Chimpanzee Communication

A brain region critical to speech and language ballooned after humans split from chimpanzees, a new study finds.

Named after French physician, Pierre Paul Broca, who identified the region in two brain-damaged patients incapable of uttering more than a few words, Broca's area usually occupies a much larger portion of the left half of the human brain than the right.

Because right-handed humans also tend to process language in their left halves – lefties' brain are flip-flopped – some researchers think that lop-sidedness in Broca's area may help explain why humans alone developed language.

Brain wiring

"There must be something unique about the wiring of the region" to explain language, says Chet Sherwood, a neuroscientist at George Washington University in Washington DC, who led the new study.

However, brain imaging studies have hinted that Broca's area also tends to be larger in one half of the chimpanzee brain than the other. What's more, this area kicks into action when chimps communicate via hand gestures, another study found.

To get a better handle on how Broca's area may have changed in the 6 million years since humans and chimpanzees last shared a common ancestor, Sherwood's team examined thin sections of Broca's area, collected from 12 chimpanzees after they died of natural causes.

Neuron count

The researchers noticed a lot of differences between individual chimpanzees in the size, location and symmetry of Broca's area. But Sherwood's team found no common population-wide differences in the number of neurons in the left and right Broca's area for chimpanzees, as is the case in humans. Furthermore, the handedness of the chimpanzees – established before their deaths – wasn't related in any way to the brain region's symmetry.

Broca's area has also swelled disproportionately during our species' evolution. Human brains are 3.6 times larger than those of chimpanzees, on average. Yet Broca's area is more than 6 times larger in humans than chimpanzees, notes Natalie Schenker, a neuroscientist now at the University of California, San Diego, who led the study along with Sherwood.

"It suggests that [Broca's area] is doing something that's important," she says. "Maybe it's taken on some increased functionality."

Language evolution

"I buy the conclusion that Broca's area underwent changes in hominins in conjunction with language evolution," says Dean Falk, an anatomist at Florida State University in Tallahassee, who was not involved in the study.

However she wishes the researchers had looked at Brodmann area 47, a nearby patch of brain important for extracting meaning from words. This area, she says, may have played an even larger role in humans' gift for the gab.

Journal reference: Cerebral Cortex, DOI: 10.1093/cercor/bhp138

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