Of the various traits separating people from other members of the animal kingdom, among the most striking is the use of language. Closely associated with this talent is a part of the brain called Brodmann's area 44, which in humans makes up part of the better known Broca's area. Now two scientists say they've discovered that Brodmann's area 44 in chimpanzees and gorillas resembles that of the human brain-a result leading them to propose that this part of the brain evolved for communication millions of years ago.
A number of studies over the years have concluded that Brodmann's area 44 is larger in the human brain's left hemisphere than in the right. Many researchers attribute this asymmetry to the area's specialization for language production during human evolution.
However, two language researchers at the Yerkes Regional Primate Research Center in Atlanta, Georgia, Claudio Cantalupo and William Hopkins, are challenging this theory. They scanned the brains of 20 chimpanzees, five bonobos, and two gorillas with magnetic resonance imaging (MRI). In all three species, they report in the 29 November issue of Nature, Brodmann's area 44 is larger in the left hemisphere, just as in humans-even though these apes can manage just simple cries and grunts.
The researchers conclude that this asymmetry must have existed at least 5 million years ago, when the human and chimpanzee lines split. But they do not claim that the common ancestor of humans and chimps could speak. Instead, they say, as others have, that this asymmetry was associated with a communication system made up of gestures and vocalizations, which later evolved, in humans, into full-blown language.
"This is a very important study," says neuroscientist Christiana Leonard of the University of Florida, Gainesville, because it "demonstrates that the structural asymmetries that accompany hemispheric dominance for language in man were probably in place for millions of years before the evolution of language."
But not everyone is buying their assertion. Some scientists dispute that an asymmetry in Brodmann's area 44 exists at all in humans; others point out that language is controlled by multiple parts of the brain. "Finding an asymmetric homolog of Broca's area in apes isn't surprising, except for people clinging to the false belief that Broca's area is the brain's left hemisphere 'language organ,' " comments cognitive linguist Philip Lieberman of Brown University in Providence, Rhode Island. Lieberman believes that other areas of the brain, including the basal ganglia, are far more central to language.