|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).
The old joke goes, "But you can't get there from here!" (This was the response given by the laconic farmer to the city motorist, who was lost in the back country and asking for directions.) I'm reminded of it because of our attempts to find a certain restaurant in the hills near Lake Balaton. We asked questions -- in German, translated from American English -- of Hungarian pedestrians and eventually discovered that we had to drive back downhill to the lakeshore and then take a different road back up into the hills.
FINDING A FAST TRACK TO THE BIG BRAIN:
How Climate Pumps Up Complexity D'où venons nous?
Que sommes nous?
Où allons nous?
Where have we come from?
What are we?
Where are we going?
Paul Gauguin, 1897
But, of course, you can get there from almost anywhere (when in the company of astronomers, one has make allowances for the improbability of ever escaping from a black hole). It's just that the path may be a little roundabout, requiring a lot of detailed description about backtracking to some other junction, rather than a simple "That-a-way." Progress sometimes requires a temporary dose of regress. The evolutionary path from an apelike ancestor to Homo sapiens also requires a lot of detailed description, including backing up a few times. Indeed, if intelligence is not one of those much-sought "general principles" of the universe, the details of the path are all-important.
Yet self-congratulatory generalizations are about all we've got in the way of explanations for hominid evolution -- such as "Man the Toolmaker" or the "bigger is smarter is better" pseudoexplanation of why brains enlarged fourfold. What we need is a good idea about each leg of the journey to humanity, the opportunities and provisions along the way, the hazards and how our ancestors coped with them, plus some notion of how we avoided drifting back to where we started. And we've got a lot more to explain than mere cleverness or brain size -- for instance, ethics, art, music, compassion.
We've come a long way from the apes. There is no animal currently around that can serve as a suitable stand-in for our common ancestor with the apes. But, if you imagine the common ancestor as a composite of the chimpanzee and gorilla, you're not likely to be far off. While all ape species have surely changed as well during the last 5-10 million years, they haven't made the major transitions that separate us from our common ancestor: walking upright, concealed ovulation, elaborate language, extensive tool use, accurate throwing for hunting, and the big brain.
Attention has naturally focussed on how we evolved from an ape level of language and intelligence to that exhibited by the remaining hunter-gatherer bands of today -- and on the acquisition of a fourfold larger brain along the way. Did toolmaking drive the brain boom, as anthropologists once proposed? The usefulness of language, as the linguists propose? The psychologists are naturally in favor of intelligence as the raison d'être. And at least one neurophysiologist thinks that it is mostly due to the brain-muscle coordination needed for hunting with projectiles (although I'm not a sports fan, we neurophysiologists are fascinated with rapid movements of all kinds). If you asked a reproductive biologist, there would surely be a key role for concealed ovulation (no more estrus behaviors advertising the time of maximum fertility, promoting pair-bonding but also social cleverness). All of us could be right. Unlike the tale of the blind men and the elephant, there is more than one right answer -- because everything in biology has multiple "causes."
There may be multiple ways to be "right" but there are even more ways to be wrong. And eliminating incorrect explanations is a key way in which science progresses in many fields. As the economist Kenneth Boulding once said,I have revised some folk wisdom lately; one of my edited proverbs is "Nothing fails like success," because you do not learn anything from it. The only thing we ever learn from is failure. Success only confirms our superstitions.
For some strange reason which I do not understand at all a small subculture arose in Western Europe which legitimated failure. Science is the only subculture in which failure is legitimate.
For example, it was once thought -- quite reasonably, I might add -- that upright posture was caused by the need to "free up" the hands for toolmaking and, in addition, that a bigger brain was required for manual dexterity. Thus toolmaking should precede upright posture and the brain boom, and parallel their changes. The sequence was even embedded into popular thought by the opening scene of the movie 2001. Now, thanks to a lot of hard work in the hot sun by the paleoanthropologists and archaeologists and geologists, we know that, instead, upright posture preceded prolific toolmaking by several million years. Some fossil footprints of a bipedal hominid dated to 3.5 million years ago are virtually identical to those of present-day South American Indians that habitually go barefoot. Though the facts eliminated that hypothesis for upright posture, they unfortunately didn't explain what "caused" the posture to shift to upright.
And we now know that about 2.5 million years ago, three major trends started up, all at about the same time: prolific toolmaking, brain enlargement, and the ice ages. Was one the "cause" of the other two? I can't imagine how anything done by a hominid could have affected the ice ages (at least back then -- we surely can cause an ice age now, should byproducts of our technology increase the marine cloud cover enough). So that reduces the possibilities to:
3) iceage-to-toolmaking, and
if one ignores such other issues as language (and the null hypothesis: That they all happened independently of one another!).
Some crucial archaeological evidence is now available concerning the first possibility. While brain enlargement accelerated to achieve the fourfold mark by about 0.1 million years ago in Homo sapiens, toolmaking had a more fitful course. From about 1.5 until about 0.3 million years ago, the brain size of our ancestors doubled -- but toolmaking suffered from a lack of major developments; the Acheulean toolkit stayed about the same. So it becomes hard to argue that innovative toolmaking was what rewarded any bigger-brain variants in the genome. So eliminate #1.
