|A book by|
William H. Calvin
UNIVERSITY OF WASHINGTON
SEATTLE, WASHINGTON 98195-1800 USA
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 its 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).
Some Consequences of a Fickle ClimateScientific inquiry is successful because it is, like the evolutionary process, a powerfully selective system. Scientific theories, by design, are always vulnerable to destruction just like a species, subjected to environmental pressure, is subject to extinction. Because of that vulnerability, scientific truth has the strength that comes of survival in a challenging environment.... Even when scientific theories fail to survive... their evolutionary progeny carry the best "genes" -- the ideas that still work -- of the previous theory intact. Ironically, it is the willingness to risk everything, even existence itself, that is the guarantor of survival.The Hungarian Academy of Sciences's library in Budapest was the scene of one of the most important, albeit unlikely, episodes in our understanding of the ice ages. It was where Milutin Milankovitch was a prisoner of war during the First World War. A Serbian mathematician who briefly saw service as an officer of Serbian troops invading the Turkish Empire, he was subsequently captured by Austro-Hungarian troops in 1914 while visiting his home town of Dalj. Taken to the Esseg fortress as a prisoner of war,
the physicist Heinz Pagels, 1988
The heavy iron door closed behind me.... After a while I happened to glance at my suitcase.... My brain began to function again. I jumped up and opened the suitcase.... In it I had stored the papers on my cosmic problem.... I leafed through the writings... pulled my faithful fountain pen out of my pocket, and began to write and count....As I looked around my room after midnight, I needed some time before I realized where I was.
His jailers later received a telegram ordering them to remove Milankovitch to Budapest. There he was paroled on the condition that he report once a week to the police station. A Professor Czuber, having learned that the mathematician had been imprisoned, had petitioned for his release.
When settled in Budapest, Milankovitch walked over to the Hungarian Academy of Sciences where he was welcomed with open arms by the library's director, the mathematician Koloman von Szilly. Milankovitch spent the next four years in the library's reading room, making a mathematical model for the climate of a planet whose axial tilt varies and whose elliptical orbit changes, stopping by the police station periodically to demonstrate that he hadn't escaped.
Milankovitch didn't know what we know now: that the first major ice buildup started about 2.5 million years ago. That means, on his 100,000 year cycle, that there were several dozen -- but each of the several dozen ice ages also contains a lot of back-and-forth in addition to a major meltoff. The last major meltoff was about 13,000 years ago, though the previous one was at 128,000 years ago -- just to show you that 100,000 years is only approximate.
The evolutionary changes that produced our big brain had mostly happened in earlier ice ages; this ice age's theme is primarily concerned with takeover rather than innovation. One can, however, see processes at work that would have been important in evolving the basic Homo sapiens body and brain during the back-and-forth of any ice age of the several dozen that Milankovitch's theory addressed.
BETWEEN BUDAPEST AND SEATTLE, there are no nonstop flights -- but Copenhagen isn't far off the great-circle route. I've managed to get a geologist's seat (defined as a window seat in front of the wing) for both legs of the trip, Budapest-Copenhagen and Copenhagen-Seattle.
After lifting off from Budapest, the plane follows the Danube upriver. The river country soon gives way to the low mountains of Czechoslovakia -- classic Neandertal country, as I explained to the Danish couple seated alongside me (they had inquired about what was so interesting outside the window). Beyond these mountains are the plains of Poland, flattened by the glaciers that periodically sat atop them -- but hardly any Neandertals have been found north of the line from the Netherlands to Czechoslovakia (nor south of the Mediterranean) and no earlier than about 100,000 years ago (nor since about 33,000 years ago).
Though we are accustomed to thinking of hominids (all of the species since our common ancestor with the chimpanzees, about six million years ago) as originating in Africa, the Neandertals seem to be primarily European -- though not like the modern Europeans. If you've ever seen a professional football team walk through an airport or hotel lobby, and compared them with some other collection of young men such as a group of soldiers out of uniform, you'll have the general idea about the size differences between Neandertals and ordinary Homo sapiens -- but you'll have to imagine the football players' faces as massively rugged around the eyes and cheeks to understand that Neandertals weren't just big.
