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鈥檚 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 鈥� lefties鈥� brain are flip-flopped 鈥� some researchers think that lop-sidedness in Broca鈥檚 area may help explain why humans alone developed language.
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Brain wiring
鈥淭here must be something unique about the wiring of the region鈥� to explain language, says , a neuroscientist at George Washington University in Washington DC, who led the new study.
However, brain imaging studies have hinted that Broca鈥檚 area also tends to be larger in one half of the chimpanzee brain than the other. What鈥檚 more, this area kicks into action when chimps communicate via hand gestures, another study found.
To get a better handle on how Broca鈥檚 area may have changed in the 6 million years since humans and chimpanzees last shared a common ancestor, Sherwood鈥檚 team examined thin sections of Broca鈥檚 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鈥檚 area. But Sherwood鈥檚 team found no common population-wide differences in the number of neurons in the left and right Broca鈥檚 area for chimpanzees, as is the case in humans. Furthermore, the handedness of the chimpanzees 鈥� established before their deaths 鈥� wasn鈥檛 related in any way to the brain region鈥檚 symmetry.
Broca鈥檚 area has also swelled disproportionately during our species鈥� evolution. Human brains are 3.6 times larger than those of chimpanzees, on average. Yet Broca鈥檚 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.
鈥淚t suggests that [Broca鈥檚 area] is doing something that鈥檚 important,鈥� she says. 鈥淢aybe it鈥檚 taken on some increased functionality.鈥�
Language evolution
鈥淚 buy the conclusion that Broca鈥檚 area underwent changes in hominins in conjunction with language evolution,鈥� says , 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鈥� gift for the gab.
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