That encephalization might have eventually facilitated toolmaking (#2) will surprise no one, but it seems a very slow path to me. That the ice ages might provide a stimulus to toolmaking (#3) is similarly possible but slow, and the argument is not as sound as one initially supposes. So did the ice ages drive the brain boom (#4), which secondarily facilitated toolmaking? That's still a very interesting proposition -- indeed, what the rest of this book will examine. This isn't some general evolutionary principle at work: that's because the ice ages do not seem to have affected other mammalian brains in a similar way. One has to examine this possibly unique journey in great detail, looking for connections between Pleistocene ecological opportunities and the hominid skills not shared with the apes. And while concentrating on "progress," one must remember to look around for opportunities conferred by regression as well.
THE CLOSER WE LOOK at apes in the wild, the more we notice that we share many features of their social lives. The African apes are clever, both in manipulating tools and in social manipulations of each other -- including both cooperation and deception, both aggression and peacemaking. Patiently trained to use sign language in the laboratory, apes exhibit the ability to use language with about the complexity shown by a two-year-old child -- a vocabulary of a few hundred words and simple sentences that make no demands on syntax -- though in the wild, the apes use only a few dozen exclamations and a variety of body postures and facial expressions. That shared body language and gesture is still used by humans -- when language fails (as by Americans in Hungary) or just redundantly (as by Italians in Italy). We have greatly elaborated communications with our serial-sequential languages -- but just how important might that have been for finding food, defending against raiders?
What about some of the ecological-niche candidates for shaping up modern man? More than a million years ago, hominids learned to live outside the tropics and subtropics, which means that they could cope with winter. This required some new skills. Clothing for insulation, fire-making, and shelter would take on considerable importance. Yet none would seem to require much more intelligence than our ape cousins demonstrate -- more patience, perhaps, but not much more cleverness.
The big problem with living through the winter is food: the choice becomes very restricted for a few months each year because the gathering is so thin, plants having shed their edible leaves, and the snow masks what's left for the taking. To get yourself, much less dependent offspring, through the winter usually means one of two strategies (though there are some exceptions): hoarding a surplus from the summer, or being able to eat grass, which remains nutritious through the winter, whether bundled as "hay" or dormant on the ground. The grazing animals manage to find enough grass beneath the snow, and they have evolved the teeth and digestive enzymes they need to utilize it. Humans haven't, but they have managed to eat animals that eat grass by becoming skillful enough at hunting.
Hoarding sometimes works, but usually only with hunting as a backup: sometimes the harvest is insufficient, sometimes the rats break into your grain hoard, or the wolves discover the frozen carcass and gobble up the meat on which you were relying to get through the last half of winter. Living in the temperate zones tends to suggest that hominid hunting techniques had become more reliable than those opportunistic snatch-and-grab tactics used by chimpanzees.
But what do hunting skills have to do with our more valued aspects of humanity: language, consciousness, ethics, music? Actually, thanks to the neural machinery needed for accurate throwing, quite a lot -- because all of the afore-mentioned happen to be aspects of the serial-order behaviors in which the brain's left hemisphere specializes. Perhaps, when not throwing projectiles or swinging a club, the neural sequencing machinery can be used for speaking a sentence, or planning for tomorrow, or feeling dismay when seeing a tragedy unfold (or laugh at the surprise ending, counter to your expectations). Some such neural machinery is secondarily useful for composing a melody, or playing chess, or dancing, or kicking. Spare-time use of machinery in the off-hours for secondary uses didn't start with playing games on the office computer: conversion of function is a major mechanism of evolution, described by Charles Darwin in the middle of the nineteenth century. Natural selection acting on any one serial-order skill might tend to improve all of them, just because they all utilize the same neural machinery for creating novel sequences of muscle commands.
WHICH WAY TO OUR BIG BRAIN? Given all the advantages of being smart, there is surely more than one way to become intelligent. Indeed, those who have thought about the SETI problem often observe that "Surely X would be useful." And the theorist of evolutionary intelligence can seldom rule out that route, in the manner that a physicist manages to rule out 99 percent of the theories offered to explain a phenomenon, just showing that X would also cause Y to happen -- and it hasn't, therefore rule that one out too. In biology, there are always multiple causes of everything we study, so we can't have the physicist's kind of confidence in our attempts to eliminate causative factors.
But one way to make progress in clearing away the minor causes and concentrating on the major movers is to ask, "Yes, but how fast would such a change happen?" In biology, fast tracks tend to preempt slow tracks. And sometimes, evolution happens so rapidly that only a fast track could have done it.
Our big brains are a prime example of rapid evolution: they have increased fourfold in size in a mere 2.5 million years. That's almost unbelievably fast by the standards of evolutionary biology. The rapidity itself is a clue as to the evolutionary causes of brain size increase. It tends to rule out slow tracks such as smarter-is-better, where the incremental payoff for each ten percent increase is small indeed.
