W. H. Calvin's THE ASCENT OF MIND (Chapter 1)
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A book by
William H. Calvin
The Ascent of Mind (Bantam 1990) is my book on the ice ages and how human intelligence evolved; the "throwing theory" is one aspect.
   My Scientific American article, "The emergence of intelligence," (October 1994) also discusses ice-age evolution of intelligence. Also see Wallace S. Broecker, "Massive iceberg discharges as triggers for global climate change," Nature 372:421-424 (1 December 1994) and his "Chaotic Climate" Scientific American article (November 1995 issue).
AVAILABILITY is challenging.
Many libraries have it (try the OCLC on-line listing), but otherwise it’s strictly used bookstores (and German and Dutch translations).
The Ascent of Mind
Ice Age Climates and
the Evolution of Intelligence

Copyright ©1990 by William H. Calvin.

You may download this for personal reading but may not redistribute or archive without permission (exception: teachers should feel free to print out a chapter and photocopy it for students).

Tracking Climate Change
and Human Evolution

Carried away, perhaps, by His matchless creation, the Garden of Eden, He forgot to mention that all He was giving us was an interglacial.
Robert Ardrey, 1976

Matching wits with the fickle climate is how we became human. Or so I reflect, while waiting for the London-bound flight to depart from New York. "Delayed by unseasonably severe weather," a disembodied voice proclaimed an hour ago. My fellow passengers speculate about whether the greenhouse climate has already arrived.
      Well, there really isn't a threshold of some sort — whenever the ice ages temporarily recede, the carbon dioxide starts climbing. It's more a question of how badly we are augmenting the overheating tendencies. And what sort of trouble we'll make for ourselves with major climate change.
     It usually works the other direction: climate change affecting humans in a big way, rather than vice versa. Major climatic changes — particularly the ice ages — have meant quite a lot, when it comes to human evolution from the apes. Back before the ice ages started 2.5 million years ago, we were upright and even looked pretty human, if seen from a distance. Yet up close, it would have been apparent that behind that large face was an ape-sized brain. Then the ice ages started. Great continental ice sheets built up and then they melted off, dozens of times. During all that, we evolved much faster than in the preceding few million years. We now have smaller faces, though with a notable forehead. Seen in side view, however, there is a big difference. That's because our brains have quadrupled in size over the early model hominid.
     Why? Nothing similar happened to any other animal during the ice ages. With the brain's enlargement and reorganization, we acquired some beyond-the-apes abilities that we value most highly: a versatile language and a plan-ahead consciousness that enables us to feel dismay when seeing a tragedy unfold, enables us to develop ethics.
     What was it about climate change that pumped up brain size, that somehow augmented intelligence? Surprisingly, severity of weather, as such, probably wasn't the key. Rather, it's those repeated boom-times that early hominids had the opportunity to exploit. Some of the stories now emerging about the ice ages demonstrate the challenges and opportunities faced by our ancestors. For example, two particularly dramatic events occurred about 11,000-12,000 years ago, just as the last ice age (the one that began 118,000 years ago) was ending and half the accumulated ice was already gone. Until very recently, no one had been aware of either the American or the European story. And while these two climatic episodes probably didn't affect brain size very much, some of their predecessors likely did.

IN CANADA, TWO GIANT ICE SHEETS had been pushing against one another, head to head: the one pushing west from Hudson's Bay, the other grinding eastward, coming down from the Rocky Mountains. They met in the eastern foothills of the Rockies. With the melting, they each pulled back a little, allowing some grass to grow. And this opened up a north-south route from northern Alaska down to Montana.
     It's called a corridor because corridors have walls: I tend to think of this as something like the biblical parting of the Red Sea. The grazing animals discovered the new grass growing in the corridor, and their predators followed them. Brown bears migrated south, as did the humans who had reached Alaska sometime earlier by crossing the Bering Strait from Asia.
     This corridor had only one exit. When the hunters reached the southern end of the corridor about 12,000 years ago, at about where the U.S. border is now, they discovered the Americas largely uninhabited by humans. It was ripe for big game hunting and, thanks to living in Arctic latitudes where gathering was scarce, they were experienced big game hunters, even felling mammoth and mastodon in addition to lighter fare.
     So they had themselves an enormous baby boom, thanks to this previously untapped resource. A few dozen generations later, about 11,000 years ago, these hunting families were all over the continent, judging by their propensity for losing their favorite spearheads, the so-called Clovis points (one has even been found in the rib cage of an extinct mastodon). Their descendants are, with the exception of a few latecomers such as the Inuit (Eskimo), the present-day Indians of both North and South America.

