Like man, like monkey
A special area of the brain believed to control language is larger in the left hemisphere than the right hemisphere in humans. Now researchers have also discovered such an asymmetry in chimpanzees. This disproves a theory that this part of the brain has only enlarged in humans. Scientists from Columbia University's Mount Sinai Medical School and the National Institutes of He alth studied the extent of a brain region responsible for controlling language in both humans and monkeys (Science, January 9, 1998 issue). Their findings question the role of the planum temporale, part of the brain's parietal cortex. In humans, the temporal planum is usually larger in the left hemisphere than in the right hemisphere, but 94 percent of the chimpanzee brains examined showed the same asymmetry.
Could the research be interpreted to mean that chimpanzees have some kind of language? "I don't think they have a language, but I agree that they have some sort of communication system that might be more complex than we previously thought," said Ralph Holloway, a professor of anthropology at Columbia University. He believes that chimpanzees communicate using a variety of sophisticated facial, body, and hand gestures, possibly aided by grunts or other vocalizations.
Patrick Gannon, director of the Paleoneurology Research Laboratory at Mount Sinai Medical School, was the first to suggest that chimpanzee brains may have the same asymmetry as humans. He enlisted the help of Holloway, who then helped him measure the planum temporale in his collection of 18 chimpanzee brains. The Columbia University anthropologist conducted comparative neuroanatomical studies on the chimpanzee brains to better understand the evolution of the human brain.
The result of the research work contradicts the scientific theory that an enlargement of the left side of the brain only exists in humans. Carl Wernicke, a 19th-century neurologist, found that patients with brain injuries to the posterior temporal lobe and parietal lobe-the same area now being studied by researchers-were unable to produce language, but could understand it. This area of the temporal cortex, also known as Wernicke's area, was thought to control language comprehension, but only in humans.
„After 100 years of comparative study of the brain, this dogma was believed to be correct. It's quite a shock to discover that chimpanzee brains share the same asymmetry as the human brain," Gannon said. When he performed magnetic resonance imaging of chimpanzee brains and found a discrepancy between the cerebral hemispheres, he was originally convinced that the results obtained were incorrect.
In addition to the possibility that communication between chimpanzees actually takes place, the scientists also propose several possible interpretations of the results obtained in the experiment in their publication. If both chimpanzees and humans have an enlarged temporal planum, then their common ancestor probably also possessed this trait. However, language function in this brain region may not have evolved until 6 to 8 million years ago, when humans split from other primates. Finally, it is also very possible that the planum temporale is not involved in language in either chimpanzees or humans.
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