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July 28, 2006
Hints of Language Origin in Rhesus Monkey
Fossil records can't tell us where the building blocks of language appeared on the evolutionary timeline. But brain imaging might. New research shows that a shared ancestor to humans and the rhesus monkey (or macaque) may have had the neural mechanisms upon which language was built. When contemplating the coos and screams of a fellow member of its species, the macaque uses brain regions that correspond to the two principal language centers in the human brain.
Non-human primates like the macaque are able to communicate about such things as food, identity or danger with vocalizations that are interpreted by others of their species and acted upon. In humans, the two main regions of the brain that are involved in encoding this type of information in language are known as Broca's area and Wernicke's area, named for the physician-researchers who discovered them. Although monkeys are not able to perform the mental activities required for language, their brains possess regions that are structurally similar to these areas in humans. The functional significance of such similarities, however, has been unclear.
An international team that included scientists at the National Institute on Deafness and Other Communication Disorders (NIDCD) and the National Institute of Mental Health (NIMH), two of the National Institutes of Health, injected water labeled with oxygen-15, a safe, fast-degrading radioisotope, into three adult macaques. As neural activity increases in a given region of the brain, blood — and the radioactive water it carries — rushes into that region. Using the brain imaging technology positron emission tomography (PET), researchers can capture an image of the radioactive areas, thus highlighting the regions of heightened activity. In this way, the researchers took brain scans of the monkeys as they listened to the recorded coos and screams of other rhesus monkeys and to assorted non-biological sounds, such as musical instruments and computer-synthesized sounds that were acoustically similar to the coos and screams but had no meaning for the macaques.
The results were published July 23 in the advance online issue of Nature Neuroscience. Although the coo of a monkey is acoustically very different from a high-pitched scream, the researchers found that both of these meaningful sounds elicited significantly more activity than the non-biological sounds in regions of the macaque's brain corresponding to Broca's area and Wernicke's area. The non-biological sounds, in contrast, activated the brain's primary auditory areas, possibly because these sounds were new to the monkeys and the primary auditory areas may be especially attuned to novel stimuli.
Based on these findings, the researchers suggest that the brains of the last common ancestor to macaques and humans, which lived 25-30 million years ago, may have had the neural building blocks for the evolution of language.