So what controls evolutionary rates? Some factors are: Producing individual variations (via mutations and permutations). The severity of selection (droughts every decade). The rate of evolutionary inventions (those conversions of function, the compounding of mechanisms). The invention of a new niche, or finding an empty one, makes for boom times (of which, more later). The prevention of backsliding (via reproductive ratchets and similar stabilities). And more.
The hominid's Great Encephalization is so rapid that one tends to look for scenarios that incorporate a number of these factors. We may not have used every possible way of speeding up our brain changes, but we likely used quite a few of them.
TO SHOW HOW SELECTION shapes up populations, short of writing a textbook on all these factors, one can try telling a bedtime story about a bear. Bears are popular here in central Europe; they're "emblematic," as the anthropologists might say. My exemplar bear will henceforth be known as Mama Bear.
At the opening of Act I, we see Mama Bear and her two baby bears, gambolling about in the summer sunshine. So ends Act I (and the traditional version of the bedtime story; the following might be considered a revisionist adult version).
Act II is a year later. Mama Bear weaned the two baby bears after a half-year of suckling them. Then after putting up with them for a winter of hibernation, she kicked them out into the real world to fend for themselves. This is Mama Bear's summer for getting pregnant again; next winter she will again give birth to another pair.
Act III is another two years later, with another two half-grown bears let loose on the world. Mama Bear may do this five or ten times during her lifetime, if she stays healthy.
Unfortunately, a little arithmetic shows that this story doesn't have a happy ending. How many bears can the environment feed? Obviously, that's the average bear population. And that means, on average, only two babies per mother get to grow up and become a parent, out of the dozen or two that she produces. The maximum population level is not set by the birth rate but by the number of job slots afforded by the environmental niche occupied by bears. And that is a complicated function of food availability, suitable nesting sites, predator populations, pathogens, parasites, and such. Only in boom times does birth rate have much to do with it.
That means the average Mama Bear is raising five-to-ten times more baby bears than can possibly survive, absent, of course, miracles -- approximately what one might call it when a new niche happens to open up, either by new territory becoming available (as when the Alaskan brown bear discovered the ice-free corridor, at the same time as the Paleo-Indians) or when a new way of making a living is discovered (as when bears learned to go fishing in streams for salmon migrating upstream). For cats and dogs, the waste is even worse: say, six per litter, and five or ten litters per mother, but only enough adult food for two of them to survive. I know one cat, living in optimal circumstances, who has given birth to more than 200 offspring, a hundred times her quota. So again (at least in nature) there are a lot of animals that are going to die of starvation, becoming food for a predator.
Why does Mama Bear spend all that effort raising many times more offspring than the market will bear (no pun intended)? It's called "keeping up with the Joneses." For Mama Bear to have a gene Q for doing less would result in fewer such gene Q's in the next generation; this gene would "work itself out of business." Indeed, this situation is like an arms race; if a variant arises that can successfully raise triplets, her genes will soon take over the population, replacing the two-at-a-time genes, making triplets the new standard, and so on to even bigger litters.
If triplets don't happen now, it's probably only because each would be so stunted in size from crowding in utero as to be less competitive; three scrawny ones being worse than two of the standard twin-size. The bigger the litter, the higher the percentage of manufacturing defects -- and the less postnatal parental attention per offspring. In boom times, all offspring might survive -- and indeed both bears and humans shift to larger litters when resources are bountiful.
This is an Alice-in-Wonderland sort of principle: the Red Queen told Alice that you have to keep running just to stay in the same place. The Red Queen is one of the reasons we talk of a naturalist fallacy. "What's natural is good," is, alas, another example of substituting upbeat wishful thinking for familiarizing oneself with the available evidence. Across many species of mammals, the amount of effort devoted to reproduction bears little relationship to niche size. Nature just hasn't developed a way of limiting overproduction of baby bears (or any other species, although humans may yet become an exception), and most animals spend nearly all their life just raising youngsters with which to feed the predators and pathogens.
THIS UNHAPPY STATE OF AFFAIRS (by humane standards) provides, however, the raw material on which natural selection operates. Though we usually focus our attention on the adult population, it's really the young (after their parents cease to care for them) that are the prime objects of natural selection. The young are where the action is. The young are comparatively inexperienced in critical areas -- and there are many more of them, compared to adults.
While predators also cull the old, that's not an example of natural selection at work: whatever genes the older animals are going to get into the next generation is determined earlier in their life, so whether they die now or later isn't going to shape up the ongoing population of genes. This is why there hasn't been much natural selection against late-developing aspects of gene repertoires, such as Huntington's chorea, or cerebrovascular disease, or Alzheimer's senility, or the inability to digest milk that sometimes develops in the midthirties.
Predators also cull the sick. That, however, promotes natural selection for the immune system's capabilities. Natural selection doesn't just work through predators and food availability but also via childhood diseases: it takes years to build up immunity to the common diseases, which is why older workers have fewer respiratory infections than younger workers. In one sense, you actually become healthier as you grow older!
