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One of the most shocking realizations of all time has slowly been dawning on us: the earth's climate does great flip-flops every few thousand years, and with breathtaking speed. In just a few years, the climate suddenly cools worldwide. With only half the rainfall, severe dust storms whirl across vast areas. Lightning strikes ignite giant forest fires. For most mammals, including our ancestors, populations crash.
Our ancestors lived through hundreds of such abrupt episodes since the more gradual Ice Ages began two and a half million years ago—but abrupt cooling produced a population bottleneck each time, one that eliminated most of their relatives. We are the improbable descendants of those who survived—and later thrived.
William H. Calvin's marvelous A Brain for All Seasons argues that such cycles of cool, crash, and burn powered the pump for the enormous increase in brain size and complexity in human beings. Driven by the imperative to adapt within a generation to "whiplash" climate changes where only grass did well for a while, our ancestors learned to cooperate and innovate in hunting large grazing animals.
Calvin's book is structured as a travelogue that takes us around the globe and back in time. Beginning at Darwin's home in England, Calvin sits under an oak tree and muses on what controls the speed of evolutionary "progress." The Kalahari desert and the Sterkfontein caves in South Africa serve as the backdrop for a discussion of our ancestors' changing diets. A drought-shrunken lake in Kenya shows how grassy mudflats become great magnets for grazing animals. And in Copenhagen, we learn what ice cores have told us about abrupt jumps in past climates.
Perhaps the most dramatic discovery of all, though, awaits us as we fly with Calvin over the Gulf Stream and Greenland: global warming caused by human-made pollution could paradoxically trigger another sudden episode of global cooling. Because of the accumulation of greenhouse gases in the atmosphere, the oceanic "conveyor belt" that sends warmer waters into the North Atlantic could abruptly shut down. If that happens again, much of the Earth could be plunged into a deep chill within a few years. Europe would become as cold and dry as Siberia. Agriculture could not adapt quickly enough to avoid worldwide famines and wars over the dwindling food supplies—a crash from which it would take us many centuries to recover.
With this warning, Calvin connects us directly to evolution and the surprises it holds. Highly illustrated, conversational, and learned, A Brain for All Seasons is a fascinating view of where we came from, and where we're going.
One of the most shocking scientific realizations of all time has slowly
been dawning on us: the earth's climate does great flip-flops every few
thousand years, and with breathtaking speed. Many times in the lives of
our ancestors, the climate abruptly cooled, just within several years.
Worse, there was much less rainfall in many places, together with high
winds and severe dust storms. Many forests, already doing poorly from the
cool summers, dried up in the ensuing decade. Animal populations
crashed-and likely early human populations as well. Lightning strikes
surely ignited giant forest fires, denuding large areas even in the
tropics, on a far greater scale than seen during an El Nino because of the
unusual winds. Sometimes this was only the first step of a descent into a
madhouse century of flickering climate.
Our ancestors lived through hundreds of such episodes-but each became a
population bottleneck, one that eliminated most of their relatives. We are
the improbable descendants of those who survived-and later thrived.
Therewas very little food after the fires. Once the grasses got started
on the burnt landscape, however, the surviving grazing animals had a boom
time, fueled by the vast expanses of grass that grew in the next few
* * *
Had the cooling taken a few centuries to happen, so that the forests could
have gradually shifted, our ancestors would not have been treated so
badly. The higher-elevation species would have slowly marched down the
hillsides to occupy the valley floors, all without the succession that
follows a fire. Each hominid generation could have made their living in
the way their parents taught them, culturally adapting to the shifting
milieu. But when the cooling and drought were abrupt, surviving the
transition was a serious problem. It was one unlucky generation that
suddenly had to improvise amidst crashing populations and burning
And improvising meant learning to eat grass and the like, because that's
about the only thing that grows in the first years after a fire. Back
before agriculture, that meant managing to eat animals that had turned the
grass into muscle. Alas, you have to catch such animals first and, whether
rabbit or antelope, they're fast and wary. Small or big, they're best
tackled by cooperative groups-and since a rabbit's meat can't be shared by
very many people, hunters would have tried for the bigger grazing animals.
