Email Calvin || Glossary || Book's Table of Contents || Calvin Home Page  


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

copyright ©2002 by William H. Calvin
ISBN 0-226-09201-1 (cloth)    GN21.xxx0     
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

Return to previous chapter


To:                  Human Evolution E-Seminar
From:             William H. Calvin
0.77217°S   36.42091°E    1,940m ASL
                        Lake Naivasha
Droughts even in good times


Back 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.


Here 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 toolmaking.

     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.


The 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.

Now the wind grew strong and hard, it worked at the rain crust in the corn fields.  Little by little the sky was darkened by the mixing dust, and the wind felt over the earth, loosened the dust and carried it away.
- John Steinbeck,
  The Grapes of Wrath, 1939

     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 next spring.

     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 vegetation.

     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 most relevant.

     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. 

[Our] results highlight the sensitivity of precipitation patterns to nonlinear or threshold-crossing climate change, and underscore the potential for future climate impacts on society via drought or flooding rather than temperature per se.
-  Jeffrey P. Severinghaus
& Edward J. Brook, 2000


Successive 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 dry.

     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.


By 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 and toolmaking.

     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, too.

     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. 

A round man cannot be expected to fit into a square hole right away.   He must have time to modify his shape.
Mark Twain (1835-1910)

The 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 multifunctional.  This 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 habitual cheaters.

     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 invented:  the 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.


I 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 curve.

     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 high-calorie meat.

     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.

     Glynn Isaac suggested a quarter-century ago that meat may have provided a social currency, and various studies since then have suggested that meat cannot be understood solely in terms of calories contributed to the diet.  My suggestion is that hunting is crucial mostly for surviving the abrupt droughts, where meat on the hoof is the major remaining resource – but that the social (and sexual selection) value of meat is what sustains the cultural aspects of hunting in between the stressful occasions – and that the boom time aftermaths allow the hunting-capable population to expand faster than otherwise.


If 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 for vegetarianism.

- Frans de Waal,

    Good Natured: The Origins of Right and Wrong, 1996


Notes and References
(this chapter
corresponds to 
pages 155 to 168 of the printed book)

Copyright ©2002 by
William H. Calvin

The nonvirtual book is
available from
or direct from
 University of Chicago Press

  Book's Table of Contents  

  Calvin Home Page

All of my books are on the web.
You can also click on a cover for the link to

Conversations with Neil's Brain:  The Neural Nature of Thought and Language (Calvin & Ojemann, 1994)

The Cerebral Code:  Thinking a Thought in the Mosaics of the Mind (1996)

How Brains Think:  Evolving Intelligence, Then and Now (1996)

Lingua ex Machina:  Reconciling Darwin and Chomsky with the Human Brain (Calvin & Bickerton, 2000)

The six out-of-print books are again available via Authors Guild reprint editions,
also available through (click on cover):

Inside the Brain

The Throwing Madonna:  Essays on the Brain

The River That Flows Uphill


The Cerebral Symphony

The Ascent of Mind

How the Shaman Stole the Moon