But in the real world of Darwinism, if an animal becomes weak from a virus, a predator eats it. If the animal becomes weak from inability to find food, a predator dispatches it. The young are comparatively inexperienced in both areas. Lacking a change in the average environment (of which, more later), the top 10-to-20 percent of the young bears survive -- which is quite a shaping-up. Some of the others make it too by accident, and some of the top ones die by being in the wrong place at the wrong time, as when struck by lightning.
Droughts make selection even more severe, but the overproduction among mammals causes a lot of severe natural selection, just as a baseline. Somehow, I doubt that more severe selection (e.g., harder winters during the ice ages) was the cause of the Great Encephalization, especially given that other animal's brains didn't similarly enlarge during the ice ages.
EVEN THE BABY BEARS that escape natural selection for a normal adult lifespan may still have a big problem. Another form of selection operates because not all get to breed.
In most animals, nearly all surviving females get to bear offspring (there are some exceptions among social insects and dog packs, where a dominant female may inhibit the reproduction of subordinate females). In many types of mating systems, however, quite a few males are locked out of propagating their genes. Harems are the most obvious example.
What determines which males get their genes into the next generation? Sometimes, brute force decides: head-to-head competitions between males for control of a harem, where male body size and armaments count heavily. In some other mating systems, there is female choice, typically for healthy-looking males. This potentially augments the tendency of natural selection to promote improvements in body styles. But health isn't always judged by something truly relevant, such as having the prospective suitors run a marathon or collect a week's food for a family. There is usually some substitute indicator of health used, some stand-in -- perhaps agility in a mating dance, or the condition of a male bird's plumage.
And this can lead to appearance mattering more than reality, with some cosmetic trimmings all-important. If shiny plumage is the criterion by which a female bird selects one male over another, you can see an arms race in plumage, such as the iridescent peacock tails. Sometimes it is feather length -- and so you see some absurdly long tails in species such as the bower birds and magpies.
So sexual selection is based, not on the elements of the natural environment such as food availability, predators, pathogens, nesting sites (those are the elements of natural selection, though it might be better called environmental selection) but on reproductive peculiarities, many of which no longer function in any reasonable way. Those absurdly long tails may impede flying abilities and those bright feathers tend to give away one's location to predators (and so sexual selection may conflict with natural selection, balancing each other out). Male gorillas are so heavy as adults that they cannot take to the trees when a predator approaches, in the manner of the adult females and adolescent males -- they have to stay and fight! One presumes that some such counterbalancing with natural selection is why sexual selection doesn't often keep proceeding to absurd lengths.
WHAT CONTROLS EVOLUTIONARY RATES, and so the length of time it takes to shape up a new feature? Most people would immediately suggest mutation rate, how fast the cosmic rays or mutagenic chemicals can introduce errors into the DNA strings. While an extra dose of radiation can indeed augment variability in offspring, gene permutation is probably the most important aspect, that shuffling of the chromosomes that takes place during crossing-over as new sperm and ova are made. From generation to generation, far more variability in offspring is created by permutations than by new mutations.
Furthermore, evolution above the one-cell level didn't really get going until crossing-over was invented by eukaryotes about one billion years ago; promptly thereafter, multicellular life developed in a big way, inventing about 50 major ways of structuring a body plan during the next half-billion years. Mutation didn't accomplish that: it was permutation. What affects the rates at which genes come to stick together, or develop new points at which to break apart during crossing-over? That's one of the unsolved problems of basic biology.
Among other factors, the reproductive arms race and its wastage must partially control the opportunity for natural selection to act on the variants thrown up by gene shuffling and mutation: everything else being equal, cats ought to evolve faster than bears because they overproduce more (their top 5 percent might survive, rather than the bear's top 10-20 percent). But fortunately we can avoid discussing the "cannon fodder" principle ("the more waste, the faster we evolve!"), because climate is the most obvious variable when it comes to fast vs. slow evolution.
The most rapid of environmental cycles are the daily ones associated with day-night and with the tides. Any planet is going to have solar tides, so long as it has oceans and doesn't keep one face always towards its gravitational attractor. If a planet has two attractors (as does ours), that's even better for speeding evolution. Thanks to the moon's tidal forces adding to (and then, for half a month, opposing) the sun's gravity, there are also monthly and yearly cycles of extreme low tides. The tides serve to select for intertidal plants and animals that can survive in a second kind of environment for longer and longer intervals -- perhaps until becoming land-dwellers.
WHILE LAKE BALATON is Hungary's largest body of water, the tides here are about as conspicuous as the Hungarian Navy. One sees only wind-driven waves of greater and lesser proportions, keeping the shoreline wet. But it reminds me of the shorelines back home in Puget Sound where the sea level varies each day over an average range of more than one story high, and so a lot of beach alternates between being underwater and being temporarily above water, drying out in the sunshine.
At some such intertidal zone of 450 million years ago, life came ashore. A species exposed to the monthly low tide series was undergoing natural selection for mechanisms that would keep it going in two different environments, both free-flowing water and up in the air. For the intertidal species, the tides provided daily waves of selection for the abilities needed to survive extreme variations in moisture, pH, salinity, oxygenation, and temperature. Had such tidal selection instead happened once per century, the fanciest land animal these days might be a floundering lungfish.
