W. H. Calvin's THE ASCENT OF MIND (Chapter 6)
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A book by
William H. Calvin
The Ascent of Mind (Bantam 1990) is my book on the ice ages and how human intelligence evolved; the "throwing theory" is one aspect.
   My Scientific American article, "The emergence of intelligence," (October 1994) also discusses ice-age evolution of intelligence. Also see Wallace S. Broecker, "Massive iceberg discharges as triggers for global climate change," Nature 372:421-424 (1 December 1994) and his "Chaotic Climate" Scientific American article (November 1995 issue).
AVAILABILITY is challenging.
Many libraries have it (try the OCLC on-line listing), but otherwise it’s strictly used bookstores (and German and Dutch translations).
The Ascent of Mind
Ice Age Climates and
the Evolution of Intelligence

Copyright ©1990 by William H. Calvin.

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


Growing Up in a Boom Time

But the chief cause of our natural unwillingness to admit that one species has given birth to other... is that we are always slow in admitting any great change of which we do not see the intermediate steps.... The mind cannot possibly grasp the full meaning of the term of a hundred million years; it cannot add up and perceive the full effects of many slight variations, accumulated during an almost infinite number of generations.... It is so easy to hide our ignorance under such expressions as the "plan of creation," "unity of design," &c., and to think that we give an explanation when we only restate a fact.

Charles Darwin, The Origin of Species, 1859

The next several chapters offer a specific set of processes that, in my estimate, might have sufficed to transform an ape into a human. In other words, I'm fairly sure that on some arbitrary earthlike planet somewhere else, they could pump up brain size and intelligence of an ape into something vaguely human. And that they could do it quickly, on the usual evolutionary time scale.
      It seems unlikely that they will turn out to be exactly how it happened here on earth, but they provide a target to aim at. When we finally understand human evolution in some detail, I feel sure that such processes will be involved, among others.
      In the Neandertal chapter was a proposal (that expandable periphery) for how to make a minority into a majority, over and over again. But it doesn't, by itself, explain why only prehumans seem to have experienced a lot of change during the ice ages. It leaves hanging the question: Why not the polar bears? Why not the other apes?
      At Mount Rainier, I consider some of the developmental processes (most known to every observant parent) that must be modified by evolution. Given boom-and-bust cycles in climate and aided by a culture able to support ever-more-helpless infants, they'll serve to enlarge an ape brain nicely. But it leaves unanswered the question: What about the reorganization of that enlarged brain for our serial-order skills? Size isn't everything (in fact, it's mostly bad news).
      And so, up on Whidbey Island, I consider an evolutionary cycle of three phases. This cycle seems to be capable of many repetitions to increment brain size; its parts include natural selection balancing acts (one of which involves serial-order skills, one of which has been recently eliminated by cesarean sections), new niches, and modifications of developmental rates. Its sterling virtue is that it doesn't seem to wear out! This re-entrant cycle may well be uniquely human, not experienced by any other species.
      Then, in the San Juan Islands, I consider the conversions of function our brains have performed in order to acquire plan-ahead consciousness and logic-and-language rules. The concluding chapter at Friday Harbor addresses the next obvious step in that evolution, which has probably become essential for our civilization's survival: Improved think-ahead.

HIDDEN IN OUR SUBCONSCIOUS are some instructions for how to behave in an ice age, a suite of behaviors (what used to be called a "racial memory") for times past. Different environments tend to bring out variations on past developmental programs so as to construct somewhat different bodies and brains, depending on the environment. This can change competition and cooperation, reproductive behaviors such as becoming more or less fussy about selecting mates, as well as height and weight norms.
      Genes are always used in combinations, usually a small subset of all the available genes. As each individual matures, the combinations may change (a "gene repertoire," as when fetal hemoglobin is superseded by adult hemoglobin). In modern repertory theaters, some actors are only rarely seen and others are ubiquitous, depending on what plays are being performed in a given theater season. Furthermore, play selection depends on the environment.
      So too with selecting a suitable gene repertoire while growing up in the new climate. There is likely a gene repertoire, also used in various warming periods of the past, that promotes a lot of parental corner-cutting in boom times, e.g., doubling up by having more twins, shortening birth spacing, perhaps pushing adolescents out of the nest earlier. Will this boom-time repertoire "speak to us" as the Greenhouse Effect upsets our climate? Or has it already spoken?
      Are such changing repertoires part of the way that prehuman evolution occurred in the ice ages? If we are intelligent enough to cope with this latest climatic challenge, will our brains again evolve? Will human mental abilities, such as our much-valued consciousness, be shaped up "higher"?

TO SAY THE ICE AGES shaped human consciousness is usually meant in a metaphorical way (those consciousness-raising awareness connotations of the word). It's another way of saying that our outlook on the world is changeable in ways that were surely "useful" in our hunter-gatherer days: changes, perhaps, in our aggressiveness, our risk-taking to exploit fleeting opportunities, our tendency to promote the interests of our immediate small group (and frequent inability to think beyond that), in our competitive attitudes toward other omnivores (such as bears), and in our predatory attitudes toward herd animals (back before horses became "pets").
      Consciousness of the less metaphorical sort must have a genetic basis, being the outcome of a variety of developmental programs orchestrated by the genome. But something so general is unlikely to be resident in some particular stretch of DNA code. We can only understand the evolution of consciousness, I suspect, by understanding the details: the details of how animal planning-ahead is carried out, the details of how really precise judgements and movements are crafted, the details of the developmental programs that shape the neural machinery, the details of the regulatory genes that influence those developmental programs. And, of course, how the gene repertoires are prompted during life: selection must choose among the available variants in the developmental programs, those "plays" that make adults out of fetuses. Those modified programs in turn have to be crafted out of new gene combinations, and remembered by the genes that survive. The genetic basis of consciousness (at least, our more-than-the-apes augmentation thereof) will lie in how our developmental programs differ from those of the apes, how those genes were shaped up by the successive environments that our ancestors conquered.
      We can begin to see a combination of regulatory genes -- those controlling the rate at which the body grows, and some others controlling how soon sexual maturity develops -- that might have something to do with the neural machinery that is so handy for planning ahead. And I can suggest how the environments might have shaped them up.