IN EUROPE at about the same time, there was a more established prosperity, as hominid hunters had been living off the grazing animals there for many ice ages, more than a half million years. By the beginning of the most recent ice age, about 118,000 years ago, Homo sapiens had probably evolved from the earlier model, Homo erectus. Brain size may have already reached the modern size by then; the main change during the last glaciation can be seen in the teeth, but only if you look very carefully.
     Teeth became about ten percent smaller, seemingly a consequence of the food technologies invented during this last ice age; they dropped another five percent when agriculture came along. Cooking came first, judging from the charcoal that appears on cave floors starting about 80,000 years ago. Food preparation involving pottery improved things even more. We start to see skulls that indicate even the toothless could survive, suggesting both food preparation and a level of care of the disabled that was not seen in earlier times. Late in this glaciation, between 37,000 and 20,000 years ago, the life of the mind grew: Carved ivory and cave paintings became popular. By 11,500 years ago, these European hunters might have been starting to practice herding and agriculture (which was certainly imminent in the Middle East).
     But, as the new Americans were thriving, the more established Europeans got a big surprise, and I doubt that they liked it very much. I wouldn't be surprised if linguists someday show that the phrase, "The good old days," dates back to 11,500 years ago.

THERE WAS A EUROPEAN GENERATION who in their youth enjoyed the warming climate. New grass was growing everywhere along the glacial margins, and the herds were gradually getting larger. It wasn't a boom time for humans, as in the Americas, but both animals and humans were probably doing well because of the North Atlantic's warming trend that had suddenly started 1,500 years earlier (this "Allerod event" was about 13,000 years ago).
     This same generation saw things change. One year, the winter rains were scant, and it seemed colder. It wasn't as cloudy as usual in the spring, and the summer was bone dry. The good grazing was exhausted early, and animals started exploring unlikely places in search of food. By the time that the winter snows started, both humans and animals were in poor condition; more than the usual numbers died that winter. Was it just a drought?
     The next year was even colder and drier. And the next. The next twenty years saw dramatic changes, far greater than in the "Little Ice Age" of a few centuries ago. Forests died and weeds took over. It became more dusty as severe storms stirred up the dry topsoil. The herds surely dropped to a fraction of their former sizes.
     And the human tribes likely did poorly in consequence. Half of all children tended to die in childhood, even in the best of times before modern sanitation and medical care, but poorly fed children succumbed even more readily to childhood diseases. If anyone had had time to notice while scratching around for food, they would have seen glaciers advancing once again. In Scotland, where glaciers had already completely melted off, they started to reform as the summers became too cold to melt much of the winter accumulation.
     People didn't live half as long as we do, back then. A forty-year-old person often looked old and worn out. Children, who had never known those warm days of plentiful food on the hoof, surely wondered what the old folks kept talking about. When the generation that had seen the transition died out, the stories may have persisted for a while, and the good old days were perhaps incorporated into the creation myths as a form of heaven on earth.
     (A few decades ago, modern scientists looked at the accumulated layers of a lake bottom in Denmark. In a deep layer, they saw the sudden introduction of the pollen of an arctic plant called Dryas that had no business being in Denmark, and named this cold snap after it: the Younger Dryas climate.)
     And then — it ended even more suddenly than it had begun. There was a generation about 10,720 years ago, the great-great-(repeat that 29 more times)-grandchildren of those people who were absolutely sure about the good old days, that experienced the change. They grew up in a cold and dry Europe, and then saw the warm rains suddenly come back over the course of just a few years and melt the ice. The grass prospered, and the remaining grazing animals began a population explosion. It became a boom time for those Europeans who had survived up in the land of hard winters, just as it had become a boom time for the Arctic-adapted hunters who reached the end of the North American ice-free corridor a thousand years earlier. North Atlantic current map
     It was as if a switch had been turned off. And then back on again. Or perhaps faucet is the apt metaphor, since the key to what happened is the Gulf Stream's European relative, the North Atlantic Current.

