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William H. Calvin, A Brain for All Seasons:  Human Evolution and Abrupt Climate Change (University of Chicago Press, 2002), table of contents and preamble. See also

copyright ©2002 by William H. Calvin
ISBN 0-226-09201-1 (cloth)
GN281.4 C293   
Available from, or University of Chicago Press.
Webbed Reprint Collection
This 'tree' is really a pyramidal neuron of cerebral cortex.  The axon exiting at bottom goes long distances, eventually splitting up into 10,000 small branchlets to make synapses with other brain cells.
William H. Calvin

University of Washington
Seattle WA 98195-1800 USA

[University of Chicago Press flap copy:]

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.

William H. Calvin is affiliate professor of psychiatry and behavioral sciences at the University of Washington School of Medicine in Seattle. He is the author or coauthor of ten books, including Lingua ex Machina, The Cerebral Code, How Brains Think, Conversations with Neil's Brain, and The River That Flows Uphill. The last third of A Brain for All Seasons is based on his cover story "The Great Climate Flip-Flop" in The Atlantic Monthly.
Author photograph
at Wits with hominid femur (Qian Wang). 

"[Calvin is] a member of that rare breed of scientists who can translate the arcana of their fields into lay language, and he's one of the best."--Marcia Bartusiack, The New York Times Book Review

"In this wide-ranging and highly readable book Bill Calvin, one of the foremost interpreters of the mysteries posed by our remarkable human consciousness, builds a dramatic yet thoughtful and very personal account of human evolution around the theme of climatic change in our emergence. Along the way he explores a wealth of fascinating byways, making this a rumination that nobody with an interest in how we came to be will want to miss."-Ian Tattersall, author of The Monkey in the Mirror

"William Calvin uses an adventure across today's earth to draw laser-sharp insights about our human past, and possibly its future. Though the climate has been exceptionally stable during the brief era that civilization moved from clay tablets to megacities, this hasn't always been the case. In fact, Calvin shows how gyrating weather patterns may have forged our ancestors' evolutionary path. And since Earth's climate may resume those catastrophic swings at any time, evolution may not be finished with us yet."--David Brin, author of The Transparent Society

"At first sight, a connection between the climate and the human brain may seem far-fetched. William Calvin makes an excellent stab, however, at convincing us that abrupt climatic changes had a profound impact on human evolution, selecting for increased cooperation that required more complex brains. The result is a rich blend of travel stories, paleontology, climatology, neurology, and of course evolutionary biology."-Frans de Waal, author of The Ape and the Sushi Master

The University of Chicago Press 
ISBN 0-226-09201-1 hardcover, ISBN 0-226-09203-8 softcover


All photos in the printed book are in grayscale; the web pages include a number of additional color photographs and many of the printed grayscale photos are shown in the original color.



                     Preamble.        ............................................... 3

51°N   0°E   Darwin’s home 
                     Catastrophic gradualism ......................... 13

51°N   0°W  Evolution House, Kew Gardens 
                     The Darwinian Quality Bootstrap .......... 21

51°N   1°E   Down among the fossils 
                     All of those chimp-human differences .... 27

49°N   2°E   Musée de l’Homme in Paris 
                     The Ghost of Habitats Past..  .................... 35

50°N   8°E   Bockenheim 
                     Tracing roots back to the Big Bang.......... 47

52°N   5°E   Layover Limbo 
                     IQ and evolution’s package deals............ 59

22°N  14°E  Contemplating the Sahara
                     Why climate can suddenly flip ................ 65

 0°N  22°E   Latitude Zero 
                     Population fluctuations and refugia  ..... 75

19°S  23°E   Okavango Delta 
                     The island advantage .    ............................ 83

25°S  16°E   Sossusvlei Dunes 
                     Hominid opportunities in deserts? ......... 97

26°S  28°E   Sterkfontein Caves 
                     The big change in hominid diet ............. 103

34°S  18°E   Cape of Good Hope 
                     The turning point that wasn’t .  .............. 111

 1°S  37°E    Nairobi 
                     Creating new species from old ones ...... 117

 2°S  36°E    Olorgesailie 
                     The easiest tool of all     ............................ 125

 0°S  36°E    Kariandusi 
                     A layer cake of handaxes .   ..................... 133

 0°S  36°E    Lake Nakuru 
                     Where droughts cause a boom time ...... 147

 1°N  36°E   Lake Baringo 
                     The earliest hominids.   ........................... 151

 1°S  36°E    Lake Naivasha 
                     Droughts even in good times ... ............. 155 

 3°S  35°E    Olduvai Gorge 
                     Degrees of separation    ........................... 169

 1°S  35°E    Maasai Mara 
                     The Crash-Boom-Boom cycle.................. 177

20°N  15°E  Libya by moonlight 
                     The last big step toward humans ...  ........ 193

52°N   5°E   Layover Limbo (again)
                     The Little Ice Age  and its witch hunts..... 205

56°N  13°E  Copenhagen’s ice cores 
                     Slow ice ages and abrupt whiplashes   .... 211

56°N  13°E  The plane where it’s always noon 
                     How ice age climate got the shakes   ........ 221

60°N  11°E  High above Oslo 
                     The ocean has a conveyor  belt    ............... 229

63°N   6°E   Out over the sinking Gulf Stream
                     Dan’s coffee cream trick     ......................... 237