MOUNT RAINIER is like no other mountain you've ever seen or are likely to see, because it looks about twice as big. It stands alone, a symmetric breast-shaped mountain, its rounded white glaciers and smooth snowfields a proud contrast to the surrounding dark green forests and light blue sky. And it is tall -- 4,405 meters, 14,410 feet, as high as a 1,200-story skyscraper.
      But, apropos discussions of brain size, Mount Rainier serves as a lesson in how it is relative size that counts. Standing near Puget Sound only 50 kilometers (30 miles) away, you see it from sea level, not from some mile-high plateau, which is the way that you might see Mount Whitney in California or some of the Colorado peaks that are technically farther above sea level (by less than 0.5 percent). Mount Rainier looks larger, thanks to its isolation and the low elevation of Puget Sound viewpoints.
      It can be quite startling, when driving around Puget Sound, to turn a corner and see this improbable volcano suddenly framed in a scenic vista. Sooner or later, you keep driving toward it.

THE LAST OF THE SNOW is melting at Eunice Lake. It is situated about halfway up Mount Rainier, just below tree line, nestled into the second range of minor mountains around the base of that giant volcano. On the hike up from Mowich Lake, one gets wonderful views of Mount Rainier -- close enough to be huge, far enough back to see the whole top half of the mountain. Eunice Lake is a delicate alpine setting, flanked by meadows full of wildflowers and a few patches of late-melting snow in August, but in September it has autumn color from all the huckleberry bushes. The growing season isn't long, hereabouts.
      Wherever you see an avalanche track, it is emblazoned with crimson. And frequented by hikers (not to mention the occasional bear), looking for the sweet blue berries that grow among the red leaves. Behind the lake is Tolmie Peak, which has colonnades of basalt, rather like the Grand Canyon's tapestries of hexagonal columns at Mile 185 of The River that Flows Uphill. Atop one is the fire lookout, and sharp pinnacles extend along the skyline.

THE CLIFFS ECHO, and one of the echos I hear is "lizard...lizard.. in the lake...lake." Come now, lizards live on land, not in lakes. Guess again, whoever you are.
      Then I hear another little girl's voice saying "It's a giant tadpole...pole...pole." Ah, I bet I know what they've found. My zoologist wife discovered them on one of our earlier trips.
      I too look into the lake, in the clear shallow water. It's downright distracting, the reflections of the autumn color on the lake's slightly rippled surface: pointillist patches of yellow, green, russet, and dark red. It is like an Impressionist painting, except slight breezes ripple the smooth surface, "smearing the paint" here and there. "Dynamic impressionism," we should call it. Both sound and light play tricks on you, hereabouts.
      Finally I get a clear view of some rather improbable things swimming around near the shore. The largest are almost as big as a rolled-up newspaper, but they have four stubby legs. Looking singularly useless, though I suppose that they can "dog-paddle" when the occasion demands.
      It's an axolotl, surely the most unusual form of the salamander. The Mexicans consider them quite a delicacy and European scientists have been worrying about them ever since the French explorers of the nineteenth century brought back a few dozen from Mexico.
      The reason for all the attention paid to them is that, as the little girl accurately observed, they look like giant tadpoles: axolotls appear permanently youthful. That certainly got the Europeans' attention: Mud puddles as the fountain of youth? A way of backing up, reversing ageing? (No, but it does demonstrate where the analogy breaks down, between "how species evolve" and "how individuals grow up".)
      The typical salamander goes through an aquatic tadpole stage, then crawls out on all fours when its pond starts drying up, loses its gills during metamorphosis, and lives thereafter on land as an adult salamander, returning to the water's edge briefly to lay eggs. But the genus Ambystoma is versatile, having acquired during the course of evolution one way of adjusting rapidly to an erratic climate: if the weather forecast is better for life underwater compared to life foraging on land, the tadpole stays in the water, retaining its gills. It does this by using early sexual maturity to lop off the gill-less stage of its developmental program; because sexual maturity may slow down somatic development to a crawl, early puberty can truncate development before implementing undesirable features.
      The Mount Rainier axolotls are relatively big; they are probably members of the several genera of salamanders that are permanently larval in morphology, called the perennibrachiate. They always keep their gills, no maybe about it. Since their legs don't have to support the body weight, thanks to the buoyancy of water, these "juvenilized" newts can grow to be larger than the land-dwelling version, which has to haul all of its weight around. From a version that backs up on special occasions, the perennibrachiate have become a permanently backed-up version.

[Ambystoma's] a giant newt who rears in swampy waters,
As other newts are wont to do, a lot of fishy daughters:
These Axolotls, having gills, pursue a life aquatic,
But, when they should transform to newts, are naughty and erratic.
They change upon compulsion, if the water grows too foul,
For then they have to use their lungs, and go ashore to prowl:

But when a lake's attractive, nicely aired, and full of food,
They cling to youth perpetual, and rear a tadpole brood.
And newts Perennibrachiate have gone from bad to worse:
They think aquatic life is bliss, terrestrial a curse.
They do not even contemplate a change to suit the weather,
But live as tadpoles, breed as tadpoles, tadpoles altogether!

