Scientists are trying to uncover the adaptive pressures that transformed primates into intelligent humans
HOW did humans acquire intelligence? Two competing theories have
attracted a lot of attention. One holds
that the complex social relations
among higher primates provided a
key driving force; the other asserts
that it was the complexities involved
in obtaining food. Recent evidence
indicates both pressures may have
been equally responsible.
It is agreed that apes are much
smarter than monkeys. Richard
Byrne of the University of St
Andrews in Scotland attributed this
to the hurly-burly of their social life
(Science, Vol 262, No 5139). But
because the social life of monkeys
was equally challenging, Byrne reasoned that there was something else
that contributed to developing intelligence. Feeding complexity offered
itself as an alternative when Byrne
studied the eating habits of gorillas in Rwanda.
Byrne discovered that much of
the diet of gorillas consists of plants
with nettles, thistle spines or thick
husks protecting the edible part.
Eating these goodies often involves
intricate procedures, such as peeling
without getting hurt. Mastering these
table manners provides a selective
pressure that, along with social
skills, could produce a more nimble mind.
Whatever the adaptive pressures
on it, the primate mind does appear
capable of remarkably advanced cognition. But not beyond a certain biological limit, as psychologist Sarah Boysen of Ohio State University discovered when she
taught a number game
to two chimpanzees,
Sarah and Sheba.
She offered two
plates to Sarah, one
with seven gumdrops
on it and the other
with one. Sarah was
supposed to indicate
which plate she wanted Sheba to have. In
theory, this would
show that Sarah could
count and reason that
her counterpart should
get the shorter tally.
But in practice, greed
clouded her reason:
she always pointed to
the plate with seven
gumdrops. Then, when
Boysen gave the chosen plate to Sheba,
Sarah got upset. "It
was the first task in 20
years I had failed to
teach a chimpanzee,"
says Boysen.
But there change when the gum-drops were substituted
with plastic number
plates: the chimp
would point to the
lower number. When the gumdrops
were put back, the chimps again had problems.
"The chimps understood the
rule," Boysen says, "but they
couldn't act on it" because of the biological imperative to get more food.
Byrne says that the first humans transcended that limitation and before
long, they developed rules for food
sharing and other underpinnings of culture.
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