AFTER LEAVING NEW YORK at sunset, our London-bound airplane followed the Gulf Stream to the northeast, up over familiar Cape Cod haunts in the dusk, then just offshore of the Nova Scotia peninsula. We saw the entrance to the Gulf of St. Lawrence, where the overflow from the Great Lakes makes its way out to sea, and saw many fishing boats as we passed over the Grand Banks fishing grounds off the large island of Newfoundland. Finally, during the night, we followed the eastbound Gulf Stream out over the North Atlantic proper.
     Before dawn, we flew over the North Atlantic Current, which sweeps northward up toward Iceland. But even after we passed over the current, I continued to see its effects, in the form of rain clouds drifting eastward toward Europe. I saw southern Ireland in the dawn light, great green patches between the storm clouds. Home of the Irish elk, the deer with the giant wingspan — at least for about 1,500 years (the Younger Dryas wiped it out, a good 1,600 years before humans arrived in Ireland).
     Seen through the scattered clouds, London at six in the morning is glowing in the early morning sunlight, and the streets shine from the spotty showers; a few delivery trucks cast long shadows while driving on the wrong side of the street. The green parks and the tennis courts are empty. But it's the London of William Shakespeare, Isaac Newton, Dr. Johnson, Charles Darwin, Bertrand Russell, and George Bernard Shaw.
     And London is a puzzle, since it is 51.5° north of the equator. It is hard to imagine any city in Asia or the Americas, that far from the equator, becoming such a center of culture and commerce. None has, so far: not Calgary, Alberta (where parking meters have electric outlets, so you can keep the car warm enough to restart). Nor Moosonee, the town at the bottom of Hudson's Bay. Or chilly Puerto Arenas at Tierra del Fuego, equally distant from the equator to the south.
     Indeed, most of Europe is at Canadian latitudes. Compared to the populous parts of the U.S. and Canada, mostly between the 30° and 45° lines on a globe, the populous parts of Europe are shifted 10-15° to the north, mostly between 40° and 60° latitudes. "Southerly" Rome lies at the same 42°N as does "northerly" Chicago. Paris lies at the latitude of Vancouver, British Columbia, about 49°N. Berlin is up at 52.5°N, Moscow at nearly 56°. Oslo, Stockholm, and Leningrad nestle up just under 60°N, where the sun makes only a brief midday appearance during December — about the same as in Alaska's coastal cities.
     The reason that Europe is warm and wet, where Canada is cold and dry, is largely due to the North Atlantic Current and how it differs from similar major currents in the Pacific Ocean. All those rain clouds I saw this morning were caused by the copious evaporation from the warm ocean surface of the North Atlantic Current.
     But what if something were to happen to the North Atlantic Current again?

THE BEST-KNOWN CLIMATE CHANGE in the offing is the global warming that is occurring from the greenhouse effect. It isn't minor, as this 1989 summary notes:

Computer-modeled predictions of greenhouse warming suggest that global mean air temperatures may rise by 5°C [9°F] over the next 30 years, with amplified rises of up to 12°C [22°F] in polar regions. This is comparable with the temperature increase from the last glacial period to the present interglacial, and the projected rate of increase is probably greater than at any time since then.
The best-known consequence is the rise in sea level that threatens coastal populations. But climate need not change gradually. We now know that, in the past, other climatic changes have flipped on and off, without much of a middle ground. The North Atlantic Current's on-and-off tendencies are only one example of the more general problem of "modes" of behavior.
     It has long been known that the climate could, in theory, become trapped in extreme states. The "White Earth Catastrophe" scenario could happen if ice extended over enough of the Earth's surface to reflect a lot of arriving sunlight back out into space: the Earth could freeze and never recover, short of volcanos covering the white surface with some dark lava. And the "Greenhouse Catastrophe" scenario would occur if the carbon locked up in the sediments (not just coal and oil but also that frozen tundra of Arctic regions) were released to the atmosphere in quantities sufficient to form a greenhouse layer of insulation, allowing the atmosphere beneath it (and oceans, and land, and us) to heat up catastrophically.
     In the 1980s, the Swiss climatologist Hans Oeschger suggested that, in addition, the earth's climate had several modes of interaction between the oceans, the atmosphere, the biosphere, and the ice sheets. These aren't extreme (indeed, we're in one mode now) but the transitions between them could be uncomfortably sudden. There had been hints of fairly sudden minor transitions. After all, people periodically rediscover that monsoons can simply be omitted some years.
     There are drought cycles that repeat every few decades, but some are much shorter: South American fisheries and the bird populations of many Pacific islands are dramatically depressed every half-dozen years by the warming changes in the ocean currents, known as El Niño. Evidence has been accumulating that North American droughts are secondary consequences of equatorial ocean currents turning colder, the so-called La Niña condition. But what Oeschger was talking about was more than minor: he suggested that the climate had major modes, some lasting many centuries. These bistable styles of operation may pose far more of a threat than the slow loss of coastal real estate to rising sea levels.
     Modifying the earth's climate with greenhouse warming may well exaggerate such mode-switching — or leave us stuck in the "wrong" mode for centuries, as has happened before. Paradoxically, you can get cold from heat, as the Younger Dryas demonstrates: a warming trend can apparently cause a prolonged cold snap. Most people have a tendency to dichotomize climate change into warming or cooling, and forget that both can happen simultaneously — but in different places.
     Ice layers preserved under Greenland's glaciers show that more than 20 regional chills, each lasting centuries, have occurred in the last 120,000 years. The Younger Dryas was simply the most recent and the longest-lasting (almost a thousand years). Though detectable along the east coast of the U.S. and Canada, it was most pronounced in Europe and southern Greenland; you won't see it in the deep ice cores from Antarctica. It was probably triggered, in part, by the dilution of the salt water by all that freshwater glacial runoff. But how were the other 19 cold snaps triggered? Might something like a greenhouse warming provoke another one? Those are the kinds of questions to which we urgently need answers.