Walter Garstang, 1951

ICE AGE CLIMATES THAT SWITCH back and forth illustrate one reason why "backing up" might be a good thing: suppress the current genes for big bodies when the climate warms up, and go back to the old set of genes that emphasized rapid reproduction instead. Just don't throw away the genes for big bodies as maybe they'll be needed again, many thousands of generations later when the unstable climate cools.
      The easy way to back up is simply to abbreviate growing up, stopping before ever implementing a presently undesirable feature. If adults have added-on features that juveniles don't (big fangs, for example, in baboons), then an easy way to get rid of them is to stop growing before ever reaching adulthood. Since sexual maturity tends to slow down body development, early puberty may indefinitely postpone them (though if you live long enough, even slow development during adulthood might eventually implement them). Because there is a lot of heritable variation in the age when puberty strikes, any environmental advantage to the more childlike adults will tend to shift the average age of puberty to earlier years as the centuries pass.
      Early puberty thus provides a way of backing up, should the overspecialized features of adulthood prove awkward. While fangs aren't such a big disadvantage, there are contrasting psychological characteristics of juveniles and adults that probably are important. Juveniles play around a lot, thus discovering new ways of doing things. When one is an adult, however, one had best be a good provider and good defender, rather than playing half the time. Adult monkeys are remarkably slow to learn about new foods compared to those juveniles that are always fiddling around. Given a change in climate, the early maturing variants might survive better, simply because they remained more childlike and thus more open to new ways of doing things.
      And, as I noted while surveying the Neandertal country of Czechoslovakia, there are also reproductive contests that reward early maturity with more such "genes for juvenil- ization" in the next generation. The boom times that follow droughts encourage such competitions, and the ice ages institutionalized them in a big way. There is a race to fill up the newly available "job slots" afforded by an environment able to feed more mouths.
      One of the fisheries problems in Puget Sound is that the salmon have been experiencing early puberty: instead of taking a few years to mature, some males mature in only one year, and so can be grandfathers before the standard-maturity males become fathers. Such precocious males remain small -- and so are more likely to be eaten by predators, presumably one reason why mature males aren't always so small (though, given that fishermen are not allowed to keep undersized fish, human cultural practices may eventually reward salmon genes that promote small size!).
      Besides this truncation of the juvenile period, another aspect of growing up quickly has been studied in monkeys: more rapid development during the juvenile period itself. Those baboons that happen to live near the tourist camps in East Africa grow up faster; the baboon troops have discovered the garbage dumps, which are considerably more reliable than the usual baboon food sources. The anthropologists Clifford Jolly and Jane Phillips-Conroy compared baboons in the wild with those in breeding colonies, showing that the captive ones consistently erupt their teeth earlier than the wild juveniles.
      Jolly cautions against interpreting this solely on the basis of veterinary care and plentiful food (captive primates are usually fed ad libitum, while wild ones suffer the ups and downs of the fluctuating availability as the seasons change and the mercurial climate creates scarcity). He suggests that we might want to view this in terms of ultimate causes rather than proximate causes: is there something built into evolutionary processes that might speed up development under some conditions, slow it down under others?
      Indeed many animals adjust their reproductive tactics to environmental conditions -- and not only the present conditions, such as whether one's stomach is full: some animals "forecast" future conditions and "plan" accordingly. The snowy owl, if its hormonal mechanisms judge that it is going to be a good year for lemmings, will lay a lot of eggs. If the forecast for lemmings is poor, the snowy owl's hypothalamus causes it to lay only several eggs, as lemmings are the favored food for feeding the chicks once they hatch. The snowy owl looks ahead (probably, I suspect, by observing the same environmental clues that the lemmings base their reproductive decisions on, one of which is similar to the Groundhog Day story), and so doesn't produce a lot of expensive eggs whose inhabitants will just starve to death later in the season.
      This short-term tactic isn't a violation of the Mama Bear strategy of "keeping up with the Joneses"; the snowy owl still tries to produce as many offspring as possible during her lifetime, but conserves resources so there will be several well-fed offspring rather than a half-dozen weak ones which may all die. Less is more, when the forecast is poor.

