Stare into the intelligent eyes of a gorilla long enough, and you may start to wonder just what it is that sets us apart from our hairy cousins. Researchers long believed the answer lies in a patch of brain called the frontal cortex, which seemed to be proportionally larger in humans than in other primates. But a new comparative study of primate brains--the most comprehensive of its type to date--finds that our frontal cortex isn't any bigger than that of the great apes.
The frontal cortex functions as the executive office of the brain. It plays a key role in abstract thought and language and enables us to plan and control our actions. Nearly 100 years ago, scientists reported that humans had an enlarged frontal cortex compared to other primates. Unfortunately, these studies, which were based on postmortem examinations of brains, compared humans only to the lesser apes, such as gibbons and monkeys. Our closest primate relatives, the great apes, were not included in these studies.
The new study examines the size of frontal cortex in great apes, including gorillas, chimps, orangutans, and bonobos. Anthropologist Katerina Semendeferi and her team at the University of California, San Diego, used magnetic resonance imaging to compare the brains of 10 humans and 24 other primates. In the March issue of Nature Neuroscience, the researchers report that the frontal cortex makes up about 36% of total brain volume in both humans and great apes. In lesser apes, the percentage is only about 30%. The new research suggests to Semendeferi that humans don't lord it over the jungle primates on the basis of an enlarged frontal cortex alone.
One substantial difference is that humans brains are three times larger overall, notes Todd Preuss, a cognitive neuroscientist at the University of Louisiana, Lafayette. He says that researchers must now recognize that regions of the human brain besides the frontal cortex have also expanded. These regions probably include the parietal and temporal lobes, which are the principal sources of information the frontal lobes act upon, Preuss says.