Humans are proud of their brains, and rightfully so. A new comparative analysis suggests however, that we should not be so obsessed with size when considering our smarts.
This week evolutionary anthropologists Robert Barton and Chris Venditti report the results of a wide analysis in PNAS Early Edition comparing human frontal lobe size to other brain structures. For an animal our size, our frontal lobes are not larger than expected.
This finding runs counter to conventional wisdom in the popular press and much scientific literature.
“One of the most pervasive assumptions about human brain evolution is that it involved relative enlargement of the frontal lobes,” the authors write. “We show that this assumption is without foundation.”
Prior studies cited as evidence for relative expansion of the frontal lobes, involved in higher-order functioning, have sometimes been based on unscaled measurements. These comparisons confuse selective enlargement with allometric scaling.
“In addition to this theoretical point,” write the authors, “unscaled measures of frontal cortex size give problematic empirical results when the comparative net is cast widely.” As an example they point to human frontal gray matter volume. In humans this volume is smaller as a proportion of total cortical volume than in several other nonhuman primates and two mustelid carnivores (members of the weasel family).
In terms of absolute size, humans have larger frontal cortexes than other measured animals. However sea lions, not considered terribly smart, exceed baboons and gibbons by this measure, animals that are noted for intelligence. And a llama’s frontal cortex is larger than a macaque monkey. Scaling must be taken into account, the authors write:
“Unless one is willing to take seriously the hypothesis that lemurs have more of the qualities bestowed by frontal cortices than do humans, or that llamas possess more than monkeys.”
The primary driver of intelligence, they suggest, is in distributed neural networks–systems coordinated between the neocortex, cerebellum, and basal ganglia, for example. “Experimental evidence now implicates such distributed networks in uniquely human cognitive capacities,” conclude the Barton and Venditti. “We suggest that natural selection selectively enlarged such distributed networks and that these–rather than more localized size change of frontal cortical regions–are likely to form the basis of human cognitive specialization.”