AN EPIDEMIC OF EARLY MATURITY in humans has certainly been happening in the last century. Girls used to start having menstrual periods when they were 16-17 years old, but now menarche has nose-dived closer to 12 or even 11 years of age. It can happen within a single generation.
      If delaying reproduction is a common response to a forecast featuring substandard prospects, then perhaps speeding up reproduction is a response to a promising forecast -- and one way to have more offspring while good conditions last is to hurry up the start of your reproductive period, to "rush the season." Shortened generation time is the more obvious aspect: you can become a grandparent in the time that your neighbor takes to become a parent. Should there be a number of new unoccupied job slots to fill, your descendants will get more of them. Assume that, thanks to booming conditions, everyone gets to produce four offspring that survive to maturity: the neighbor ("Standard Mom") gets four offspring into the next generation, but Fast Mom's rushing the season yields sixteen in the same time (assuming the characteristic is heritable). Early maturity also allows more pregnancies in a mother's lifetime, at least if the standard lifetime isn't also shortened by the speedup: the speedy mothers might have six surviving offspring, each of whom also produces six -- and so Fast Mom has thirty-six grandchildren in the same time as Standard Mom's four children. In short order, the population characteristics are skewed toward the speedy ones. Lacking the boom time expansion, the population characteristics change little.
      But why not always have a longer reproductive span, e.g., routinely start childbearing at age 12 rather than 18? At least in mammals, early pregnancies tend to be associated with more problems, such as low birth weight. If there isn't going to be enough extra food for a gestating fetus, the prospective mother is better off putting on some weight herself, getting better prepared, rather than building a baby. She will need to be able to nourish the baby off her own fat supply, should famine strike.
      All of this seems to be part of the evolutionary diversity that we call the r-K spectrum. It concerns parental strategies: r and K are just somewhat abstract variables in an equation used by population ecologists (I'd rename it the q-Q spectrum, since the emphasis shifts from quantity to Quality). Animals that lay great quantities of eggs (such as mosquitos) but invest nothing in caring for them or raising the juveniles are called r-selected; just remember "lay them and leave them" if you prefer. Other animals (primates are good examples) have relatively few single (not twin or triplet) pregnancies, carefully gestate the fetus during early development and nurture it after birth, often for years. They are called K-selected, mostly because such a strategy is associated with conditions where the species is operating close to the carrying capacity of the environment: they are exploiting the resources about as fully as possible, and so the extra advantages associated with quality become important. The ultimate version of K-selection so far (though it is due to cultural evolution, not biological) is when parents put their offspring through not only college but postgraduate education as well.
      Some K-selected animals, however, will vary their strategy as the climate fluctuates: let the prospects improve and they will r-shift, not all the way to the mosquito's lay-them-and-leave-them strategy but certainly cutting a lot of corners, producing extra offspring because it looks as if there will be room for them. When times are good (or when they can predict that the climate is improving), they go on a reproductive binge.
      They may start having more twins, despite the hazards of crowding in utero on development (the pediatricians say that twins have a harder time, both during gestation and afterwards). Sheep ranchers have discovered that they can increase the number of twins born, just by feeding the ewes a high-calorie diet for a few weeks before mating season. Both ovaries let loose an ovum: the ewe fires with both barrels into the uterus.
      In addition to doubling up, parents may devote less care to each offspring in other ways as well, as the offspring may be able to manage on their own with such improving environmental conditions. Instead such parents have as many offspring as they can by starting early and repeating just as quickly as possible. When prospects turn sour, they may K-shift back toward the more conservative strategy of sinking one's bets on a few well-placed shots. When your species is already exploiting the environment near the limits of its carrying capacity (which includes food availability but also nesting sites and such), play it safe by waiting until you are better prepared, then raise only a few offspring and devote a lot of care to them.
      If this also applies to humans, then two questions immediately arise: How is the "boom time" r-shift implemented? (Is sexual maturity sped up, or is juvenile growth rate, or perhaps both?) And what triggers it, what aspects of the environment are "read" for the forecast? If we are ever to replace this corner-cutting "Quantity is Better than Quality" philosophy of nature and effectively combat its fatalistic "Life is Cheap" corollary, we need to understand what drives it (the "hangover" that follows a reproductive "binge" is better known as a population crash).
      What's natural isn't always good, as David Hume pointed out two centuries ago, but the Pope still holds to the "Naturalist Fallacy." One wonders how many other "natural" reproductive behaviors the church would care to endorse?

Every man is to be respected as an absolute end in himself; and it is a crime against the dignity that belongs to him as a human being, to use him as a means for some external purpose.

Immanuel Kant, Metaphysics of Morals

BEHIND EUNICE LAKE IS TOLMIE PEAK, and there is a fire lookout tower atop it, just two stories high, enough to look over the tops of the stubby trees growing around it. Tolmie Peak rises several hundred meters above Eunice, shadowing the lake from the north winds of winter. From the top of Tolmie, there is a better view of Mount Rainier -- and of the Olympic Mountains to the west facing the Cascades across Puget Sound. One sees the other Washington State volcanos to the north, even Mount Baker up on the Canadian border. Looking south just to the right of Mount Rainier, I can see a much-diminished Mount St. Helens, plus even more volcanic peaks across the Columbia River into Oregon. There's nothing quite like seeing a long-familiar mountain, now missing its top, to make you realize that the natural world can change abruptly.
      This string of volcanos in the Northwest, mostly in a band about the same distance in from the coastline, is due to what happens to the bottom of the Pacific Ocean as it is recycled. The ocean floor slowly creeps east from where it is formed in the mid-Pacific upwellings. When it reaches the west coast of North America about 100 million years later, it dives under the continent to rejoin the molten magma of the Earth's core. This "subduction" process is associated with volcanos of the type we have in the Northwest, sitting atop the zone where the ocean floor is folded back into the hot interior, a series of escape valves for the excess steam pressure.
      If islands are carried along with the sea floor conveyor system, they may be appended to the west coast rather than subducted down to Hades: all of North America west of the Rocky Mountain chain seems to be a hodgepodge of different rocks from different places (the firm granite in the Cascade Mountains is very different than the unreliable granite of the Olympics, as mountain climbers around here soon learn). Of course, adding coastal mountains poking up into the clouds attracts lots more rainfall to the coast while producing a "rain shadow" inland. The islands that are subducted probably cause giant earthquakes hereabouts, as they snag and then pop free. The ceaseless motion of the sea floor and the continents means that plants and animals are constantly having to adapt to changing conditions.
      Tolmie Peak also has a hummingbird, performing the usual disappearing act -- now you see it, now you don't. Hummingbirds haven't yet made an evolutionary adaptation to the false alarms caused by the bright jackets favored by hikers, and usually come over to inspect the big flower. Bees make the same mistake; to keep them from swarming around my head, I once had to take off a bright neck scarf and throw it aside. They followed it. It makes me worry that we humans have such senseless attractions too, following things for reasons we don't understand. And following to excess some of the "natural" attractors we do understand, such as sugars and fats: my colleague David Barash points out a number of supernormal releasers in The Hare and the Tortoise and Annie Dillard discusses supernormal attractors in The Writing Life (not to be confused with the strange attractors of chaos!).