While their brains were somewhat larger than ours (perhaps 15-20 percent), their bodies enlarged even more. As did the brains and bodies of their European competitors, early modern Homo sapiens. Most of us today are smaller than both types of Europeans that battled the ice, just as most aboriginal groups are smaller than the Eskimos. As a consequence, we have smaller brains (if size counts, it's probably relative to body size).
Though Europe was their home (and there is no sign of them having originated in, or spread to, Africa), Neandertals are occasionally found elsewhere. Like the birds pushed into ice-age refuges in the Balkans and the Iberian Peninsula, ice-age Europeans might have also been forced to the southeast and southwest. Certainly they occasionally spread to the southeast as far as the Caspian Sea. One of the most rugged-looking Neandertal skulls I've ever seen is from Mount Carmel, near Haifa, and is one of the first exhibits to greet you as you enter the Israel Museum in Jerusalem. The Crusaders weren't the first Europeans to invade the Middle East.
To the southwest of central Europe, there are many Neandertal remains, and one can imagine them isolated on the Iberian peninsula, trapped between the ice and the sea. An "island" is a classic setup for developing specializations such as robustness, such as behavioral predispositions that tend to minimize interbreeding with a parent population once the geographic isolation ends. Europe was being covered and uncovered by glaciers all the time; it was all frontier country, grassy steppes near the glaciers giving way to forests a bit further away. Megafauna such as mammoth grazed and browsed on those glacial frontiers. Homo sapiens, both modern-type and Neandertal, probably made a good living by hunting the big animals.
But the Neandertals lived with much more stress, probably due to episodes of starvation diets; you can see evidence of disrupted growth in their teeth. Certainly, they died far earlier; only 8 percent of adults made it past 35 years of age, compared to about 50 percent in aboriginal populations. A possible reason: three out of four adults that lived past 25 years of age had some evidence of healed bone fractures, compared to perhaps one in four in modern aboriginal populations. Trauma that quickly killed the individual isn't counted, as it is difficult to tell fresh breaks from postmortem breakage; only partially healed fractures are distinctive enough to count with confidence, so the percentage is undoubtedly even higher. And, of course, infant and child mortality meant that many didn't even make it into adulthood at all, to be counted by such statistics.
That so few Neandertal adults survived to 35 years, and that so many of them were maimed by injuries, suggests a life full of hazards not faced by the groups to which we compare them. Perhaps their methods of hunting involved getting too close to the angry animal. Perhaps they hunted only deer and the like rather than a wide range of species (and so were hurt by their bad years). Perhaps there was bloody rivalry between different bands of Neandertals for hunting territories. Conflict with early modern Homo sapiens, the invariable focus of popular discussion, is unlikely to explain very much of the Neandertal's short, brutal lives, as most would live their lives without ever seeing the other version of Homo sapiens.
The early modern Homo sapiens were doing something better than the Neandertals: they certainly lived longer as individuals and ultimately survived as a subspecies, whereas the Neandertals disappeared as an identifiable group. Starting about 50,000 years ago, early modern Homo sapiens began moving west from the plains of Poland into Neandertal's home grounds. The Neandertals were last seen about 33,000 years ago in western Europe. After surviving two major glaciations, Neandertals died out before the last one (which peaked 20,000 years ago); they lasted at least 70,000 years.
MODERN-TYPE HOMO SAPIENS was around during most (if not all) of this last ice age, which started 118,000 years ago; we might well date from the 10,000-year-long population boom that probably occurred during the major respite between glaciations, when all this land of northern Europe and Asia opened up to hominid habitation. We have more information on this ice age than the several dozen earlier ones, but it is well to remember that the last 100,000 years is merely the last two percent of the evolutionary period for getting from the apes to humanity -- and that Neandertal-like variants lasting only about 70,000 years might have often happened in earlier times, confusing our view of the "progression."
