Brain Size Surprise: All primates may share expanded frontal cortex

Researchers have traditionally theorized that the frontal cortex, a brain region linked to mental faculties such as planning and reasoning, expanded to an unprecedented extent during human evolution. However, a new analysis of brains from many different mammals takes the uniqueness out of our frontal cortex.

Lemurs, gibbons, chimpanzees, and other primates have roughly the same proportion of brain tissue devoted to the frontal cortex as people do, say Eliot C. Bush and John M. Allman of the California Institute of Technology in Pasadena. Lions, hyenas, and other carnivores display a substantially smaller frontal cortex relative to the rest of the brain.

“People aren’t special in regard to frontal-brain size,” Bush says, “but there appear to be important differences between primates and carnivores in the way the frontal cortex is put together.”

The new findings, in the March 16 Proceedings of the National Academy of Sciences, elaborate on a 2002 study led by anthropologist Katerina Semendeferi of the University of California, San Diego. Using magnetic resonance imaging, Semendeferi’s team found similar relative volumes of frontal cortex in 10 people and 24 great apes. The latter group consisted of orangutans, gorillas, and chimps.

Bush and Allman widened the scope of frontal cortex analysis, focusing on primates and carnivores. They compared 25 primate species with 15 carnivore species. Computerized analyses of a series of brain slices identified various neural regions and yielded volume estimates for them.

In all the primates, the frontal cortex displayed about the same relative size, approximately 36 percent of the total brain volume, while carnivores had less than 30 percent of their brain in this area.

Intriguingly, lemurs and other prosimians—regarded as the most primitive primate suborder—exhibited a slightly greater frontal cortex proportion than people and great apes did. That finding challenges the influential theory that the frontal cortex progressively expanded in our primate ancestors, Bush holds.

During primate but not carnivore evolution, he speculates, the frontal cortex and a few other neural structures expanded simultaneously and became brain networks devoted to sharper vision and other primate-specific capabilities.

Anthropologist Robert A. Barton of the University of Durham in England agrees. Specialized functions in the brains of various primate species depend on networks encompassing far-flung brain regions, in Barton’s view. For instance, on the basis of brain data that he reported last year, Barton suspects that areas of the frontal cortex, thalamus, cerebellum, and brain stem jointly expanded during primate evolution to regulate movement and balance.

Although other researchers have previously noted frontal cortex expansion in primates, Bush and Allman report an interesting neural contrast between primates and carnivores, comments neuropsychologist Barbara L. Finlay of Cornell University.

Semendeferi says that the next step for researchers is to compare various parts of the frontal cortex in people, apes, and monkeys.