WHAT ASPECT OF THE ENVIRONMENT is "read" to predict boom times ahead? In the days before market surveys, shoe manufacturers contemplating expanding their factory probably used secondary indicators such as the strength of the baby-carriage business. Since even owls can forecast, r-shifting is surely an unconscious business; owls don't go out and take a market survey, a census of owls and lemmings, divide, apply a safety factor, and reset their factory. If it is like other things biological, r-shifting probably operates on secondary indicators such as light and humidity and social crowding.
      But speeding up juvenile growth rates suggests that even children might be "judging" the future market for babies. What might they be judging? Much has been made of the deleterious effects of overcrowding on reproduction (e.g., aborted pregnancies) -- but a lesser degree of crowding might work the other way. Cliff Jolly suggests that the number of other children -- how many playmates a child has to choose among -- could serve as a secondary indicator that the resources were improving, enough so that people can successfully live at somewhat higher population densities. These are mechanisms below the level of consciousness that nonetheless have the same effects as conscious ones -- such as "keeping up with the Joneses" in terms of family size, e.g., the Israelis worry about the neighboring Arabs outnumbering them and encourage large families. Economic conditions are the more familiar birthrate determinants in industrialized societies; what we are here concerned with are pre-economic life-styles, and what might lead to corner-cutting.
      More babies encouraging even more babies sounds like a positive feedback loop which would become unstable, given the aggression that comes with overcrowding. We might not design such a system ourselves, knowing about control systems and harmful oscillations, but maybe our reproductive system wasn't well designed.
      Another possibility for the mechanism of early human maturity is that lighting conditions could be serving as an indicator. The length of daylight is one cue used by birds in determining how many eggs to lay; for example, the European robin raises a clutch of three or four at Mediterranean latitudes, but a clutch of six in Sweden where the summer days are longer (since robins forage for food only during the day, the amount that they can collect during a working day determines how many offspring they can raise). Lighting affects growth rate too: farmers have long known that one way to speed up growth in farm animals is to leave the barn lights on at night. Is there a human equivalent of this? (Many humans now tend to rely on artificial lighting during the evening, getting 16 hours per day of light, year-round).

ACCELERATED MATURITY isn't merely a speedup in the time scale: to the extent that the maturation of the reproductive organs gets ahead of the speedup of general somatic development, you get a juvenilization of adult body styles. Since puberty tends to send signals that slow down somatic development, such individuals tend to be smaller than average (rather like those precocious salmon, though the axolotls demonstrate how other factors may cause the juvenilized version to be larger instead). For example, girls with early menarche are typically (though not always) short. Some tall girls have gotten in a few years more growth before puberty slowed body growth.
      Facial characteristics are also likely to be affected, since the lower face and jaw get in a lot of late growth when not terminated by early puberty. Though the brain itself is about full-grown by age seven, the skull adds thickness and the sinuses fill out in later childhood; one might expect early puberty to affect them too. General robustness is likely affected, once the more gracile adolescent build becomes the new adult standard.
      There is a lot of hidden change too: the brain size may change little after age seven, but much still happens internally. If not already myelinated, some axons will gain insulation during later childhood and adolescence. And synapses are being edited all through childhood, the elimination rate decelerating at puberty. Juvenile brains have many more interconnections between nerve cells than adults; a third to a half of all cortical synapses are lost during childhood, the peak occurring about eight months after birth and it's all downhill thereafter. Should those extra connections be crucial for some task (I suggest that they're very useful for throwing), early puberty might help out, saving some synapses from being disconnected.

IF r-K STRATEGIES regulate growing up, then surely they are involved in the making of a new species. What modifications make us grow up to be humans, rather than chimpanzees? Might some of the evolutionary changes in the hominid lineage be explained by early maturation too?
      Juvenilization has apparently played a large role in the evolution of humans from the apes, just as it played a large role in the evolution of apes from Old World monkeys, just as it played a large role in evolving the vertebrates from the invertebrates known as ascidians. Backing up from overspecialization, then evolving some new specialization, backing up a bit from that, and striking out in a new direction once more -- we've done a lot of that, and at major turning points in our evolutionary history. The French have a phase, reculer pour mieux sauter ("step back to leap better"), that epitomizes a crucial evolutionary principle.
      This does not mean, of course, that humans are merely infant monkeys, or that the whole human genome was present in the ascidian, just waiting for repeated juvenilizations to come along (keeping us all from finally growing up to be sand dollars!). Animal development isn't one-dimensional (like our usual train of reasoning).