While Neandertals may have evolved out of early modern Homo sapiens in some isolated refuge such as the Iberian Peninsula, both forms could have evolved out of archaic Homo sapiens, which date back to the penultimate ice age and somewhat earlier (400,000 to 100,000 years ago, very roughly). Archaics are a large-brained transitional form, not very well defined, which evolved out of the somewhat smaller-brained Homo erectus. These ancestors got started about 1.7 million years ago in Africa, appeared in the role of "Java Man" about a million years ago, and were last on stage as "Peking Man" 225,000 years ago.
Between the DNA-dated common ancestor with the chimpanzees at 6-7 million years and the earliest of Homo erectus at 1.75 million years are several major types of upright but small-brained australopithecines. Between 2.0 and 1.8 million years ago, there was Homo habilis with its enlarging brain. But who is ancestor to whom? That's always controversial, and Homo habilis is not the best-defined of species either. The australopithecines were seen almost 4 million years ago, overlapped with Homo erectus for awhile, and died out by about 1 million years ago. They are frequently subdivided into a gracile and a robust form (slender and thickset, e.g., the difference in appearance between basketball and football players); indeed, a hyper-robust form of australopithecine developed and died out.
The two types of the australopithecines are somewhat reminiscent of the difference between the slender "pygmy chimpanzee" (often called the bonobo) and the more robustly-built common chimpanzee. And between the early modern Homo sapiens and the Neandertal. Such contrasting body types are actually a fairly common theme in zoology: while bigger is often better, boom times may allow lightweight variants to arise and come to dominate. A not uncommon mechanism for this switchover involves early sexual maturity.
Because there is less time between generations and it's a boom time, this allows the fast maturing to outreproduce the standard-maturity main population in the race for the extra allowable population slots; that's how their percentage in the population increases over the centuries. The other major consequence is that the adult body size and shape of the fast maturing becomes "gracile," reminiscent of how the robust adults looked when adolescent. That is because sexual maturity tends to slow down body development to a crawl (just compare ages 10-19 with 20-29!), and so one gets a "juvenilized adult."
In average times, the gracile variants may not be able to feed their dependent offspring as well as the average type. Among mammals such as the hominids, you can see some reasons why. While bigger males might be more successful in something such as hunting, I suspect that the females are the key because, back in the good old days, fasting was more than just a voluntary religious exercise. A woman's body fat can be converted into mother's milk, and so protect an infant from the bad weeks when the adults involuntarily go hungry. A bigger mother has more resources and can keep an infant healthy for longer, despite going hungry herself; a steady diet is particularly important for infants because they are so often sick, not having built up their immunity yet. Fat was surely beautiful.
In hard times, the robust forms may come to dominate again despite their slow maturity. Shorter generation time may count for little when the overall population isn't booming but rather contracting. And so, even if the juvenilized graciles have become a majority, their dominance slips a little in average times, even more in hard times. The Neandertals seem to have typically lived in hard times, given those appalling trauma and longevity percentages, given that tooth pathology caused by going hungry so often -- not at all the sort of situation depicted in novels about the ice age.
NEW SPECIES presumably arise from just such boom-time broadening of species characteristics. The old climate might only support a robust type of body and a conservative sort of brain. The boom time might broaden out the species, so as to support both robust and gracile bodies, both shy and bold personalities, as extremes of a bell-shaped distribution. After the boom time is over, so the usual reasoning goes, the graciles ought to shift back toward the robust standard.
Should the extreme graciles discover a new way of making a living, thanks to their juvenile playing-around tendencies or some other aspect of gracile bodies or juvenilized development, then they might persist even when the climate worsens once again. The two extremes of the bell-shaped distribution might continue, but the middle disappear, not optimized for either niche.
