Brain Study Casts Doubt on Theory of How Human Intelligence Evolved

Of all the characteristics that distinguish humans from other primates, those related to intelligence hold special interest for scholars and laypeople alike. It's an appropriate enough fascination. After all, what could be more uniquely human than the desire to understand how our big, curious, narcissistic brains evolved? That said, a lot of the story is entirely unknown, and the results of a new study suggest that some of what scientists thought they knew may actually require revision.

According to a popular view of human cognitive capabilities, much of what sets our species apart from the other primates can be attributed to a disproportionate enlargement of a part of the brain known as the frontal cortex that occurred at some point in human evolution. But the evidence traditionally used to support that argument, say Katerina Semendeferi of the University of California at San Diego and her colleagues, comes from small studies that in many cases did not include data from apes, our closest relatives. Furthermore, the studies varied in the way they defined the region of the cortex.

The team's own findings, detailed in the current issue of the journal Nature Neuroscience, led to a rather different conclusion. Using magnetic resonance imaging, the researchers obtained brain scans of 15 living great apes, four lesser apes, five monkeys and 10 humansa sample larger than any used in the previous studies, which looked at the brains of deceased individuals. They found that, in fact, "the human frontal cortex was as large as expected for a primate brain of human size." That is, the size of the human frontal cortex is, relatively speaking, comparable withnot larger thanthat of the great ape frontal cortex.

The investigators note that their results do not necessarily imply that the frontal cortex is less important in human cognitive specialization than previous studies have suggested, however. "The frontal cortices could support the outstanding cognitive capabilities of humans without undergoing a disproportionate overall increase in size," they write. It may be that reorganization allowed certain cortical areas to grow at the expense of others, thus maintaining the same overall size. Alternatively, the authors offer, modification of the brain circuitry may have led to richer interconnectivity between these cortical areas.