ADULT DOMESTIC ANIMALS are paedomorphic (literally, "child-shaped"), exhibiting a lot of juvenile characteristics: flatter faces, smaller teeth, and more behavioral plasticity than their wild predecessors. While most of this is probably the early puberty of a boom time (being fed regularly), there might also be some selective survival: there is a lot of natural variation in the time of sexual maturity, and humans could have selected those wild animals that were more juvenilized. Since juvenile animals solicit attention, juvenilized adults are the ones more likely to hang out around humans and get fed. But that's probably not the route used for human juvenilization.
      Human juvenilized appearance ("neoteny") might, of course, have occurred because of natural selection for its usefulness. In particular, sexual selection might have done it: the novelist David Brin suggests that prehuman females competed for the attentions of those variant males most likely to be nurturing of their offspring (in our African ape relatives, males have no special relationship with their own offspring). And as part of this competition, juvenile-appearing females preferentially attracted the nurturing males; together they made a good team, getting more offspring to maturity because of two adults to share the work load. And so juvenilized appearance per se could have been under natural selection in females (that's where it is most pronounced), and not just a hauled-along secondary consequence of boom-time opportunism or some other advantage of juvenilization (of which, more later).
      But consider the boom-time argument a little further. Just within the species span of Homo sapiens -- and almost-modern-looking people seem to extend back through most of the last ice age to the last interglaciation 120,000 years ago -- there have been a number of changes in the "carrying capacity" of the human environment (think of it as one of those "maximum occupancy" signs erected by the fire marshall). First came the ice age itself, but that changes so slowly it would be hard to detect an improving trend. Even during the rapid meltback phase, the average rate of uncovering new land was only 0.4 percent per century. The rapid warmings (such as the Allerød event 13,000 years ago, or the end of the Younger Dryas at 10,720 years ago) might, however, have caused a few generations to experience rapid change.
      But boom times aren't caused only by changes in the natural environment. There have been some technological improvements during the last ice age that have markedly changed human prospects. As each was introduced, it might have permitted boom times until the new niche filled up.

      ** First was probably the invention of big-game hunting. The attachment of spear points to shafts goes back to the penultimate ice age. Following herds of big game around and exploiting them for meat is probably what carried Homo sapiens into every reach of the temperate zone. Loren Eiseley used to say that meat provided the energy that took man around the world. While opportunistically eating meat is very old, eating big game regularly may have been one of the innovations of the more recent ice ages. Boats were also invented sometime during the last glaciation, greatly improving the prospects of coastal peoples by allowing offshore fishing.
      ** Second was cooking, which allows many kinds of flora and fauna to be eaten that cannot be consumed raw in any quantities. That too seems to date back to within the last glaciation, not as an initial invention but as a widespread and improving technology that left behind more and more charcoal and burnt hearthstones for archaeologists to find.
      ** And food preparation got fancier with the introduction of baskets and especially pottery, again expanding the range of foodstuffs that could be exploited.
      ** Finally, with the meltoff 12,000 years ago, we start to get agriculture -- which, combined with all the improvements that followed, is said to have increased the human population a thousandfold.

With the adoption of each of these technologies, it would be just as if the hunter-gatherers' environment had improved: just as more rain brings better foraging, so an improving food technology may have repeatedly stimulated an r-shift in human ontogeny (not to mention economic inflation!). It depends on what is actually sensed unconsciously by growing children, but it would not be surprising to find boom-time reproductive strategies coming to the fore on each of those occasions when technology expanded the food that could be captured, prepared, and digested.

TOOTH SIZE REDUCTION has been a big puzzle to the physical anthropologists: during the last ice age, as I mentioned earlier, human tooth size slowly and steadily decreased, late ice age teeth being ten percent smaller than the standard earlier in the last ice age. C. Loring Brace says that a steady decline occurred between 60,000 years ago and the great melt-off 10,000 years ago -- but thereafter the reduction sped up even more, perhaps due to the agricultural technologies that developed. This worldwide decline in tooth size became more pronounced in each region as pottery was introduced. The domestication of animals also reduced their tooth size (just compare your dog's dentition to a wolf's!).
      Thus the anthropologists are asking: Are small teeth an adaptive response (Are large teeth a disadvantage? Are smaller teeth better for something?) or the result of the removal of selective pressures that were keeping average tooth size large (maybe the rougher food from before cooking and fancier food preparation was selecting against those variants with smaller teeth -- but with easier foods, the tooth size returned to its inborn value). I would suggest yet a third alternative: maybe small teeth are just another manifestation of more rapid maturity, that the boom-time juvenilization has struck again! Juveniles have smaller teeth, just as they have flatter faces, and larger brain/body ratios.
      Note that early maturity is comprised of several processes: somatic growth rates (as seen in tooth eruption speedups upon domestication of wild animals) and also the speedup in sexual maturity. You can have one acceleration without the other, e.g., a speedup in sexual maturity without a speedup in somatic growth (the medical use of the term precocious puberty tends to refer to six-year-olds with pubic hair, but I am talking about down-shifting the adolescent growth spurt by several years). The two rates are regulated by somewhat different hormonal systems, melatonin from the pineal being important for sexual maturity and growth hormone from the pituitary being important for somatic growth, among many others. But some hormonal systems affect both somatic and sexual developmental rates, as in the common effect of testosterone on muscle building and maturation of secondary sexual characteristics (and, of course, testosterone effects the primary sexual differentiation of the month-old fetus -- the prime reason why potentially-pregnant women should never use muscle-building steroids that can mimic testosterone's actions, as masculinization of a female fetus can result even before pregnancy is detected).
      The relative speedup of sexual development, compared to somatic, is the common cause of juvenilized appearance (at least, in the days before eye-enhancing cosmetics and for-appearances-only dieting!). If both sexual and somatic rates were equally accelerated, presumably the adult form wouldn't change. But smaller teeth, low birth weight, twinning, and less robust long bone development suggests that corners are being cut -- and that sexual development might have sped up even more than somatic. The situation is not unlike a boom-time economy where quality control slips: there is so much demand for the product that no one is being choosy about the product's durability.
      In addition to the progressive reduction in tooth size during the last ice age, has there been an increasing flattening of the face? Is it all due to one boom time after another (big game, cooking, pottery-related food preparation -- and then agriculture)? Even the ice-age meltback itself is potentially a boom-time event, and there have been lots of those during the 2.5-million-year period of encephalization. Could boom-time physiology alone have pumped up brain size, simply through the reproductive contests during expansionistic times?
      Or has natural selection also worked on one or another aspect of early maturity as well? Perhaps all those synapses that are being eliminated during late childhood turn out to be handy for something in the context of the hunter-gatherer life styles, so that early puberty serves to conserve them. That's been my proposal for what happened even earlier, in the 2.4 million years preceding the last ice age and modern-style Homo sapiens, though it may not apply to the most recent glaciation. Repeated juvenilizations may serve to enlarge relative brain size, but they won't necessarily reorganize the brain at the same time, to give us those serial-order skills that we have in such great abundance.

