Given two candidates, both capable of getting here from there, one is likely to be faster. This doesn=t mean that the other is pre-empted (they=re not really alternatives); it can surely continue to play a modifying (and perhaps stabilizing) role as the faster one sprints along, both of them modifying brain structures as improved functions pay off. But the faster is likely to have caused more profound alterations in the brain circuitry B and perhaps in brain size.

I especially like the Bickertonian scheme for evolving syntax from a nonlanguage foundation in the social calculus of reciprocal altruism. That=s because altruism has such an extensive growth curve, and I=m always judging evolutionary advantages by their potential for repeated growth. So many evolutionary inventions are Aone shots@: once you invent a digging stick or a carrying bag, you can=t just keep reinventing it for additional credit. You have to do something different for your next act. Even inventions with growth curves will often plateau: aquatic mammals, for example, find they can swim faster by reducing their body hair. This reduction can be repeated for further advantage, but there=s a limit, as you can only become so naked. Most evolutionary Good Tricks do not have long growth curves.

A long growth curve is one of the nice features of accurate throwing: no matter where on the growth curve you currently are, throwing twice as far or twice as fast will improve the payoff still further (more high-calorie, low-toxicity meat is usually a dietary improvement). So one of the things that I like about cooperation is that it too, at whatever stage you=re at, keeps being advantageous in so many circumstances. Doubling the size of the sharing group always has another payoff: eventually one could get such social organization as systematic sharing with the disadvantaged, a system of laws to minimize conflict, and chancy joint endeavors with an agreed upon sharing of any yields.

Being able to repeat the course for additional credit is an important consideration given that, in an evolutionary system where there are multiple plausible Acauses,@ the fast track may be the important one, even if the slow ones might have also succeeded given enough time. There are many plausible candidates for what enlarged our brains, elaborated our social behaviors, and gave us structured language and thinking; the issue may come down to which was fastest B or which had the best Acurb cut@ potential for secondary uses.

Furthermore, extensive altruism is on the short list of major inventions since our last common ancestor with the chimps and bonobos, five million years ago B another major improvement that needs an evolutionary explanation. In my opinion, the big beyond-the-chimps developments are:

The two major preadaptations we=ve been discussing, planning for accurate throwing and role categories for altruism and argument structure, both strike me as being good fast track candidates because they=re capable of being re-used in a further round of improvements. I have no idea which was faster, but I think that it will prove useful to compare either of them to the many other candidates for what was important in hominid evolution that lack long growth curves.

Furthermore, both elaborated sharing and accurate throwing are likely to have been under strong selection during the ecosystem crashes that occurred every several thousand years during the ice ages when the climate abruptly cooled or warmed. These repeated events were not the ice sheets themselves, let me hasten to say. Rather, they were the fastest of the many types of climate instability that the planet has suffered in the last three million years or so.

Let me summarize briefly, using the introduction that I gave at Bellagio when doing my stint on the after-dinner circuit. Each resident usually gives a brief show-and-tell in the music room or upstairs in the conference room, some time during his or her month at the Villa Serbelloni, on some past or current project. Derek did syntax, so I did the Aphysics for poets@ version of the abrupt climate change story that I wrote for The Atlantic Monthly.

Ever since the damming up of the original Panama Canal about three million years ago, when North and South America finally drifted together, the Earth=s climate has been unstable. There had been a tropical path for the ocean currents between the Atlantic and Pacific Oceans but, with the rise of Panama, there=s now a long loop of ocean currents between the North Atlantic and the southern oceans. Sometimes this loop (it=s actually more like a conveyor belt, going north on top and going south near the ocean bottom) shifts into an alternate mode of operation, with drastic climate consequences.

The most vulnerable part of the current path is when it turns around in the North Atlantic Ocean, via diving from the surface to the ocean bottom in the whirlpools of the Greenland and Labrador Seas, and then heading south. When this turnaround shifts to occur well to the south of Iceland, things change elsewhere too. Our ancestors saw such an occasion as a catastrophic cooling, even if they lived in Africa rather than Europe. The cooling wasn=t necessarily frigid, especially in the tropics B the new daytime temperatures were more like the nighttime temperatures before the shift, with the new nighttime temperatures shifting to proportionately cooler. The real problem with the climate change was how abrupt it was: just a matter of a decade or two.

Coolings like these are the equivalent of jacking up the landscape a thousand meters in elevation. Even the plants on the valley floors are unhappy, not only because of the cooler temperature extremes but because of less rainfall. Plant species growing up on the mountain sides would be suitable for the valley floors, but there isn=t time for them to get there before lightning strikes set the dry forests ablaze. The landscape in many places must have soon looked like those pictures of Brazil and Southeast Asia we saw during the 1997-98 El Niño, where lightning and forest-clearing fires got out of control and burned vast areas. Familiar ways of making a living vanish, even in the tropics.