This had an interesting corollary. Even if a single hunter killed a big
grazing animal, it was too much to eat-meaning that it was best to give
away most of the meat and count on reciprocity when someone else
succeeded. Even chimpanzees do this if they kill a bush pig or small
monkey-and the handouts aren't limited to those that took part in the
Such a climate-induced downsizing temporarily exaggerated the importance
of such traits as cooperation, hunting, and innovation. We might call the
survivors the Phoenix Generation, after the big bird of myth that arose
from the ashes, over and over again. Centuries later, with the return of
other resources, the hominid population numbers would have recovered and
the traits essential during the bottleneck would have slipped in
But several thousand years later, after the stories about the hard times
had disappeared from the word-of-mouth culture, it happened all over
again: another generation got surprised by a downside episode of the
boom-and-bust climate. And this generation had to conduct another search
for how to eat grass indirectly. Fortunately, their ancestors had survived
the same challenge and some of the genes for the relevant behavioral
traits were still present, waiting to be tapped again.
For the ones with the right stuff, the temporary savanna even offered a
window of opportunity for expansion, a brief version of the expansion
opportunity that the mammals experienced after the dinosaur extinction.
And so this latter-day Phoenix Generation promoted those genes a little
bit more, another stroke on the pump.
The ice-core record of temperature suggests that this phoenix scenario
recurred hundreds of times, that the Phoenix paleoclimate pump is the
longest-running rags-to-riches play in humanity's history. Even if each
individual window of opportunity only changed the inborn abilities for
hunting or cooperation by a mere one percent, 200 repetitions of this same
selection scenario would (just like compound interest) be potentially
capable of explaining seven-fold differences between our inborn abilities
and those of our closest relatives among the great apes.
* * *
Yet how did such abrupt coolings happen on a worldwide scale? And can such
population oscillations account for the enormous increase in altruistic
and cooperative behaviors in humans, compared to our closest cousins among
the apes? Might they have set the stage for the emergence of language? The
structured thinking needed for planning ahead or logical trains of
reasoning? The survival skills of being able to regularly eat large
grazing animals? For our reflective consciousness? And why didn't other
land animals experience the same boost, given that they must have been put
through the same trials? Why just us? (Short answer: Though they suffered
from the bust, they weren't tuned into the grasslands boom time aspect.)
Such are the questions tackled here during a trip to hominid settings in
Europe and Africa, followed by an over-the-pole flight that looks down on
the probable origins of the abrupt climate changes: great whirlpools in
the North Atlantic Ocean near Greenland. They flush the cooled surface
waters down into the ocean depths, part of a giant conveyor belt that
brings more warm surface water into the far north. This keeps Europe-and,
surprisingly, much of the rest of the world as well-a lot warmer, much of
the time. Except, of course, when the northerly whirlpools fail. There are
likely multiple ways in which this climate collapse can be triggered. The
best-understood one is via the greenhouse effect. Gradual warming,
paradoxically, can trigger abrupt cooling.
Many climate changes are not gradual affairs, like turning up a thermostat
or ramping up a dimmer switch. A gradual greenhouse warming over several
centuries is not how things usually happen. As when tilting a table,
there's a point when things start to slide off, and a tipping point when
the table flips into a sideways mode. Abrupt (by which I mean the
year-to-decade time frame) climate changes are more like a light switch
that suddenly, at some pressure, flips into an alternative state. Just as
when a power surge injures a fluorescent light tube and it starts
flickering between bright and dim, so warming can cause air temperature to
start abruptly flickering between warm and cool-and so produce a madhouse
Were a cold flip to happen in our now-crowded world, dependent on
agricultural productivity and efficient supply lines, much of civilization
would be ruined in a series of wars over the shrinking food supply. With
death all around, life would become cheap. Millions of humans would
survive but those left would reside in a series of small countries under
despotic rule, all hating their neighbors because of recent atrocities
during the downsizing. Recovery from such antagonistic gridlock would be
Surprisingly, these large fast climate changes may be easier to prevent
than a greenhouse warming or an El Nino. Maybe. Maybe is the good news.