[A woman must store a minimum] amount of body fat in order to begin and maintain normal menstrual cycles. Activities that reduce fat below the threshold, such as serious dieting and intensive exercise, can delay the age of menarche... to as late as 20 years. Such a loss can also "silently" halt ovulation... in someone who menstruates every month.... [This] helped to explain [why] American girls now begin to menstruate when they are 12.6 years old; a century ago the age was 15.5 years [and that is about when athletic girls now begin their periods]. [Roger] Revelle and I postulate that the earlier menarche is explained by the fact that children now become bigger sooner because they are better nourished and have less average disease.
the reproductive biologist Rose E. Frisch, 1988

HIKING BACK DOWN THE TRAIL, I almost ran into a young deer on the trail -- we both jumped in surprise, and the doe leaped nimbly uphill and then eyed me while munching on the greenery again. It looks like one of this year's, almost grown, trying to find enough food to get it through its first winter. A mild winter will mean a boom time for deer next year, because winter and wolves are what keep the deer population from expanding exponentially.
      Body fat certainly makes a big difference in when teenaged girls start to ovulate. That raises a lot of questions: How much of that accelerated ovulation is inheritance, how much can be manipulated by diet and exercise? And, of course, how much does early ovulation affect general body development thereafter -- especially the brain? We've always thought of the prenatal period as the particularly important period for shaping brain development, but cortical pruning and labile menarche together raise the possibility that the early adolescent period might also be an important one for biasing adult brains one way or another.
      I got to thinking more about human boom times, trying to summarize it all in my head. Culturally, I know what is meant: people save less, borrow more, open more risky small businesses, cut corners on quality because demand is so high that buyers aren't fussy, and generally become less concerned about the future. We get inflation, a bullish stock market, and a party atmosphere -- at least in comparison to the downside of the business cycle.
      That's the psychology of boom times. Not much is really known about boom-time physiology, especially in humans. We do know that the apes are very K-selected (perhaps too much so) because of their long birth spacing (nearly five years and no twins). Humans are near the K-extreme: we have relatively few offspring and devote a lot of postnatal care to each one, but still manage to reduce birth spacing to four years (down to two years in agricultural societies, with bottle feeding reducing it even further). And we manage twins on occasion, something apes do not.
      But what might shifting away from this average position involve? Assume for the moment that this r-shifting is due to "selfish" genes, doing their usual behind-the-scenes manipulation in an effort to produce more grandchildren carrying the gene -- use that for a moment rather than our usual humane concerns about infant mortality and over- population. To the selfish gene, the name of the game is not simply a matter of having more children than average -- if they die in childhood at a higher-than-average rate because of lack of parental attention or not enough food to go around, one will not end up with more grandchildren than average.
      Since half of all children do not survive childhood (except in certain parts of the modern world), the calculating gene has to, in effect, weigh the potential of a child dying before reproducing itself -- and compare it to the chances of getting by on less parental care, while the parent tries raising yet another infant. Mothers who simply have additional children do so at some risk to the existing children; the statistics show that all children in a big family grow up (gain height and weight) more slowly, perhaps because of not as much food per child, perhaps from a more complicated effect of the r-shift mechanisms.
      In an improving environment, the calculating gene might well decide to decrease birth spacing: if the environment is easy, maybe the existing children will make it anyway while Mom is busy with another pregnancy and another infant. One way of implementing such a hurry-up-while-the-getting's-good strategy is to stop breast-feeding early, so as to build up maternal body fat once again and thereby resume monthly ovulation. Or the selfish genes might decide to try for twins or triplets, a somewhat risky business because of in utero crowding and less individual attention while the children are growing up.
      And a calculating gene in a child's body might make reproductive decisions too, speeding or slowing somatic development and sexual development. The number of offspring per mother is also a function of her reproductive span, starting several years after puberty and ending with menopause. Maximizing one's offspring is not simply a matter of becoming pregnant as young as possible, as babies born to young mothers have low birth weight and higher mortality; the new mother is more inexperienced than if she had watched child-rearing techniques a little longer before trying it herself.
      Even selfish genes cannot take a "So what if I lose the first one?" attitude toward a teenage pregnancy. The mother loses something permanently by trying early: Since early menarche tends to cut short the mother's growth, she won't have the bodily resources to devote to her later offspring that she would otherwise have by waiting until the usual year to become fertile. But if the environment was obviously improving, a child's selfish genes wanting to maximize her number of grandchildren might "decide" to reach puberty sooner and let the babies take their chances.
      Selfish genes being what they are, any tendency to shift reproductive strategies toward exploiting a boom-time environment would result in more such genes in the subsequent generation (provided the gamble paid off), compared to genes reflecting a static strategy that maintains parental investment per offspring regardless of the environmental quality. Both groups will get more grandchildren since the booming environment will somewhat reduce childhood mortality, but the shifting-strategy group could easily get twice as many grandchildren before the next downturn -- and so increase their proportion in the population with each boom-and-bust cycle.

