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Thursday, March 14, 2002
Evolution of intelligence
By ROWAN HOOPER
Woody Allen once famously said that the brain was his second favorite organ. And it is well-established that having a big one, as with Allen's "first favorite" organ (I'm guessing he wasn't referring to his liver), confers high status on its owner.
But what are big brains for?
The intuitive, straightforward response -- that big brains are linked with higher intelligence -- had, surprisingly, not found adequate support until recently.
Certainly, the question is not an easy one to get to grips with, not least because "intelligence" is a notoriously difficult property to define, let alone measure. Now, however, Simon Reader and Kevin Laland, two biologists from the department of zoology at the University of Cambridge, have taken on the task.
Intelligence is usually tested experimentally, which has led to criticism that tests are not fair to different species and that such tests are not relevant to the animals in the wild, in their day-to-day life. Furthermore, lab-testing can't provide data on large numbers of species. To get around these problems, the researchers went through hundreds of scientific papers published in specialized primate journals and collated reports of three measures of intelligence that are relevant to each species and that can be compared across different species.
The three measures are innovation (the ability to find novel solutions to environmental or social problems), social learning (the ability to learn skills and acquire information from others) and tool use. These measures, say Reader and Laland, are "ecologically relevant." They accumulated data on 116 primate species and then looked at the brain size of each species relative to its intelligence.
Brain size is also a problematic measurement, because as the species' body size gets bigger, so does the brain. It becomes meaningless to make comparisons of the absolute size of the brain when you've got species like tiny lemurs and giant gorillas, whose brains are vastly different in size.
The researchers avoided the problem by measuring the size of the "executive brain," the combined area of the neocortex and the striatum. These two brain areas are centrally involved in social learning and innovation and, as such, are good candidate areas for measures of intelligence. (The development of the neocortex and the striatum, incidently, is strongly influenced by the genetic contribution of the mother, which is why "intelligence" has been said to be inherited from females.)
Reader and Laland found that primate species with larger executive brains exhibited more behaviors from all three categories of intelligence.
"Our findings support the view that social learning and innovation may have been important processes behind the evolution of large brains in primates," the scientists wrote in Tuesday's issue of Proceedings of the National Academy of Sciences.
Previously it was believed that primate intelligence evolved as the result of selection in a single "brain domain," such as that for technology or for the Machiavellian strategy of manipulating and deceiving others. But Reader and Laland found that species that were good at using tools, for example, also tended to be good at social learning and innovation.
"Our results do not diminish the findings of advocates of Machiavellian intelligence [or those of the social learning and innovation], but they do suggest that other processes are likely to have also been important in the evolution of large brains," said Reader in an e-mail interview.
There is another evolutionary advantage to having large brains, said Reader. "Large-brained animals will expose themselves to more novel selection pressures than small-brained groups, because they innovate and socially learn more," he said.
"We tend to think of human intelligence as entirely bound up with language, and of course in many respects it is," said Robert Seyarth, a psychologist at the University of Pennsylvania, commenting in an e-mail interview on Reader and Laland's paper. "But what, exactly, is it that language endows us with that other creatures lack? At the moment we don't exactly know. A complete understanding of human intelligence requires that we understand what intelligence was like before language appeared, and this is what Reader and Laland are trying to do."
E-mail Rowan Hooper at firstname.lastname@example.org