* * *
Human evolution from an apelike ancestor started about 5-6 million years
ago. This ancestor probably looked a lot like the modern bonobo and
chimpanzee, with which we share this common ancestor. It probably had a
pint-sized brain and only occasional upright posture. We are, in a real
sense, the third chimpanzee species, the one that made a series of
The first departure from this chimplike ancestor was probably some
behavioral change-but behavior doesn't fossilize very well, and so the
first change we can observe in retrospect was that of the knees and hips.
They shifted toward our present form, well adapted to a lot of two-legged
locomotion. Then, much later, when the ice ages began, toolmaking became
common and the brain began to enlarge and reorganize. So the period of
hominid evolution breaks neatly into two halves, each several million
years long: the period of adaptation to upright posture (plus heavens
knows what else), and the period of toolmaking and brain enlargement (plus
language and planning).
I'm one of the many scientists who try to figure out what's behind an
interesting correlation: What did the ice ages have to do with ratcheting
up our ancestor's brain size? Our australopithecine ancestors, though they
were walking upright, had an ape-sized brain about 2.5 million years ago.
Ape brains probably hadn't changed much in size for the prior 10 million
years. But when the ice ages began 2.5 million years ago, brain size
started increasing-not particularly in the other mammalian species, but at
least in our ancestors. About 120,000 years ago, in the warm period that
preceded our most recent ice age, modern type Homo sapiens was probably
walking around Africa with dark skin-and sporting a brain that was three
times larger than before the first ice age chatters 2.5 million years ago.
Now, it's not obvious what ice, per se, has to do with brain size
requirements. Our ancestors would simply have lived closer to the tropics,
were it too cold elsewhere. And it's not that much colder in the tropics
during an ice age (most of us would likely rate it more comfortable).
Something about the ice ages probably stimulated the brain enlargement,
but neither average temperature nor average ice coverage seem likely to be
Climate change is, of course, a standard theme of archaeology, all those
abandoned towns and dried-up civilizations. Droughts and the glacial pace
of the ice ages surely played some role in prehuman evolution, too, though
it hasn't been obvious why it affected our ancestors so differently than
the other great apes. The reason for our brain enlargement, I suspect, is
that each ice age was accompanied, even in the tropics, by a series of
whiplash climate changes. Each had an abrupt bust-and-boom episode-and
that, not the ice, was probably what rewarded some of the brain variants
of those apes that had become adapted to living in savannas.
When "climate change" is referred to in the press, it normally means
greenhouse warming, which, it is predicted, will cause flooding, severe
windstorms, and killer heat waves. But warming could also lead,
paradoxically, to abrupt and drastic cooling ("Global warming's evil
twin")-a catastrophe that could threaten the end of civilization. We could
go back to ice-age temperatures within a decade-and judging from recent
discoveries, an abrupt cooling could be triggered by our current
global-warming trend. Europe's climate could become more like Siberia's.
Because such a cooling and drying would occur too quickly for us to make
readjustments in agricultural productivity and associated supply lines, it
would be a potentially civilization-shattering affair, likely to cause a
population crash far worse than those seen in the wars and plagues of
history. What paleoclimate and oceanography researchers know of the
mechanisms underlying such a climate "flip" suggests that global warming
could start one in several different ways.
For a quarter century global-warming theorists have predicted that climate
creep was going to occur and that we needed to prevent greenhouse gases
from warming things up, thereby raising the sea level, destroying
habitats, intensifying storms, and forcing agricultural rearrangements.
Now we know that the most catastrophic result of global warming could be
an abrupt cooling and drying.
* * *
The Sahara down below gets no rain at all. It doesn't even get dew from
offshore fog drifting inland, like some deserts near a coast. It is "hyper
Not always, however. There have been "pluvial" periods when the Sahara got
enough rain. Between about 14,800 and 5,500 years ago (except for the
Younger Dryas), it was a verdant landscape covered with grasses and
shrubs, with numerous lakes. There were grazing animals of many kinds,
There have also been periods in the ice ages when the arid area was even
larger than at present. So why is there a Sahara at all?
Well, we're into the "horse latitudes," those bands of fickle winds and
dryness that surround the globe near 30° North and 30° South. Lack of
vegetation makes them brighter-looking. The Sahara is an example (the arid
band actually extends east across Asia), and the Southern Hemisphere has
the Kalahari and Australian deserts, plus Patagonia.
