HOW did humans acquire intelligence? Two competing theories haveattracted a lot of attention. One holdsthat the complex social relationsamong higher primates provided akey driving force; the other assertsthat it was the complexities involvedin obtaining food. Recent evidenceindicates both pressures may havebeen equally responsible.
It is agreed that apes are muchsmarter than monkeys. RichardByrne of the University of StAndrews in Scotland attributed thisto the hurly-burly of their social life(Science, Vol 262, No 5139). Butbecause the social life of monkeyswas equally challenging, Byrne reasoned that there was something elsethat contributed to developing intelligence. Feeding complexity offereditself as an alternative when Byrnestudied the eating habits of gorillas in Rwanda.
Byrne discovered that much ofthe diet of gorillas consists of plantswith nettles, thistle spines or thickhusks protecting the edible part.Eating these goodies often involvesintricate procedures, such as peelingwithout getting hurt. Mastering thesetable manners provides a selectivepressure that, along with socialskills, could produce a more nimble mind.
Whatever the adaptive pressureson it, the primate mind does appearcapable of remarkably advanced cognition. But not beyond a certain biological limit, as psychologist Sarah Boysen of Ohio State University discovered when shetaught a number gameto two chimpanzees,Sarah and Sheba.
She offered twoplates to Sarah, onewith seven gumdropson it and the otherwith one. Sarah wassupposed to indicatewhich plate she wanted Sheba to have. Intheory, this wouldshow that Sarah couldcount and reason thather counterpart shouldget the shorter tally.But in practice, greedclouded her reason:she always pointed tothe plate with sevengumdrops. Then, whenBoysen gave the chosen plate to Sheba,Sarah got upset. "Itwas the first task in 20years I had failed toteach a chimpanzee,"says Boysen.
But there change when the gum-drops were substitutedwith plastic numberplates: the chimpwould point to thelower number. When the gumdropswere put back, the chimps again had problems.
"The chimps understood therule," Boysen says, "but theycouldn't act on it" because of the biological imperative to get more food.Byrne says that the first humans transcended that limitation and beforelong, they developed rules for foodsharing and other underpinnings of culture.