It takes a swamp-and-tide-flat zoologist to tell you about life; it is in this domain that the living suffer great extremes, it is here that the water-failures, driven to desperation, make starts in a new element. It is here that strange compromises are made and new senses are born.... [In] the mangrove swamps by the Niger, fish climb trees and ogle uneasy naturalists who try unsuccessfully to chase them back into the water. There are things still coming ashore.Loren Eiseley, The Immense Journey, 1957.
ONCE ASHORE, there are some yearly variations in environment outside the tropics -- better known as the seasons. Thanks to axial tilt and land surface in the temperate zone (mostly Northern Hemisphere these days, the tip of South America excepted), we have had yearly cycles of selection for species able to survive both summer and winter weather (most species simply stick to the tropics).
But for those who do evolve the mechanisms to endure both winter and summer extremes, there will be yearly waves of selection, operating upon that huge overproduction of the Mama Bears of the temperate zone. While not as frequent as the daily and monthly cycles of the tides, wintertime selection cycles might cause more rapid evolution in the temperate zones than in the tropics -- at least for winter-related body features. And for the behavioral traits needed in the wintertime (predation skills are particularly important in many animal species, as plant life becomes dormant).
THANKS TO CYCLES in the atmosphere-ocean system, we have multiyear cycles of drought. Somewhat understood are the monsoon variations in the Indian Ocean, and El Niño's twice-per-decade cycle in the weather systems of the Pacific Ocean. Among human populations, the families of South American fishermen are most affected -- but the bird populations of many Pacific islands crash to ten percent of their usual numbers. Recently, some of the U.S. midwestern droughts have been linked to El Niño as well.
Pathogens also have multiyear cycles, as in the shellfish population crashes. Forest fires occur every few decades and, near a shoreline or watercourse, floods occur several times each century (if not more often). Less systematic are the meteor strikes and volcanos that darken the atmosphere (though some of the more famous examples of post-volcanic cooling, such as the year-without-a-summer in 1816, may yet turn out to be unrecognized visitations of El Niño).
Those trying to live on the margins of a habitat are the hardest hit. In Europe, most traces of people who lived at low population densities have been lost by the reuse of sites by the peoples that followed. But in the New World, one can do better: many "stone age" sites have been discovered relatively intact. For example, in the U.S. Southwest, rainfall improved about A.D. 1050 and many new Anasazi villages sprung up all over the area; by 1130, they were all abandoned and even the major population centers were dwindling. So the archaeologist gets to see a window in time, largely uncontaminated by what followed. Boom and bust is common in nature, not just in economies.
THANKS TO VARIATIONS in the earth's axial tilt and the drift in the season when perihelion is reached, we've had 100,000-year-long major climatic cycles to shrink and expand the temperate zone populations. Of which, more later.
There is also lots of back and forth within each ice age, with perhaps five minor retreats in the ice sheets between each major meltback. Back and forth. So that means a major climate change about every 10,000 years (and within just the last 120,000 years, "cold spikes" have also occurred about every 6,000 years in the North Atlantic region alone).
And we know that there are centuries-long fluctuations within this as well. The Little Ice Age was a period of cooler climate between about 1200 and 1800, made even worse on occasion by some volcanic eruptions that clouded the atmosphere and cooled the weather for several years at a time.[There] are two ways in which a creature can seek to survive in a jungle environment. One way [known as wedging] is to compete fiercely and successfully for an existing niche with other creatures that are trying to occupy it. The other way is to find a wholly unoccupied niche....
Herbert A. Simon, 1983
MASS EXTINCTIONS also affect evolutionary rates, thanks to whatever happens every 28 million years (volcanos, asteroids, and meteors are possible, but some favor comets since a scheme has been suggested whereby the recurrences would cycle every 28 million years). After each mass extinction, wherever its cause turns out to be, there are opportunities for new species to fill vacated niches. This is an extreme case of boom and bust, and so it is worth examining it in more detail.
Darwin realized that evolution could be slow if efficiency was the only factor. An improving species (say, one better able to utilize a particular food) would have to wedge its way into the niche of another species already utilizing the resource. This metaphor of the wedge is very useful, but some of the major advances in evolution occur when no wedging is needed -- because of the empty niches. Some empty niches are just there, waiting to be found: There are no woodpeckers on New Guinea despite dozens of species of woodpeckers on Borneo and Sumatra in similar forests. And filled niches can be emptied, as occurs in an extinction.
In periods of rapid diversification, a whole series of empty niches are discovered -- as when some uninhabited islands are discovered, as has happened on both land and sea after the mass extinctions of 250 million years ago and 65 million years ago (those were merely the biggest extinctions; minor ones seem to happen about every 28 million years, the last one about 10-11 million years ago). After the dust clears, competition within a species is not important for awhile; nearly all the offspring get to live and reproduce, allowing for variations that would ordinarily be culled by natural selection to survive and, indeed, be elaborated in successive generations. And so body styles vary widely; when the niches start to fill up, many will be eliminated and a more standardized model will take over.
