Human intelligence might have evolved in response to the demands of caring for infants who are born far more immature than the offspring of other species, suggests new research.
This theory is based on a novel evolutionary model in which the development of high levels of intelligence may be driven by the demands of raising offspring.
"Human infants are born far more immature than the infants of other species. For example, giraffe calves are able to stand up, walk around, and even flee from predators within hours of their births. By comparison, human infants cannot even support their own heads," said one of the researchers Celeste Kidd, assistant professor at University of Rochester in New York.
"Our theory is that there is a kind of self-reinforcing cycle where big brains lead to very premature offspring and premature offspring lead to parents having to have big brains," noted Steven Piantadosi, who is also from University of Rochester.
"What our formal modeling work shows is that those dynamics can result in runaway pressure for extremely intelligent parents and extremely premature offspring," Piantadosi said.
In other words, because humans have relatively big brains, their infants must be born early in development while their heads are still small enough to ensure a safe delivery.
Early birth, though, means that human infants are helpless for much longer than other primates, and such vulnerable infants require intelligent parents.
As a result, selective pressures for large brains and early birth can become self-reinforcing -- potentially creating species like humans with qualitatively different cognitive abilities than other animals.
Their study appeared online in the journal Proceedings of the National Academy of Sciences' Early Edition.
The researchers tested a novel prediction of the model that the immaturity of newborns should be strongly related to general intelligence.
"What we found is that weaning time -- which acts as a measure of the prematurity of the infants -- was a much better predictor of primate's intelligence than any of other measures we looked at, including brain size, which is commonly correlated with intelligence," Piantadosi said.
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