This sort of scenario (and it is hardly restricted to the robust-to-gracile spread of characters) illustrates how a species might split into a Y. If the original species dies out simultaneously, the new would seem to replace the old. "Windows of climatic opportunity" can be quite rapid on the geologist's time scale, and so such boom-time splits might seem sudden. The "tempo and mode" of evolutionary change is especially important, as Darwin recognized:
[The] periods during which species have been undergoing modification... have probably been short in comparison with the periods during which these same species remained without undergoing any change.
Charles Darwin, from later editions of On the Origin of Species
There was a "boom" in punctuated equilibrium research until it was realized that a geological time resolution of only several centuries was probably going to be needed to test alternative hypotheses; stratigraphic layering just isn't that precise. Surely, however, species characteristics need not take a jump: in most cases, there was likely a hard-to-resolve broadening of characters that preceded the "new species."
THE OBVIOUS WAY TO GET A BIGGER BRAIN has always been juvenilization. Juveniles have a larger brain/body ratio than adults, which suggests this simple recipe: Declare the juveniles adult (good old early maturity), slow down their somatic development to a crawl as usually happens after puberty, and presto! You'll have an adult with a relatively bigger head. An example:[T]he talapoin, a drawfed relative of the rhesus monkey, has the largest relative brain weight among monkeys. Since within-species brain curves have substantially lower slopes than the two-thirds value for the marmoset-to-baboon curve, evolution for a smaller size by backing down the within-species curve would yield a dwarf with a far larger brain than an ordinary monkey of the same body size.Stephen Jay Gould, 1986
Backing up isn't just biological; these days, it is also a major cultural phenomenon. One often remarks on adults who are attempting to look younger than they really are, using a variety of cultural strategies such as eye-size-enhancing makeup, dieting to an adolescent profile, and hair dyes. Evolutionary processes can also make an adult look juvenile, by the standards of earlier generations.
This idea is usually subsumed under the term neoteny, though, in many people's minds, that term means more than juvenilization of features: it also indicates a concomitant slowing of somatic development (so that it takes a child weeks longer than average to gain the next centimeter of height). But I think that, in the hominid case, precocious puberty eventually leads to such slowing of development, which is why we often see them as linked.
THERE ARE EASILY a hundred features by which humans differ from the apes. Thanks to various linkages, some of the features group into families, such as neoteny's group of linked traits. Then there are the fat-salt-water features of the aquatic theory, though the savannah theory (a proposal that hominids went through a period of making their living in open country away from the forest, and that this shaped up a group of features such as eating seeds and perhaps partially upright posture) is probably the best-known example of a common cause for many features.
Although they accept the linkages implied by the savannah theory, most anthropologists seem to be generally dismissive of the aquatic ape theory and its analogous group of linked traits. The physiologist Alister Hardy proposed in 1960 that early hominids must have gone through a period of making their living by shoreline foraging, given all those unapelike features that we share with the more thoroughly aquatic mammals such as seals and whales. We have lots of subcutaneous fat (and those rounded body contours of the aquatic mammals), we have the salt-and-water wasting kidneys (which force us to stick close to fresh water, and seek out salt if not eating enough meat), plus a variety of other oddities such as tearing (likely related in part to the salt glands used by aquatics to get rid of excess salt).
Dismissing this makes about as much sense as totally dismissing the savannah theory, merely because a few facts can be found that cannot be explained by the savannah's selection pressures (such as those subcutaneous fat layers). I think it a mistake to consider aquatic and savannah as competing theories, which is reminiscent of the blunder seen in post-1900 genetics. Those pioneering geneticists made the false dichotomy of mutations or Darwinian selection; we now know that both are correct -- but still not a complete view.
I suspect that we will come to see that both aquatic and savannah are correct but not complete. I'll bet that aquatic came first, then a savannah phase (and that a more recent temperate zone, winter-driven phase will eventually be recognized once the fossil record improves). Certainly from the viewpoint of evolutionary theory, it is a serious mistake to treat either aquatic or savannah as anything other than a partial theory.
