Why don’t chimpanzees have language the way humans do? Researchers are confident that it has something to do with differences in their brains that arose sometime in the past 5 million to 7 million years, when the chimp and human lines went their evolutionary ways. But exactly what differences account for human language are not entirely clear. Size might have something to do with it, because the modern human brain is about 3.6 times as big as that of a chimp. And yet chimp brains appear to share many features with their human counterparts, including a frontal lobe region called Broca’s area (purple in image) that in humans is closely associated with speech and language.
Most studies of Broca’s area in human brains have concluded that it is larger on the left side than the right, which seems to correlate with the finding that 94% of right-handers do most of their speech and language processing on the left sides of their brains. Scientists had long assumed that this asymmetrical enlargement of Broca’s area in humans was key to language abilities. But in 2001, researchers led by William Hopkins, a primate neuroanatomist at Emory University in Atlanta, began reporting that the brains of many apes also had asymmetrical Broca’s areas. The first such report, in the 29 November 2001 issue of Nature, found enlarged left-side Broca’s areas in the brains of chimpanzees, bonobos, and gorillas. And in a 2008 paper in Current Biology, Hopkins and his colleagues reported that Broca’s area is activated in chimp brains when they communicate with gestures or vocalizations. Hopkins and his co-workers concluded that the enlargement of Broca’s area, and its role in communication, began before the chimp-human split and was not unique to humans.
Yet a new paper, published online last week in Cerebral Cortex, challenges some of these findings and argues once again that the language centers of human brains are special.
The new study, led by evolutionary neuroanatomist Chet Sherwood of George Washington University in Washington, D.C., finds that Broca’s area in the chimpanzee brain does not show a clear asymmetry. And here’s a surprise: One of the co-authors is none other than William Hopkins. How can researchers come up with such conflicting results on such a fundamental issue, one important to understanding the evolution of language?
The answer lies largely with the difficulties in studying Broca’s area. For starters, researchers do not always agree about exactly what part of the frontal lobe should be included under that name. Thus, many brain experts define Broca’s area as two adjacent regions, called Brodmann’s areas 44 and 45, which have been associated respectively with the verbal production of speech and giving meaning to speech. But others have included a somewhat larger area in their studies, called the inferior frontal gyrus (IFG), which includes areas 44 and 45 but a few other structures as well, such as Brodmann’s area 47. Hopkins’s 2008 study, for example focused on the IFG, whereas his 2001 study targetted only area 44.
Another obstacle is that there is great variation among both humans and chimps in the sizes of their Broca’s areas, how asymmetric they are, and even their exact location in the brain. Thus, one key study in 10 humans (five male and five female), led by neuroanatomist Katrin Amunts of the Institute of Medicine in Jülich, Germany, found that area 44 was larger on the left side of the brain in all subjects, but the difference was only statistically significant for the men—whereas area 45 was only significantly larger on the left side for the women! (Nevertheless, this study did confirm the overall enlargement of Broca’s area on the left side.)
For the new study, Sherwood and his colleagues focused primarily on areas 44 and 45. The team studied the brains of 12 chimpanzees that had lived at the Yerkes National Primate Research Center in Atlanta. Within 14 hours of each chimp’s death, its brain was removed and preserved to avoid shrinkage and other artifacts; it was then scanned with MRI and eventually sectioned into tissue blocks. The boundaries of Brodmann’s areas 44 and 45 were defined using special software, and parameters including the volume of Broca’s area and the number of neurons it contained were measured.
The team found that although some of the chimps had what they called “notable asymmetry” in their Broca’s areas—defined as more than a 10% difference in volume or number of neurons between the two sides of their brains—there was no consistent pattern in the direction of the asymmetry. In other words, there was no evidence that chimps have enlarged Broca’s areas on the left side of their brains as humans do.
As for why the new study differs with Hopkins’s previous work, the researchers suggested, among several possibilities, that the earlier studies—which relied upon MRI but not additional tissue sectioning to more exactly define areas 44 and 45—might have picked up differences in underlying white matter, which forms connections with other parts of the brain rather than restricting itself to the gray matter of Broca’s area, where speech processing takes place. If so, the team concludes, the common ancestor of chimps and humans may have evolved symmetries in white matter connectivity before the chimp-human split, possibly associated with gestural or vocal communication, and only afterward did the speech- and language-processing areas themselves become asymmetric.
Although Hopkins, as a co-author of the paper, signed off on this suggestion, he told me in an e-mail that he thinks the jury is still out on whether the Broca’s area of chimps is really asymmetric. For example, he thinks that the team’s data could still be interpreted to indicate that area 44 in chimps is larger on the left side, although probably not area 45. “I think there is much more to the Broca’s story,” Hopkins says. Dean Falk, an anthropologist at Florida State University in Tallahassee, told me that although the paper has “important implications,” she is not convinced by the authors’ suggestions about why the new results differ from Hopkins’s earlier work. Falk thinks the researchers should have included Brodmann’s area 47 in their study because, she says, this area “is now known to be important in semantic processing and might even be more important for evolutionary studies” than areas 44 and 45.
Nevertheless, the authors say that their results argue anew for human uniqueness. Although the human brain is 3.6 times larger than the chimp brain, they say, the new work shows that on average the left-brain Brodmann’s area 44 is 6.6 times larger in humans and left-brain Brodmann’s area 45 is 6.0 times larger than in chimpanzees. “Because areas 44 and 45 are among the most greatly expanded cortical areas yet identified in humans,” the team writes, “this evidence supports the conclusion that enlargement of Broca’s area on the left side is an evolutionary specialization.”
Photo credit: Patrick Hof