But some variants may become associated with a stable niche. If they no longer interbreed with the parent species, as often happens in such situations, then they will constitute a new species. This serves as a ratchet, preventing backsliding to some average body style. Much evolution, in the sense of change in body form, has been temporary because it did not find a new stable point from which backsliding was prevented. Speciation, i.e., inability to interbreed effectively or reproductive isolation, is the prime (though not the only) means by which stratified stability occurs.
BUT BEFORE SPECIATION OCCURS, isolation has usually happened by more temporary means. Because the environment is often patchy, there are subpopulations of a species that mostly interbreed within their group. They could interbreed between groups, given the opportunity, but they don't -- usually because of some geographical barrier such as a mountain range, or a wide expanse of territory lacking their kind of food resources. And so the members of a species are often found in dozens of relatively independent colonies: call the effectively interbreeding group a deme if you like, though population or subpopulation or colony will also suffice.
When natural selection has been episodically severe, a population may be completely eliminated and the territory's resources may go untapped until straying members of another population discover it -- and have themselves a population explosion, founding a new deme.
Consider, however, what happens in less severe population crashes. Anyone who regularly visits a natural setting from year to year, as I do at the Friday Harbor Labs's nature preserve in the San Juan Islands, must be impressed with the yearly fluctuations in the number of common species such as deer and rabbit. Some years, there will be a dozen rabbits outside the front door of the main labs, mowing the lawn; other years, you're lucky to see two rabbits when walking around the entire several acres of lab buildings and housing units (and the maintenance personnel have to crank up the lawnmowers that year). Some years, there will be a dozen deer grazing amidst the buildings; other years, none are seen for days at a time. Some years, the raccoons are particularly prominent.
These fluctuations are not due to variations in food resources, nor nesting sites. Some are due to hard winters. But most, I suspect, are due to diseases that nearly decimate the island's population within several years. The survivors are the few whose genes allowed their immune systems to cope with the infection. Some of their offspring will inherit those genes, and so the population may start rebuilding even in the continued presence of the virus.
Over a century, a population will be exposed to quite a few similar episodes. If the population is modest in size, its genes may be severely edited by such episodes, and not just those for the immune system. Those whose constitutions are particularly robust will survive illness better, and so the plague years (as we would call them, were this a human population) also shape the genome in the direction of a specialty in this island's environment: its climate, its food resources, etc. Illness, by weakening the individual, sharpens the importance of that island's natural environment. Populations on other islands may be shaped somewhat differently, both by chance and by the differing environment there.
Boom and bust cycles edit small island populations far faster than they would a mainland population. You can see why a Mama Bear that produced triplets rather than twins would spread her triplet genes around rapidly, given the occasions where an island's bear population had been depleted by a virus. On the mainland, the adjacent populations unaffected by an epidemic could later move into a depleted area, the merely lucky competing with the offspring of the survivors who had "earned" the chance. Large mainland populations buffer rapid change -- and incidentally help insure against the loss of valuable genes, as can easily happen in a small island population.
Back in the millennia long past, when humans were sparsely located over the surface of the earth, each tribe was effectively on an island. Now evolution is slowed down to a crawl, simply because there is little isolation anymore. A large population evolves slowly in comparison to a small deme.
AGRICULTURE GREATLY REDUCED the influence of natural selection on most humans (while we still die, the evolutionary issue is who dies when). "Being better" now shapes up the human gene pool rather slowly. And that's probably been true on most continents for the last few millennia, perhaps most of the period since the continental ice sheets melted about 12,000 years ago. Which is not to say that evolution ("change") won't occur in the future, only that traditional darwinian processes will perhaps play a minor role in guiding it, compared to biotechnology and such environmental novelties as air pollution.
So the combination of transportation (allowing different demes to intermarry more easily, which can have the advantage of hybrid vigor) and agriculture (serving, along with sanitation and medicine, to allow most variants born to survive and reproduce, for yet another round of variation) give rise to a conclusion about future human evolution that is very different from the Social Darwinism views of the late nineteenth century and the eugenics views early in this century. The eugenicists (not, interestingly, most geneticists) thought that artificial selection was important to improve human bloodlines, preventing the mentally ill and the epileptic from reproducing, and encouraging marriages between those possessing favored traits.
Given the successes of animal breeding, eugenics was in many ways a perfectly reasonable hypothesis, given how little was known at the time. I think few people realized how slowly things worked -- absent that Mama Bear scale of wastage, absent new niches to explore. Incorporating this early scientific speculation into German nationalistic slogans says more about the need for morale-building than it does about science.
By the time that the Nazi government got hold of the eugenics idea (or at least its vocabulary), many geneticists and evolutionary biologists had backed off from any support of the eugenics movement. Scientifically, concepts were changing. Geographic isolation leading to reproductive isolation and speciation, etc., are insights from the thirties and forties, particularly from mathematical types such as Sewell Wright, and the early Forties saw the emergence of the Modern Synthesis of darwinism with genetics. It took a while for this news to spread. The major German societies of physical anthropologists indeed collaborated with the Nazi program of racial hygiene. And in the United States, awareness was not markedly better: Even in 1939, the American Association of Physical Anthropologists tabled a proposed resolution condemning Nazi racial myths (as the 1989 president of that association, Matt Cartmill, has recently noted in a sobering book review).