Both aquatic and savannah theories are an effort to reduce the hundred features to a few processes, each of which affects a multitude of features via linkages. And so is the neoteny theory, though in a different way: the neotenic features may be tied together by developmental linkage, not by a common environment such as aquatic or savannah. A selection pressure involving one feature (such as shortened generation time, when the niche is expanding in a boom time) may haul along a number of other features, such as smaller teeth and flatter faces, even though there is no positive selection pressure on those ancillary features.
The aquatic theory, the savannah theory, and the neoteny theory together serve to "explain" many of the hundred features, and so focus attention on the remaining features for which some additional selection pressure was probably required. To say that neoteny cannot explain the descent of the larynx (ours is located several vertebrae lower down than in apes, making the throat a more versatile vocal filter), or all that unapelike fat that is added to a fetus in the last months of gestation, is not to say that "neoteny is wrong" but only to raise the possibility that yet another selection pressure may be needed besides the ones leading to the major groupings.
Reducing the laundry list to manageable proportions, and so singling out the oddities, is an important function of such partial theories. Some scientists make the mistake of assuming that the function of neoteny or aquatic ape theories is like that of a theory in physics: to be an all-purpose explanation ("One thing explains all!"), so that finding something "wrong with it" can be used as a pretext for dismissing it completely. They do not, fortunately, make the same mistake with the savannah theory; they have not discarded it when finding some comparable feature that they cannot explain. That the aquatic ape theory derives from comparative physiology, and the neoteny theory from developmental biology, probably has something to do with this unequal treatment as most anthropologists are far more comfortable talking about comparative anatomy and primate behavior (such as changing a monkey's anatomy and social behavior into those of a baboon by moving from the trees to a savannah, as a model for what hominids might have once gone through).
Simple adaptationist reasoning is usually the first thing we try, but we must examine the possibility that ape-to-human mosaic evolution involves only a half-dozen important selection pressures, rather than a different one for each of the hundred distinguishing features. And we must examine the processes that spread a new feature around to other parts of the world.
GLACIERS ADVANCE AND RECEDE like a disease with occasional remissions. That has some important consequences for demography, thanks to another aspect of episodic boom times. The fluctuations in the climate serve to magnify the effect of natural selection in certain locales, so as to make a small minority into a widespread majority. One implication: natural selection in one locale affects population characteristics elsewhere, indeed in places where such natural selection is quite weak. The ice ages provide an easy example. When Europe and northern Asia were uncovered, a lot of new land was opened up for human habitation -- about 20 percent of the hominid-habitable land area of the Old World lay under the ice sheets at their maximal extent, so the meltback expanded the remainder by 25 percent. That is a lot; one would expect quite a population boom (say, for the sake of illustration, a 25 percent population increase).
Still, by today's standards (Kenya's population is expanding at the rate of 4.2 percent per year), it was a very slow population boom, so sluggish that no one could have known it was happening. Even in the fastest part of the meltback, the land area was only increasing at the rate of 0.4 percent per century. It wasn't like the Oklahoma Land Rush of 1889, when the existing population suddenly spaced itself out to occupy the "vacant" land. The new land surely went to the offspring of the people who lived near the ice-age frontier: they would have been able to successfully raise somewhat larger families than the world average because of the increasing productivity of the nearby land (more grass, therefore more grazing herds, consequently more meat for hunters).
And those inhabitants of the ice age frontier likely had a gene pool somewhat different from the world average: because the frontier was in the temperate zone, they may have required some adaptations to get through the winters, such as somewhat larger bodies. The large bodies of the Neandertals and early modern Homo sapiens are suggestive of such an adaptation to life on the ice age frontiers of Europe during the most recent glaciation, for about the same reason that polar bears have especially large bodies: when caught out in a blizzard, core temperature loss is slower if you're big. Stature tends to increase with latitude among present aboriginal populations. Note, however, that height isn't always the best indicator of "body size"; the tall and slender Masai on the equator demonstrate how body shape can be important for heat loss by sweating (the more skin area, the better), and the more compact bodies of the Inuit suggest heat conservation. What, besides thermal adaptability, does one need in order for an ape to live in the temperate zone year-round?
