with the areas important for hand-arm movements.
Were throwing and hammering particularly useful for making a
living, then there might have been some spare-time use of the same
circuitry in the evenings for gossip.
The growth of language abilities might have, in the initial
stages, simply been a free, spare-time consequence of natural selection
for hunting skills. Language
could have been bootstrapped by precision ballistic skills, though
eventually paying for further improvements via its own virtues.
Since I proposed that in 1981, another such carryover with
bootstrapping potential has been suggested.
In our book Lingua ex Machina, the linguist Derek
Bickerton points out that the evolution of altruism could have provided
the mental categories needed for argument structure in syntax.
Freeloading is the big problem with sharing; everyone loves a
freebie, a social objet trouvé.
Happenstance clustering of individuals likely to share helps to
overcome this evolutionary problem.
Another aid is to share mostly with individuals themselves
likely to share. But how
do you know an individual is a sharer? It would be handy, as Richard Dawkins once noted, if they all
wore red beards or some other mark that distinguished them from the
One strategy is to share initially, but keep track of
reciprocity, refusing to share on some subsequent occasion if
repeatedly disappointed. “But
you owe me!” requires some mental categories for debt.
The task of remembering who owes what to whom is amazingly like
linguistic argument structure (those word categories involving actors,
recipients, beneficiaries, and so forth), which provide major clues to
understanding a story-like sentence about who did what to whom.
The same categories that are so handy for minimizing freeloading
in sharing are very similar to those needed for fancy sentences.
Language structure could have been bootstrapped by the
categories needed for altruism to succeed.
If our ancestors already had protolanguage (words and short
sentences like two-year-olds), then it isn’t a big step up to
structured speech or sign, given you’d already have a language-ready
listener preadapted with the mental categories to parse your
structuring into phrases and clauses.
Fragmentation into shrinking subpopulations, thanks to an abrupt
cooling, would place a lot of importance on versatility in food
finding. And the
happenstance clustering of small groups selected from a larger
population would have occasionally created groups where altruism had
enough practitioners to make a difference in the crunch. Language, too, needs a critical mass if children are to be
exposed to sufficient examples, so that they pick up enough by
imitation to be useful later.
That “something,” which made abrupt climate changes
different for our ancestors than for the other omnivores, isn’t
really a settled scientific question.
But it may well have to do with the tools that our ancestors
action-at-a-distance of projectile predation, the sharp tools needed
for food preparation, and the “debt tools” of altruism.
Other things built upon them, such as the wonderful toolkit that
we call our vocabulary, such as our abilities to speculate about the
future and engage in beyond-the-apes levels of social manipulation.
But the basics are exactly what might make a big difference in
subpopulation survival during the fragmenting population crashes –
and they are things that the other omnivores haven’t also invented.
have been casting some doubt on the traditional Darwinian
interpretation of gradual “progress,” not because I think it is
incorrect but only because I think that, unaided, it is usually too
slow and too easily reversed. First
I said that running in place – “automatic gradualism“ –
is pretty slow. Second,
that the expand-and-contract population cycles also might not
accomplish much in the way of adaptations.
These everyday and every-century aspects might be minor in
comparison to periods that do most of the “evolutionary work.”
Then I said that it was when even refugia came under pressure
that adaptations really mattered.
Furthermore, it may take a lucky combination of adaptation and
speciation to keep an adaptation from backsliding when immigrants
arrive (and dilution of the “progress“ threatens), and small groups
are a better setup for speciation as well.
Now let me revisit the problem of repeating the course for
additional credit – some improvements have growth curves, and others
do not. As a generation of anthropologists emphasized, back when Man
the Hunter was out of fashion and Woman the Gatherer was being
emphasized, the carrying bag must have been a very important invention
for both gathering and for small-game hunting.
I agree. Yet one
cannot reinvent the carrying bag for extra credit.
Fortunately, a few types of invention can be repeated, much as
the occasional college course (say, undergraduate research or music
technique courses) can be repeated for additional credit.
The standard example is that many aquatic mammals have
discovered that a small reduction in body hair buys them greater
swimming efficiency. Another
reduction buys them even more. No
matter where along this “growth curve” they are, another increment
has additional rewards. Yet you can only become so naked. Some growth curves plateau.
Some growth curves are also steeper than others, faster at
driving evolution than other candidates.
There are at least two aspects of the abrupt boom-and-bust
scenario that have long growth curves.
They involve things from that chunnel list, where we humans have
considerably enhanced abilities over the great apes.
The side-of-the-barn accuracy needed for flinging branches into
waterhole herds doesn’t have much of a growth curve by itself (it
doesn’t matter which one you trip up), making it more like the
carrying bag. Important, but you need something entirely different for your
next act. Still, it could
have gotten hunters on to the bottom on the precision-throwing growth
Being able to hit smaller herds has an even more frequent
payoff. Once herds become
wary, then an ability to hit the target from a greater distance becomes
important. This has the
incidental benefit of reducing risk to the hunter.
No matter how many times you successfully double your throwing
distance, it has an additional payoff, with no plateau in sight.
Eventually you can become accurate enough to hit lone animals
from the distances achieved by modern baseball pitchers. Important technological inventions improve throwing further,
such as spears and launching sticks.
Note that each improvement confers an additional payoff: additional days that you and your offspring can eat
Sharing has a similarly long growth curve.
You can share more things, over longer periods of time, with
more people, and so forth. Yet
there are few examples in the primates, except for mothers sharing food
with their offspring. There
is one stunning exception to this mothers-only rule:
fresh meat is shared by chimpanzees, and not just with the other
chimps who took part in the chase.
The palm out begging gesture is directed toward the possessor of
the meat, even if lower in the usual dominance hierarchy (were it
fruit, high-ranking animals would likely plunder the food).
Almost everyone that is persistent gets a scrap of raw meat.
No theorist of social behavior would have dared to hypothesize
such an exception for meat, for fear of being laughed off the stage –
yet there it is, in one field study after another.
carnivory was indeed the catalyst for the
evolution of sharing, it is hard to escape the conclusion that human
morality is steeped in animal blood.
When we give money to begging strangers, ship food to starving
people, or vote for measures that benefit the poor, we follow impulses
shaped since the time our ancestors began to cluster around meat
possessors. At the center
of the original circle we find a prize hard to get but desired by many.
. . .
This small, sympathetic circle grew steadily to encompass all of
humanity – if not in practice then at least in principle. . . . Given
the circle's proposed origin, it is
profoundly ironic that its expansion should culminate in a plea
Good Natured: The
Origins of Right and Wrong, 1996
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