If hot air tends to rise, then what goes up at the equator has to come
down somewhere else. All of those tropical rains are because the moisture
drops out, once the dew point is reached during the ascent to cooler
levels. By the time the tropical air comes back down from the stratosphere
hereabouts, it is dry. These are examples of what the atmospheric
scientists call the Hadley Cell circulation, named after the 1735 analysis
by the British lawyer George Hadley (scientists used to make their livings
in more diverse ways).
It is now known that each hemisphere is divided into three cells: rising
air at the equator falls between 20° and 35° North creating the Hadley
Cell). Air rises again at roughly 55° to 60° North and descends over the
North Pole (creating the Polar Cell). In between the descent at about 30°
and the rise at about 60° is the third one (called the Ferrell Cell after
a nineteenth-century American meteorologist). All of this varies with the
season. Ditto for the Southern Hemisphere. This six-cell general
circulation pattern is one of the reasons why the North and South Poles
are so dry, as they are being flushed by moisture-free air that descends
from on high, just like the Sahara.
They are, of course, high-pressure areas. In low-pressure areas, air rises
and any moisture may precipitate out when the dew-point temperature is
reached. Thunderheads are vast upwellings and can carry some heavier
molecules (like refrigerator coolants) into the upper atmosphere; they'd
never diffuse there on their own, but they go with the flow, another one
of those package deals like brain size.
Another consequence are the bands of fickle winds ("the doldrums"), cursed
by sailors for centuries, that occur near the equator and at the horse
latitudes. They are because winds tend not to cross between cells, thanks
to the vertical curtain of air separating adjacent cells.
Excerpted from A Brain for All Seasons
by William H. Calvin
Copyright © 2003
by University of Chicago.
Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
51°N 0°E Darwins home—Catastrophic gradualism
51°N 0°W Evolution House, Kew Gardens—The Darwinian Quality Bootstrap
51°N 1°E Down among the fossils—All of those chimp-human differences
49°N 2°E Musée de lHomme in Paris—The Ghost of Habitats Past
50°N 8°E Bockenheim—Tracing roots back to the Big Bang
52°N 5°E Layover Limbo—IQ and evolutions package deals
22°N 14°E Contemplating the Sahara—Why climate can suddenly flip
0°N 22°E Latitude Zero—Population fluctuations and refugia
19°S 23°E Okavango Delta—The island advantage
25°S 16°E Sossusvlei Dunes—Hominid opportunities in deserts?
26°S 28°E Sterkfontein Caves—The big change in hominid diet
34°S 18°E Cape of Good Hope—The turning point that wasn't
1°S 37°E Nairobi—Creating new species from old ones
2°S 36°E Olorgesailie—The easiest tool of all
0°S 36°E Kariandusi—A layer cake of handaxes
0°S 36°E Lake Nakuru—Where droughts cause a boom time
1°N 36°E Lake Baringo—The earliest hominids
1°S 36°E Lake Naivasha—Droughts even in good times
3°S 35°E Olduvai Gorge—Degrees of separation
1°S 35°E Maasai Mara—The Crash-Boom-Boom cycle
20°N 15°E Libya by moonlight—The last big step toward humans
52°N 5°E Layover Limbo (again)—The Little Ice Age and its witch hunts
56°N 13°E Copenhagens ice cores—Slow ice ages and abrupt whiplashes
56°N 13°E The plane where its always noon—How ice age climate got the shakes
60°N 11°E High above Oslo—The ocean has a conveyor belt
63°N 6°E Out over the sinking Gulf Stream—Dans coffee cream trick
71°N 8°W Jan Mayen Island—Flushing the Gulf Stream
72°N 12°W The Greenland Sea—Losing the first Panama Canal
74°N 19°W Greenland fjords—What stops the conveyor
75°N 40°W Atop Greenland—Why melting can cause cooling
78°N 69°W Thule—Rube Goldberg cause-and-effect
73°N 95°W Somerset Island—North Poles arent what they used to be
68°N 105°W Crossing the North American coast—How we might stabilize climate
62°N 114°W Yellowknife, Northwest Territories—Feedbacks in the greenhouse
49°N 123°W Bumpy border crossing—Managing high-risk situations Afterthoughts Acknowledgments Glossary Recommended Reading Chapter Notes Subject Index