All temperate zone aboriginal peoples utilize hunting in the wintertime (even if hunting is currently out of fashion among anthropological theorists). Obviously, somewhere along the way, we've improved the typical ape's flinging skills into the baseball pitcher's throwing skills. Projectile predation is very rewarding, compared to snatch-and-grab hunting of the kinds that baboons and chimpanzees practice -- action at a distance is safer for the hunter, and accuracy improves the yield.
While a nice invention, hunting isn't usually essential; most preagricultural aboriginal populations get the majority of their calories from gathering plant products such as leaves, fruit, and nuts. Hunting is of restricted importance in the tropics because hunting failures can usually be offset by more gathering. While this is also true in the temperate zones for most of the year, wintertime means sharply restricted gathering opportunities: plants become dormant, snow hides much of what is left. This creates an annual do-or-die wave of selection for hunting skills.
While hunting involves all sorts of skills, most of them are shared with the carnivores and baboons and chimpanzees: teamwork (and the body-language communication that goes with it) and outsmarting the prey have often been mentioned as aids to selecting for prehuman communicative skills and plan-ahead intelligence -- but they were well-developed in other social animals (dogs, baboons, etc.), presumably before hominids came along. The only area in which humans seem to have enormously augmented primate hunting skills is in projectile predation: we seem to have invented "action at a distance" killing (well, reinvented: archer fish spit at insects flying just above the water surface). So the temperate zone population might also have had some of the brain reorganization and enlargement that facilitates precision pitching.
Still, the temperate zone must have supported only a small fraction of the total hominid population -- say, 15 percent. The annual wave of selection for wintertime skills would keep the subpopulation shaped up, and people moving around would spread some of those genes south to the tropics.
Diffusion back from the frontiers is, however, a very slow way to change the world-average hominid -- absent, that is, a little pumping of the periphery. Therein, I suspect, lies the true importance of the ice ages per se for human evolution.
ONE STROKE ON A PUMP HANDLE usually doesn't yield any water. You just hear the water trickling back down. Only by repeated strokes on the pump does any water flow. The second stroke catches the water from the first before it can escape, and boosts it a little higher.
While evolution doesn't work exactly like that, it has some properties that you'd never guess just from studying static populations, just as you might never guess how to pump water uphill by contemplating a still pond. As Darwin noted, relatively static situations mean that new species have to "drive a wedge" between old species in order to eventually take over. It is very slow going. But climatic fluctuations provide pumps. They not only promote complexity but they speed up change via boom times. They occasionally even have ratchets to prevent backsliding.
"In the variations lay the insights" is a scientific principle that stands in contrast to all post-Platonic efforts to define the "essence" of what makes a monkey a monkey, and so forth. Many of the most important insights into nature have been made by scientists who looked instead at deviations from the "average type." Newton and followers discovered that the really workable way to define a force was as the rate of change of momentum (itself the product of mass and velocity), that anything which caused momentum to change would henceforth be defined as a force. Darwin and followers looked at the variations around the "typical" and saw in them the needed raw material for evolution. Einstein and followers looked at Brownian motion -- that random walk of bits of dust illuminated by a sunbeam -- and saw in it a way to derive the gas laws that predict your tire pressure increase on a hot day. And in the 1980s, climatologists took a second look at those cold spikes in the ice-core records of the last glaciation that they had previously discarded as just "noise" and discovered in them a mode-flipping tendency of the earth's climate.
Over and over again, in matters large and small, the variations turn out to be more important than we thought. Now we look at the back-and-forth ice ages and see in them not just overblown winter but a way of amplifying the effect of the wintertime natural selection that takes place up on the sparsely populated frontiers.