71°N   8°W  Jan Mayen Island 
                     Flushing the Gulf Stream   ......................243

72°N  12°W The Greenland Sea 
                     Losing the first Panama   .............. 247

74°N  19°W Greenland fjords 
                     What stops the conveyor   ....................... 253

75°N  40°W Atop Greenland 
                     Why melting can cause cooling     ............. 257

78°N  69°W Thule 
                     Rube Goldberg cause-and-effect   ............ 261

73°N  95°W Somerset Island 
                     North Poles aren’t what they used to be... 269

68°N 105°W Crossing the North American coast
                     How we might stabilize climate   ............. 275

62°N 114°W Yellowknife, Northwest Territories 
                     Feedbacks in the greenhouse.  ................. 283

49°N 123°W Bumpy border crossing 
                     Managing high-risk situations    .............. 289

Afterthoughts.................................................... 297

Acknowledgments........................................... 299

Glossary............................................................. 301

Recommended Reading.................................. 312

Chapter Notes................................................... 317

Subject Index..................................................... 338

Directions, Distances, Temperatures:

      A westerly wind comes from the west, moving eastward.

      Ocean currents, however, are described by the direction that the current would carry a ship toward.

      An east wind and an east current thus flow in opposite directions.  Don’t blame the scientists for this one (it’s an old maritime tradition); our peculiarities can be found in the extensive glossary starting at page 301 .

      For those who have to mentally recompute when traveling outside the United States, Liberia, and Burma:  16 kilometers is 10 miles, and a rise of 5°C is the same warming as 9°F.  In the rest of the world, water freezes at 0°C and boils at 100°C, a comfortable outdoor air temperature is 25°C (77°F), and normal body temperature is a mere 37°C.


Corrections to the first printing (substitute entire paragraphs)

On p.201:
...periods when the monsoon rains extended many degrees farther north into the
Sahara and grass became widespread.  Major lakes and many archaeological sites were found throughout the Sahara , dated to the most recent one.

          Various factors contribute to the extent, but what dominates is the month of closest approach to the sun (now in January).  When this is in the northern summer (every 19,000 to 26,000 years or so), about ten percent more sunlight is delivered.  The greater afternoon heating of the Sahara (also the Arabian peninsula and points east) means more moist ocean air is attracted into the interior by the rising hot air.

          There was a pluvial back during the prior warm period about 130,000 years ago, and another one reported sometime between 50,000 and 30,000 years ago.  During that interval, there were a dozen times when the worldwide climate kicked out of cool-and-dry into warm-and-wet within several decades time.  And, of course, a dozen collapses back into cooler-and-drier that likely affected the entire Sahara .

           In the Sahel cycle,....

On p.222, the arrow for the trade wind strength should be inverted (a colder North Atlantic is associated with increased trade winds near the equator).  The figure will be inverted in the softcover edition.

On p.271

You can see a mode of operation in the Saharaas the monsoons penetrate farther and farther into arid areas.  Just within decades, less dust blows offshore because the Sahara got major amounts of grass:  small additional changes create major consequences.  This started about 14,800 years ago and soon there were lakes and archaeological sites all around the Sahara.  Then the arid conditions abruptly reappeared about 5,500 years ago as the particularly hot Sahara summers moderated.  

pp.313 replace with

Richard B. Alley, The Two-Mile Time Machine:  Ice Cores, Abrupt Climate Change, and Our Future (Princeton University Press 2000).  It is written for nonspecialists and won the 2001 Phi Beta Kappa book award “for contributions to the literature of science.”  This book contains far more detail on abrupt climate change than the others.  So if you find yourself asking, “But how could they possibly know that?” you’ll find most of the answers in Alley’s excellent book.  For an update, see R. Alley, et al “Abrupt Climate Change,” Science 299: 2005-2010 (28 March 2003).


          If anyone needs a quick reference about the importance with which the scientific community views the revelations about abrupt climate change, see Abrupt Climate Change: Inevitable Surprises (U.S. National Academy of Sciences, 2001), at  For a good textbook on the earth sciences, covering atmospheric sciences, geology, and oceanography, let me suggest:

On p.336:
     Abrupt change from wet to dry in the Sahara  (at least, as measured by offshore dust) as the summer sun gradually changes:  Peter B. deMenocal, J. Ortiz, T. Guilderson, J. Adkins, M. Sarnthein, L. Baker, and M. Yarusinsky, “Abrupt onset and termination of the African Humid Period:  Rapid climate responses to gradual insolation forcing,” Quaternary Science Review 19: 347-361 (2000).

            Quote from Alley (2000), p. 83.


To the memory of my late friends,

DONALD N. MICHAEL (1923-2000)

DONALD J. REIS (1931-2000)

PENN G. GOERTZEL (1947-2001)

PATRICK D. WALL (1925-2001)






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 Niño 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.

          There was 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 decades.


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 ecosystems.

          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 chase.

          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 importance.

          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 century.

          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 very slow.

          Surprisingly, these large fast climate changes may be easier to prevent than a greenhouse warming or an El Niño.  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 important innovations.

          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 the stimulus.

          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.

 [This book will appear in Spring, 2002.
Reviewer requests should be faxed to the
Publicity Department at the
University of Chicago Press, 1-773-702-9756.]

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Notes and References (this preamble corresponds to pages 3 to10 of the printed book)

Copyright ©2002 by
William H. Calvin

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