Part of the problem was that news travels surprisingly slowly between different disciplines -- in this case, evolutionary biology and anthropology. Even today, I am surprised at how little anthropologists concerned with hominid evolution seem to know about evolutionary biology of the kinds summarized thus far; their major concepts such as the savannah and scavenging are seldom evaluated in terms of anything other than slow adaptations, Darwin's original valuable insight. Fortunately, ignoring fast tracks and isolation opportunities (or developmental linkage, or compounding, or conversions of function) has none of the potential for societal misuse, at least when compared to transplanting animal breeding considerations into the human sphere.
Ignorance doesn't merely slow science down: ignorance also leads to mistakes. One of the seldom-realized benefits of science has been what knowledge has allowed us to avoid: the quack remedies and their tendency to delay effective action until too late, the buildings that collapse from trial-and-error construction methods, the invasion of pests because of having ignorantly killed off a natural predator. If history is any guide, our changing concept of human origins will enable us to avoid some of the problems in education and health care occasioned by our ignorance of how we humans function.
EVOLUTIONARY CHANGE is not only more rapid in small groups, but it is more likely to become "permanent" in a temporarily isolated setting. That is to say, something may happen that makes interbreeding less likely, even when the geographic barrier is removed (as when an ice age's lower sea level reconnects all of the islands in the San Juan Archipelago with one another).
Sometimes this is a chromosomal rearrangement in an island's population; mating between individuals with the rearranged and regular chromosomal patterns may have a high rate of spontaneous abortion or, if there are offspring, they may be sterile and so fail to propagate the lineage. Other times, there is simply less of a tendency to mate with a member of an out-group: behavior can effectively keep descendants of an island's population, now expanded over a broader landscape, from mating except with one another. The classic example is when mating seasons have shifted: on a mountainous island with late-melting snow, the mating season may have become a month later than usual, as those variants had offspring which survived better. And so that island's population, when mixed up with the general population by lowered sea level, still tend to mate with their original group for millennia thereafter, simply because the two groups are never sexually receptive at the same time.
Such prevention of backsliding might be called a reproductive ratchet. While mere "attractiveness" of physical appearance contributes to this tendency, culture tends to augment it: Erik Erikson noted the "excessive" amount of human energy that preliterate peoples spent in simply being different from one another. He calls it "pseudospeciation."
Reproductive isolation that is truly persistent is another way of saying that a new species has been formed: the traits shaped up by that island have to stick together, because they are simply unable to mix with those of the parent population. Various traits may have altered in an island's population that have nothing to do with reproduction -- but they too will be protected against the dilution caused by mixing with the main population. A reproductive ratchet speeds evolution.
SPECIES TEND TO DIVERGE a bit after reproductive isolation occurs because of the Exclusion Principle: one species per niche. If two species tend to make their living in exactly the same way, are subject to about the same viruses and parasites, and so forth, then after enough time has passed, one of those species will probably decline in numbers and eventually go extinct in the area where the two species' ranges overlap. They won't be equally adept at utilizing the resources: one species will be more efficient.
There may, of course, also be some antagonism between such closely related species: if you see two species peacefully coexisting, they are probably not competing with one another for resources or nesting spots, etc. The antagonism speeds up the decline of one species, but efficiency is the most fundamental cause of such an extinction. This competition between species, certainly the usual nonbiologist's image of natural selection at work, is, however, fairly infrequent; most competition is within a species, involving things such as superior child-rearing.
Animals with a broad ecological niche, such as monkeys that can efficiently eat many different kinds of fruit but at different seasons, may so exclude more species than a narrow specialist like the bamboo-eating panda does. And humans have one of the broadest ecological niches of all, so it is not surprising that we have few close relatives left (and all five of the ape species are now endangered by human activities). We have created a wide swath of exclusion, and can only lessen the damage by substantial conservation efforts.
When bigger-brained species of prehumans formed on some island (real or virtual), they were probably capable of making their living in new ways or of exploiting former resources more efficiently, e.g., through food preparation technologies. If they were reproductively isolated from their parent species, they would tend to take over in local regions after a few millennia, even without antagonism between the two groups.
WHEN THE ICE AGES came along, the lowering sea levels caused a lot of islands to be reconnected with each other. And melting later caused new islands to form -- not only literal islands, but some virtual islands as well: those mountain-top "islands" in the tropics to which the mountain gorilla is limited (by its need for 60 pounds a day!) tend to shrink and expand with the climatic changes seen even in the tropics when the ocean currents rearranged themselves.