If this global cooling had taken 500 years to happen, it would have been difficult but not a catastrophe, as the mix of plants and animals would have gradually changed to emphasize the new climate conditions. Each generation could have made their living in the manner their parents taught them. But instead of ramping down to the new temperature, it stepped down (so say the isotopes and air bubbles in the ice cores from Greenland, which preserve a tree-ring-like series of layers from the last ice age). With major changes in just a few years, innovation was the only way to survive. Hominid populations must have crashed B and, in doing so, fragmented into many isolated groups.

As the population fragmented, some of the subpopulations turned out to possess a majority of what, beforehand, were minority traits. It was just the Aluck of the draw.@ These days, we are surprised when the court clerk draws a jury with a majority of a minority group, given that the jury pool had only 20 percent of the minority group. Similarly, out of a jury pool with the usual 10 percent left-handers, you can sometimes randomly draw a jury, most of whom are left-handers. Some of the hominid subpopulations, just as improbably, found themselves with a majority of food sharers, or with an unusual number of individuals with a passion for throwing things.

For a few generations, the only plentiful food was likely grass. If you couldn=t eat grass directly, there was a premium on being able to eat animals that predigested the grass for you. And some groups would also, by the luck of the draw, have had more altruistic tendencies than others. Those groups assembled by chance would have had, in such circumstances, an interesting advantage: they would have wasted a lot less time arguing over a food discovery and spent more time looking for additional ones after sharing it out. A better-fed group might survive the crash whereas the groups that fought constantly did not.

Though serious objections have been raised over the years to both hunting and reciprocal altruism scenarios in hominid evolution, the objections may not apply with any force during the abrupt decade-long population bottlenecks , simply because of the chance sorting and selective group survival that occurs in bottleneck times. Though some of the changes might be lost as selection pressures relax with the return of less transitional ecosystems, the abrupt disruption repeats a hundred generations later to surprise a culture that has forgotten the survival tricks of the previous episode. Still, some will possess the epigenetic traits that helped their ancestors to survive similar conditions: the tendencies to share, or to practice aimed throwing.

It all suggests an interesting scenario. Just imagine starting with chimpanzee-like tendencies to share meat and fling branches.

Such Arelaxation@ after a selection pressure is lessened is a standard feature of evolutionary theory, but this one comes with a climate instability that repeats the stress, pumping it back up ever further, concentrating the already concentrated even more. This gets around the well-known weakness of group selection scenarios, that they backslide via cheaters B it=s like a leaky tire, all right, but you can keep pumping to maintain a minimum level, thanks to repeated fragmentation and recovery. Furthermore, there is something of a ratchet that counteracts backsliding: meat is a prized food among the chimpanzees (and likely our common ancestor), and the social prestige of the successful hunter likely provided more reproduction via sexual selection.

Group selection happened, not because of groups competing like football teams but because most subpopulations starved, done in by the abrupt environmental change. Groups that had only average collections of traits likely perished. Only those groups lucky enough to wind up with Athe right stuff@ got through the terrible times.

Such fragmented bottlenecks, where lots of experiments occurred in parallel, likely happened many hundreds of times as brains grew larger. Any one episode only had to change things a little bit, because of this pump. Several dozen abrupt coolings have happened since our species, Homo sapiens, arrived on the scene, and they shocked people just like us: suddenly, the name of the game changed, and the way your parents taught you to make a living no longer sufficed, making it imperative to discover new ways within just a few years.

There wasn=t much change in our brain size with the episodes during the most recent ice age, however, probably because something even more important had happened in the prior ice age that give our ancestors even better mental tools with which to get through the abrupt hard times: it was Dennett=s slingshot. My guess is that structured thinking became much easier because of the coming together of the preadaptations, thanks to enough improvement in corticocortical coherence. We not only began to grow our culture with the aid of a truly versatile language, but we could use the same structured mental abilities to plan ahead and to reason more logically. Success at hunting and cooperation yielded the limelight to the higher intellectual functions that we so prize: syntax, planning, logic, games, and music. Together, they made possible that combination of foresight and altruism that we know as ethics.

For a glimpse of what happens to the higher intellectual functions without syntax, consider the case of Joseph, an 11-year-old deaf boy. Because he could not hear spoken language and had never been exposed to fluent sign language, Joseph did not have the opportunity to learn syntax during the critical years of early childhood. As Oliver Sacks described him:

Joseph saw, distinguished, categorized, used; he had no problems with perceptual categorization or generalization, but he could not, it seemed, go much beyond this, hold abstract ideas in mind, reflect, play, plan. He seemed completely literal B unable to juggle images or hypotheses or possibilities, unable to enter an imaginative or figurative realm . . . . He seemed, like an animal, or an infant, to be stuck in the present, to be confined to literal and immediate perception, though made aware of this by a consciousness that no infant could have.