THE ICE AGES PROVIDE THE PUMP -- though not, as most people initially assume, because of more severe selection during the ice age itself. Rather, it is because of those population booms that occur with a meltback.
When the population increases by 25 percent, it all comes from the temperate zone version of hominids: they are the ones living up near the new land as it becomes available. And so the winter-specialized hominid subtype goes from being 15 percent of the population at the peak of the glaciation to being 32 percent of the expanded total (15/100 expands to 40/125) when the meltback is complete.
With the next advance of the ice, the habitable land shrinks. Plants cannot pick up and move; they are plowed under. But animals can move south as the glaciers cover up land. There will be more hominids than the reduced land area can support, so the population will drift back down to the original total over the generations. Will the percentage of temperate zone body styles also change?
If there were farmers defending their fenced-off fields, the frontier people might be caught between a glacier and a hard place. But both temperate and tropical types were mobile populations of hunter-gatherers, always milling around, and their territories were likely flexible in the manner of the American Indians. The crowding would force some temperate-zone hunters into lower latitudes, where they didn't absolutely require their temperate zone adaptations. Yet the hunting skills they had would be handy for exploiting semitropical resources that the less winter-adapted hominids weren't utilizing, such as meat that can run away from you, such as the gathering high in the subtropical hills where the chilly weather discouraged the tropical types. The larger body size of the temperate veterans would make them more likely to win arguments when defending desired hunting territories, but one need not make an analogy to the "Out of the North" hordes such as the Viking and Mongol invasions of Europe (facilitated by boats and horseback riding, respectively): simply running off competitors for hunting grounds with thrown rocks and the like would have sufficed. The temperate zone types would be better at that, whether the competition was a wolf or another hominid.
So in the population contraction that accompanied the advancing glaciers, the winter-specialized subtype might well stay at 32 percent -- and perhaps they even did better. And now a majority of the winter-specialized subtype is no longer living in the temperate zone but at lower latitudes without winters, mixing with the general population.
LET THE CYCLE REPEAT: a meltback slowly expands the population by 25 percent once again, and the 32 percent winter-specialized subtype increases to 46 percent via all those "extra" frontier babies. When the population contracts during the next ice advance, they remain 46 percent of the reduced total if they hold their own. Expand again, and they go to 56 percent -- and 65, 72, 78 percent and so forth. Thanks to the slow meltback giving them the extra babies, the temperate zone subtype soon becomes the dominant type in the world population.
Actually, it probably went faster than that. You have to remember all of those "minor" back-and-forth movements of the glacial frontier during each major ice age. Four expand-and-consolidate cycles of only 10 percent each takes the original 15 percent winter-specialized subtype up to 42 percent of the world population, then the big 25 percent meltback jacks it up to 54 percent. So the winter-specialized subtype goes from being a 15 percent minority to being a 54 percent majority in only one 100,000 year ice age cycle.
And all this while, the annual round of natural selection for getting-through-the-winter abilities is further changing that 15-to-32 percent of the population that still lives in the temperate zone. So as fast as hunting abilities can be shaped up into biological adaptations via form-follows-function on the frontier, they can be spread to the lower-latitude main population with a lag of considerably less than an ice age or two.
Fluctuations can be very important, given a pump -- and this pump is even simpler than Darwin's successive shaping up of genome variations by a static environment.
PUNCTUATED EQUILIBRIUM arguments are essentially about acceleration and stasis, the extremes in the rate of gradual evolution. This expand-consolidate cycle suggests that the rate of gradual change seen in the central population (the typical source of the fossil record) is markedly affected by sometimes-distant frontier conditions, not merely their severity (e.g., winter culling) but also the slowness of habitat expansion (e.g., ice ages) and of gene pool mixing.
Though the present model emphasizes that repeated inundations of the central gene pool may alone be sufficient to accelerate the otherwise slow pace of gradual evolution associated with large established populations without speciation, loss of the ability to create fertile hybrids between the two populations may have also occurred during some cycles. This might have happened on offshore islands which were later reconnected to the mainland by falling sea level, or on peninsulas serving as refugia. Though such new species are most vulnerable (as are all specialized small groups) to haphazard extinction, they might have occasionally "taken over" and replaced the previously dominant type.
