DURHAM, N.C. -- Some primates have evolved big brains because their
extra brainpower helps them live and reproduce longer, an advantage
that outweighs the demands of extra years of growth and development
they spend reaching adulthood, anthropologists from Duke University and
the University of Zurich have concluded in a new study.
investigators compared key benchmarks in the development of 28
different primate species, ranging from humans living free of modern
trappings in South American jungles to lemurs living in wild settings
"This research focused specifically on the balance between the costs
and benefits of growing a large brain," said Nancy Barrickman, a
graduate student in Duke's Department of Biological Anthropology and
Anatomy, who is first and corresponding author of a report now posted
online for a future print edition of the Journal of Human Evolution.
rates are much slower in large-brained organisms, and that causes a
delay in reproduction," Barrickman said. "If individuals wait too long
to reach maturity then they run the risk of dying before they've had
the chance to reproduce. So there must be some benefit to large brain
size at the same time these costs are incurred.
"Is larger brain
size causing life histories to become extended and slowed down? We
think so," Barrickman added. "That obviously fits in very well with
humans, who take forever to grow up and live a really long time. So we
have the opportunity to have lots of offspring over that long period."
drew these conclusions working with Carel van Schaik, a Duke adjunct
professor on her doctoral studies committee who directs the University
of Zurich's Anthropological Institute and Museum. Other coauthors
include Duke graduate student Meredith Bastian, and Karin Isler, a
collaborator of van Schaik's in Switzerland.
"Our main finding
is that brain size is a far better predictor of the duration of
immaturity than body size, at least among primates," said van Schaik.
"This study is also useful because it allows us to understand why
humans develop so slowly and live so long -- we have no other choice!"
studies have linked primate brain size to life span and other factors,
but those results have been contradictory, according to the new report.
Previous studies were "polluted" by mixing data on captive and wild
animals, van Schaik said. "Because development and survival are highly
responsive to conditions, this variability made it impossible to do
Their study was supported by the scientific
research society Sigma Xi, the American Museum of Natural History and
the Ruggles Gates Fund for Biological Anthropology in the United
Barrickman and her colleagues focused on primates
living in the wild because "animals tend to grow up faster in
captivity," she said. In the case of humans, they studied the Ache, a
tropical forest culture in eastern Paraguay.
"Their food is
exclusively wild food they forage from the forest," she said of the
Ache. "And they don't have other things like modern birth control
methods that you'd find in an industrial population like ours. My
argument is that we're basically captive primates by comparison."
analyzing available data on life history benchmarks such as length of
pregnancy, years from birth to maturity, pre- and post-natal brain
development and lifespan, the researchers found that humans and other
big-brained species such as chimpanzees share certain survival traits.
takes longer to grow a bigger brain, thus leaving immature offspring in
need of extra care for longer periods. But larger brains also provide
adult caretakers with "more complex foraging techniques, predator
avoidance and social skills," the researchers wrote.
skill allows adults to live longer, which in turn gives them longer
reproductive lives. Humans have added to this adaptive advantage by
using their cognitive and social skills to work together in providing
shelter and nourishment for the young, they said.
human females can live well beyond their reproductive years. And the
contributions of non-reproducing grandmothers may further enhance their
own children's reproductive effort and decrease infant mortality,
Barrickman said. That's because grandmas offer extra assistance in
child rearing and food gathering.
Studies of some primitive
societies, such as the Hadza in East Africa, show that "grandchildren
are more likely to survive if they have a grandmother present," she
Some studies suggest that starting life with a brain that
is still developing itself confers some survival advantages to
offspring, according to Barrickman. Extended interactions with mothers
and their surroundings can help "wire their brain" as it grows, she
"They wind up with very plastic brains that can adjust to whatever environmental stimulations come at them," she said.
Duke University. August 2008.