SUDDEN REGIONAL COOLING during a global warming trend probably happens because the circulating ocean currents switch into a new mode, as when the North Atlantic Current no longer warms and waters Europe in its customary way. And Europe without the North Atlantic Current would be about like Canada: they both have a comparable amount of fertile agricultural land at similar northerly latitudes. Indeed, Europe gets Canada's air second-hand, a week or so later, as Europe periodically rediscovers whenever a forest fire in Canada makes European skies hazy and sunsets dark red.
     You might surmise that Europe's population ought to be something like Canada's 27 million people. But France alone has twice as many people as Canada. Europe, to the west of the Soviet Union, totals more than 500 million people (twice the U.S. population), and there are another 200 million people in the western parts of the Soviet Union that share Europe's climate (the Younger Dryas climate reached as far as the Ukraine). That Europe presently supports about 26 times as many people as Canada is largely attributable to the beneficent influence of the North Atlantic Current, warming all that cold Canadian air crossing the North Atlantic, before it reaches Europe. And thus loading it with a lot more moisture, to be dropped on Europe as rainfall.
     What will the "extra" half-billion people of Europe do, should the North Atlantic suffer another hiccup, returning Europe to a Canadian climate? If one could reliably forecast this situation, with a lead time of a hundred years or so, perhaps those Europeans would move elsewhere peacefully or develop a reciprocal symbiotic economy with some Third World countries that could feed them. Yet mode-switching cooling can happen as quickly as the onset of a minor drought, and no one knows how to predict it, much less control it. The first few years, there would be an "economic response": Europeans would buy grain elsewhere and ship it in, cut back on meat. But what would happen in the long run?
      Remember how poorly the economic response worked for Ireland in the mid-nineteenth century when the potato crops failed? And what happened during Europe's last Great Depression a half-century ago: Germany's lebensraum excuse for territorial expansion, a professed need for "more living space"? Europe is technologically competent, compared to today's Third World or nineteenth century Ireland, and a starving population isn't going to die quietly. They will move instead. A little glitch in the North Atlantic, similar to those of the past, is the most serious, least avoidable scenario for global warfare that I can imagine.
     Whether it is a greenhouse-induced rise in sea level threatening the half-billion people relying on low-lying areas of the Indian subcontinent, or a cooling-and-drying Europe in need of lebensraum for a half-billion people, or the projected return to dust bowl conditions in the American Midwest and the loss of irrigation water in California (whose agriculture already helps feed Eastern Europe and the USSR in their bad years), climatic change is not likely to be peaceful. "Disruptions" is hardly the word for it.
     We are very overextended, with far more population than we can support (even in the off-years of our current climate, as those Third World famines have repeatedly demonstrated). Major climate change, whether ice age or greenhouse warming, means a considerable "contraction" in the human population that the planet can support, unless new technologies fix up things very well indeed. An abrupt Dryas-like climate change, however, could easily destroy the stable civilizations that such large-scale innovative technologies require.