And then, of course, there are the geographic barriers created by the ice sheets themselves. As Ernst Mayr notes, they are likely to induce speciation:During the height of the glaciation, the ranges of many temperate-zone species contracted into small pockets, so-called glacial refuges, which persisted south of the area of glaciation. In Europe, for instance, the Alpine and northern ice caps approached each other to within 300 miles, separated by icy wind-swept steppes. The forest animals retreated into southwestern or southeastern Europe. When conditions improved at the end of glaciation and the populations in the refuges expanded northward, the isolates in southwestern and southeastern Europe had, in many cases, become sufficiently distinct from each other to form hybrid zones in central Europe.
Animals that could survive in the steppes were on small "islands" indeed. But the animals that needed forest were pushed toward the Iberian Peninsula and Greece, effectively divided by the Alps from opportunities to interbreed. These "islands" were much larger than the ones between the ice sheets, but apparently still small enough for speciation to occur. "Pleistocene forest" refugia also occurred in the Americas to the south of the ice sheets: mountains in Arizona, for example, were a refuge for temperate-zone species forced south from Canada.
In fact, Hungary is right near the boundary between the group of birds associated with the Iberian refuge and the group associated with the Balkan refuge. The experienced bird-watchers among us could probably find some hybrids right around Lake Balaton, crosses between Iberian and Balkan peninsular species, living evidence of the icy wedge that disappeared 12,000 years ago.
SO A SPEEDY SCENARIO for hominid encephalization would likely be set in the temperate zone, where every year the winter speeds up natural selection. The temperate zone would provide exposure to the ice age's tendencies to create islands on which evolutionary change is faster -- and incidentally enhance speciation opportunities and reduce backsliding.
The ice ages also provide a lot of empty niches to fill, simply because they are forever changing the landscape. Near a glacier, only grasses grow. Farther back, forests get started. Birds find them quickly, but mammals take a little longer. Each species that comes upon a big uninhabited area enjoys a population boom. Sometimes an "adaptive radiation" occurs, diverse forms arising while the competitive rules are inoperative. The big brain is expensive (not just in terms of blood supply, but apparently in terms of a long vulnerable childhood), and it might have taken a profound dose of "good times" to allow it to develop, some major new resource becoming attainable (such as being able to eat meat every day, rather than once a month).
We tend to emphasize the conditions in Africa when talking of hominid evolution. The older fossils are found there and adaptationist theory tends to emphasize local adaptations to local environments (rather than carryovers from more distant former environments). One of the minor points that this book has to make is that ice ages cause temperate zone traits to become far more important than one might initially think. Except near a few mountains, cold weather plays little role in Africa; during an ice age, while equatorial regions may house glaciers high on volcanos, most species can easily escape natural selection for cold climate by retreating to a lower elevation, an option not always available in the temperate zone (especially Europe, where southern retreat is often limited by the Mediterranean).
Hominids spread out of Africa, on the present evidence, about 1.4 million years ago and, as more digs are conducted in the temperate zone, the dates will likely become even earlier. Indeed, we are faced with the probability that after such a date, the African models of hominid may have been developed elsewhere: that some of their features were shaped up in the temperate zone, and later spread back into Africa. That would mean that such "African" hominids had some temperate zone specializations that weren't really essential in the tropics. And the big brain may be one of them.
I JUST LAUGHED, remembering the time when I was teased about "walking on water." It was in the bottom of the Grand Canyon, rafting on the Colorado River. Trying to pull the boats into the beach near Phantom Ranch, we encountered a large sandbar offshore -- basically a standing wave of sand, providing a narrow ridgeline just under the waters' surface but rapidly falling off into over-your-head depths. And so I finally got out of the boat, stood ankle-deep atop the sandbar, and pulled the boat down to where there was a break in the sandbar. Then I sloshed back up the sandbar to help a second boat that was stuck. One of its occupants couldn't resist: "Say, Doc, while you're at it, suppose you could change this canteen of water into some wine?"
This is surely standard repartee among even devoutly religious fishermen, repeated many times over the centuries. And so I couldn't help wondering, when I visited the Sea of Galilee, if there were sandbars. One winter day, I sat there on the southern shore near its outlet into the River Jordan, eating a picnic lunch while facing into a cool on-shore breeze, looking out over the large lake with the snow-covered uplands of the Golan Heights as its backdrop, remembering the 1.4 million year old traces of Homo erectus found a few kilometers to the south in the Jordan Valley, between the Galilee and the Dead Sea -- right on the path out of the bottleneck from Africa into Asia.
And I remember contemplating the shallow bottom and the wind-driven waves that often produce sandbars. Walking across sandbars is, when you know your way around, a shortcut that saves much time over the long circuitous route along the shoreline. A hidden standing wave, no less. If visitors don't know about the submerged sandbar, it must look pretty strange.
In paleoanthropology, one concentrates on the stones and bones, hoping that they will yield some clues to function, what was serving to shape up the new species. But functions have shortcuts too, hidden structure that supports a new way of making a living.
We have learned all the answers, all the answers:
It is the question that we do not know.
At the moment we are an ignorant species, flummoxed by the puzzles of who we are, where we came from, and what we are for. It is a gamble to bet on science for moving ahead, but it is, in my view, the only game in town.