Sometimes animals try out a novel combination of search image and movement during play and find a use for it later. Many animals are playful only as juveniles; being an adult is a serious business (they have all those young mouths to feed). Having a long juvenile period, as apes and humans do, surely aids intelligence. A long life further promotes versatility by affording more opportunities to discover new behaviors.
A social life also gives individuals the chance to mimic the useful discoveries of others. Researchers have seen a troop of monkeys in Japan copy one inventive females techniques for washing sand off food. Moreover, a social life is full of interpersonal problems to solve, such as those created by pecking orders, that go well beyond the usual environmental challenges to survival and reproduction.
Yet versatility is not always a virtue, and more of it is not always better. As frequent airline travelers know, passengers who have only carry-on bags can get all the available taxicabs while those burdened by three suitcases await their luggage. On the other hand, if the weather is so unpredictable that everyone has to travel with clothing ranging from swimsuits to Arctic parkas, the jack of all trades has an advantage over the master of one. And so it is with behavioral versatility and brain size.
When chimpanzees in Uganda arrive at a grove of fruit trees, they often discover that the efficient local monkeys are already speedily stripping the trees of edible fruit. The chimps can turn to termite fishing or perhaps catch a monkey and eat it, but in practice their population is severely limited by that competition, despite a brain twice the size of their specialist rivals.
Whether versatility is advantageous depends on the time- scales: for both the modern traveler and the evolving ape, it is how fast the weather changes and how long the trip lasts. Paleoclimatologists have discovered that many parts of the earth suffer sudden climate changes, as abrupt in onset as a decade-long drought but lasting for centuries. A climatic ip that eliminated fruit trees would be disastrous for many monkey species. It would hurt the more omnivorous animals, too, but they could make do with other foods, and eventually they would enjoy a population boom when the food crunch ended and few of their competitors remained.
Coping with Climate Change
ALTHOUGH AFRICA was cooling and drying as upright posture was becoming established four million to six million years ago, brain size did not change much. The fourfold expansion of the hominid brain did not start until the ice ages began, 2.5 million years ago. Ice cores from Greenland show frequent abrupt cooling episodes superimposed on the more stately rhythms of ice advance and retreat. Entire forests disappeared within several decades because of drastic drops in temperature and rainfall. The warm rains returned even more abruptly a few centuries later.
The evolution of anatomic adaptations in the hominids could not have kept pace with these abrupt climate changes, which would have occurred within the lifetime of single individuals. Still, these environmental uctuations could have promoted the incremental accumulation of mental abilities that conferred greater behavioral exibility.
One of the additions during the ice ages was the capacity for human language. In most of us, the brain area critical to language is located just above our left ear. Monkeys lack this left lateral language area: their vocalizations (and simple emotional utterances in humans) employ a more primitive language area near the corpus callosum, the band of fibers connecting the cerebral hemispheres.
Language is the most defining feature of human intelligence: without syntax--the orderly arrangement of verbal ideas--we would be little more clever than a chimpanzee. For a glimpse of life without syntax, we can look to the case of Joseph, an 11-year-old deaf boy. Because he could not hear spoken language and had never been exposed to uent sign language, Joseph did not have the chance to learn syntax during the critical years of early childhood.