Still, I have a hard time imagining that there have been dozens of such replacement events during the gradual quadrupling of hominid brain size during the last 2.5 million years -- particularly when there is such a simple alternative available, where ice age fluctuations merely pump the periphery and spread around the newly shaped-up subtype. Modern-type Homo sapiens (the model year that probably came in with the previous interglacial about 120,000 years ago) could be the exception to the rule, however -- big game hunting and food preparation technologies might have facilitated a fairly complete takeover, for once, during the last of the two dozen ice ages.
Winter, then, is likely to be of great importance in hominid evolution, once hominids started attempting to live year-around in the temperate zone (which, on current fossil evidence, is between 1.1 and 1.4 million years ago -- call it a dozen ice ages). Without the ice ages to expand and contract the habitat size, the evolution of hominids might have been slow going – just as without the daily tides augmented by the moon, it would have been a much slower process for fish to make the transition to land-dwelling.
The fluctuations in hominid-habitable land surface meant that frontier-type genes could be readily spread around the tropics. This provided our ancestors with both the physical advantages of the frontier-type and the cultural advantages developed in lower-latitudes where population density was higher and making a living was somewhat easier, allowing for more cultural innovations to be invented and passed around.
THE BALTIC COMES INTO VIEW from the plane's window; the Earth looks thoroughly flattened for as far as one can see. The Baltic is not very deep; it emptied out during the ice ages as sea level dropped. Only 13,000 years ago, Sweden and Norway were a walk from western Europe, as were the British Isles (if you don't count the runoff rivers!).
Poland's landscape bears a striking resemblance to Minnesota on the U.S.-Canadian border, which is also much flattened by glaciers and pock-marked by shallow lakes. Farmers love the good soil in both places, though they complain about the big rocks that the glaciers dropped as they melted back. The Cape Cod fishermen, while cursing such glacial erratics on the shallow continental shelf when they snag their nets and lobster pots, usually stop short of actually moving the rocks, in the way that farmers do when they bend their plow on one.
Getting off the plane in Copenhagen, I noticed a sign that said:
INSwhich tells me exactly where I am on the surface of this planet, with an uncertainty of only about a city block. That's the latitude and longitude of this particular gate at Kastrup Airport (we're 55° 37.6' north of the equator, and 12° 39.1' east of the Greenwich meridian), and it is posted such that the sign is the first thing that a pilot sees when looking out of the cockpit window, as the plane sits at the gate. The pilot punches those numbers into the Inertial Navigation System, so that its computer will know where the flight starts from; by sensing accelerations in the three directions and keeping track of them, a computer can calculate exactly where an airplane is at any moment by the history of the accelerations since the starting point. Punch in the wrong numbers for the starting point, and you'll wind up going to the wrong place if using the INS to run the autopilot. Knowing where you are, and where you're headed, has changed since Neandertal times.
E 012° 39.1
I indulged myself in an evening of sampling the exquisite products of Danish bakeries and breweries, then set off late the next morning in a 747, a nine hour flight to Seattle that crosses nine time zones -- so it stays midday all the way. It reminded me of a wonderful old wristwatch languishing in a drawer back home, since it was the perfect occasion to wear a watch that always stops -- for once, it would remain pretty accurate.
[E. D. Cope, the nineteenth-century paleontologist,] first enunciated what he called the "law of the unspecialized," the contention that it was not from the most highly organized and dominant forms of a given geological era that the master type of a succeeding period evolved, but that instead the dominant forms tended to arise from more lowly and generalized animals which were capable of making new adaptations, and which were not narrowly restricted to a given environment.... [But] who is to say without foreknowledge of the future which animal is specialized and which is not?
Loren Eiseley, The Immense Journey, 1957.