SOME HUMAN MOTHERS consciously choose to have a dozen children. But if a related strategy (rushing the season via early puberty and growth speedups) can be implemented even by children as they grow up, perhaps we'd better rethink our assumptions and search for a more widespread "cause" than many independent rational choices. What is it about the booming environment that is sensed for the boom-time cue by the selfish genes? What hormonal systems implement the shift?
      Implementation first -- maybe it will provide a clue as to what is being sensed to trigger it. Let me start by simply making a list of all those possible ways of cutting corners to increase grandchild production rates. For the mother, it could involve early weaning and encouraging early mating by her offspring via sexually permissive advice. She could also cut corners by double ovulations or other means of having twins. For the child mother-to-be, it would involve "rushing the season," hurrying up sexual maturity so as to get in several extra pregnancies.
      Now mothers are not usually thought to be able to dictate the characteristics of offspring, but if selfish genes are at work using indirect means, what else might increase the number of grandchildren? Shifting the sex ratio toward more males than females -- since females are almost guaranteed a minimum number of offspring and so are the conservative strategy, males (who can in effect have multiple pregnancies going at the same time in different mothers) are a risky investment that pays off best in boom times.
      Inherited personality traits might shift too, not just reproductive traits. For example, about 15 percent of modern infants fall into the extremes of very shy and very bold, when exploring a novel situation. Might r-shifting increase the number of bold infants, so as to better exploit a more benign environment? In rats, there is even a brain structure difference between the shy and bold rats: the very timid ones have markedly larger right cerebral hemispheres compared to their left sides. The very bold rats tend to have somewhat larger left hemispheres; the average rat (like the average human) has a somewhat larger right hemisphere. Is this heritable? Have humans followed this pattern during boom times of the past? Have human hemispheric asymmetries changed in the last century, concomitant with the earlier puberty -- might r-shifting be affecting fetal brain development? Between the physical anthropologists studying old graveyards that construction crews accidentally discover, and the neuroradiologists studying modern human brains with the various imaging techniques, we might actually be able to answer such questions.
      Social behaviors might shift too. Males might shift toward even more of a "love them and leave them" strategy to spread their genes around as widely as possible, abetted by a relaxation of the usual female tendency to be choosy about selecting a good provider. If maximizing grandchildren is the name of the gene-propagation game, and not the quality of the grandchildren, these are all ways of cutting corners to increase production rate. But, at least for humans, they're not "fate."

      Our learning biases and emotional responses... are not random or manufactured from thin air; they are the products of the unbroken process of evolution by natural selection that extends across the whole of history, into our prehuman past, and millions of years before that. This is why even a seemingly "purely cultural" phenomenon, such as an arms race, may be most effectively dealt with from a perspective that includes a thorough understanding of our history of natural selection.... Moths fly to their deaths around electric lights; this maladaptive response to an environmental novelty is understandable... only by knowing the nocturnal behavior of moths prior to the introduction of electric lights.
Richard D. Alexander, 1987

SOCIAL COMMENTATORS have been remarking on developments in our culture that involve quantity rather than quality of offspring, though I think that we must be careful in establishing links, lest the overenthusiastic inflict another eugenics monster on us.
      I have a hard time believing that all such changes can be accounted for by a hidden biological strategy like r-shifting -- some are surely just cultural phenomena or selective recall about the "good old days" instead -- but culture isn't going to account for everything on the boom-time corner-cutting list. Boom-time r-shifts is a hypothesis that needs testing if we are to get control of the situation and encourage quality as the preferred strategy. The human cultural version of K-shifting is "quality, not quantity" -- and that's very different from mere birth control. If we wait for biology to do the K-shifting for us, there will likely be a dramatic crash in human population that accompanies it -- potentially including Neandertal-like problems, a return of the life that is "nasty, brutish, and short."
      What price success? Heretofore, we have seen it as a big boom-and-bust cycle, our ice age genes remaking our world into an overcrowded, famine-ridden place, likely to be followed by a population crash. People often discount dire predictions, believing that (as often in the past) some new invention will come along and change the whole situation. But the price of ignoring boom-time shifts in human reproductive strategies may also be some undesirable shifts in human characteristics during the boom itself: corner-cutting on quality. Whatever the pleasures of becoming the first 28-year-old grandparent on your block, they may not outweigh the immediate losses associated with the decrease in parental competence and attention per child.
      Our ice-age selfish genes may not have been concerned with the quality of life, but they did provide us with an amazing ability, never seen before in evolution, to look ahead and correct our course.

[Jorge Luis Borges' Labyrinths] exploits the paradox of making the future present through foreknowledge. As the god simultaneously sees the world's fate and possibly deflects it, so do our perceptions of the inevitable and the possible sometimes alter the very soil from which the future springs. This is one of the reasons I have argued so vehemently against the doctrine of biological fate. Beliefs may fulfill themselves not by virtue of their truth but by virtue of their fixity, and we are only too ready to disavow responsibility for what we perceive as biologically imposed.

the ethologist Susan Oyama, 1985

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