BUT WE HUMANS THRIVE on challenges, and a prolonged series of climatic changes probably played a leading role in how we evolved the neural mechanisms for those aspects of our consciousness that exceed those of the apes.
     The most unique aspect of our consciousness is "thinking ahead," our ability to spin scenarios that try to explain the past and forecast the future. Often these strings of concepts make little sense (such as our nighttime dreams); other times, we shape them up into a thing of quality (such as a poem or a logical argument) and then act on it. Planning ahead in other animals is mostly a hormonal thing, hoarding behaviors being triggered by the shortening daylight hours of autumn which prolong the nighttime release of melatonin from the pineal gland. But we humans are capable of planning decades ahead, able to take account of extraordinary contingencies far more irregular than the seasons.
     Since the prehuman brain enlarged only when the ice ages came along, the betting is that climatic challenge had something to do with the Great Encephalization — probably not so much because of a more severe climate but because the constant disruptions created opportunities and slowed "optimizing." Shaping up a body plan to the environment, efficiently dealing with its opportunities and hazards, is the usual anthropological concept of darwinism, but fickle climates can add another dimension to the story.
     Give evolution enough time to shape up things for efficiency, and jack-of-all-trades abilities will be eliminated — we'll get a stripped down, lean-mean-machine version optimized to the existing climate. Fortunately, evolution is slow. Climate often changes faster than biological evolution-for-efficiency can keep up — and so a brain that can function in various different climates has an advantage over one that is merely efficient in a single climate. Retaining those jack-of-all-trades abilities is a lot easier if the climate keeps switching around unpredictably.
     Ever since the major buildup of ice caps started 2.5 million years ago, the world climate has been oscillating markedly every 10,000 years or so (and more often in some regions), with major meltbacks of the northern ice sheets every 100,000 years (like the one 13,000 years ago that heralded the development of agriculture and then civilizations). This book makes the argument that we owe our versatile brains to these first-one-thing-and-then-another challenges of the ice ages — and the boom times that often followed.
     That doesn't mean, however, that another major climatic challenge will pump up the brain a little more. There has been a little change in scale. The human population has increased a thousandfold since the end of the last ice age: that's what agriculture, animal breeding, and technologies have made possible, compared to the days of hunter-gatherer bands wandering around. Having large numbers of individuals tends to buffer biological change, to slow it down.

EXPLAINING THE CLIMATIC PAST, forecasting our climate's future — those are some urgent tasks for our newfound mental abilities. But since human behavior plays the major role in generating the problems we now face — all those boom-time birth rates that lead to more population than can be fed in the drought years, our live-for-today and let-tomorrow-take-care-of-itself mentalities that lead to more pollution — understanding our evolutionary past may be just as important as building those big computers that will make working models of the global interactions between ocean, atmosphere, and ice. The way to make plausible plans for the future is to know what's worked in the past, and what hasn't. Navigating in tight spots means knowing the currents.

The inhabitants of planet Earth are quietly conducting a gigantic environmental experiment. So vast and so sweeping will be the consequences that, were it brought before any responsible council for approval, it would be firmly rejected. Yet it goes on with little interference from any jurisdiction or nation. The experiment in question is the release of CO2 and other so-called "greenhouse gases" to the atmosphere.... Because of our lack of basic knowledge, the range of possibility for the greenhouse effects remains large. It is for this reason that the experiment is a dangerous one. We play Russian roulette with climate, hoping that the future will hold no unpleasant surprises....
     My impressions are more than educated hunches. They come from viewing the results of experiments nature has conducted on her own.... Earth's climate does not respond to forcing in a smooth and gradual way. Rather, it responds in sharp jumps which involve large-scale reorganization of Earth's system.... Coping with this type of change is clearly a far more serious matter than coping with a gradual warming.

the geophysicist Wallace S. Broecker, 1987

1995 Update: The current dating of the Younger Dryas onset is 13,000 years ago rather than the 11,500 mentioned in this 1990 manuscript. And the abrupt climate changes are even more frequent than then appeared: see
Wallace S. Broecker, "Massive iceberg discharges as triggers for global climate change." Nature 372:421-424 (1 December 1994) and the November 1995 issue of Scientific American (two months later in translation editions).
For updates on El Niño, see El Niño tuitorial from NOAA.
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