A 'Handy' Guide to Primate Evolution: The Hands of Monkeys and Apes May Hold Clues to the Riddle of Human Handedness and Brain Organization

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A 'Handy' Guide to Primate Evolution

A monkey sits expectantly in psychologist Patricia K. Kuhl's laboratory. The animal wears earphones that keep its head in a fixed, upright position. Its hands rest at its waist near a telegraph key that the monkey can feel but not see. A green light begins to blink and the monkey presses the key to signal readiness. It hears two speech syllables through the earphones, realizes the sounds are different and correctly lifts the key.

Hardly an amazing feat, but Kuhl, of the University of Washington in Seattle, has noticed something remarkable about the exercise. Although either hand could be used to press the telegraph key, every one of 30 monkeys trained in the procedure by Kuhl and her colleagues uses its right hand.

"In 10 years, we have not seen a single animal use the left hand," Kuhl says. "Each animal is tested daily for about two years, and we have never seen an animal reverse this hand preference."

Yet the same animals use either hand in other situations, such as grabbing monkey chow from a feeder and taking apple pieces offered them.

It is generally assumed that a specieswide tendency to favor one hand over the other is characteristic only of humans and is related to the development of brain hemispheres with specialized functions. The right hemisphere guides the left arm and hand, and the left hemisphere controls the right arm and hand. Important mental abilities, such as speaking and understanding language, are largely handled by the left hemisphere in most people, making that side "dominant"; this is thought to have promoted a surplus of right-handers.

However, a growing number of researchers are challenging the long-held notion that nonhuman primates have no hand preferences. These scientists also point to accumulating evidence for specialized brain-hemisphere functions in several primate species. Their argument generates considerable controversy among primate researchers, but if it holds up, the implication is that traits thought to be uniquely human -- including handedness, specialized brain hemispheres and language -- can be traced back to primates living tens of millions of years ago.

Kuhl is one proponent of the idea that monkeys have hand preferences. Based on laboratory observations such as the one described above, Kuhl suggests monkeys use their right hands to perform well-practiced, precise manipulations, especially when they cannot clearly see what they are handling.

Kuhl's work--as well as recent findings by several other investigators -- lends support to a controversial theory devised by three linguists, Peter F. MacNeilage (also a psychologist) and Bjorn Lindblom of the University of Texas at Austin and Michael G. Studdert-Kennedy of Yale University. They contend hand preferences do indeed exist among nonhuman primates and have gone largely unnoticed by researchers. Furthermore, say the linguists, patterns of hand use among prosimians, monkeys and apes hold clues to the evolutionary forces promoting human right- and left-handedness, as well as the functional development of the human brain with its important language abilities regulated in the left hemisphere and critical types of perception handled by the right hemisphere.

MacNeilage and his co-workers dispute the longstanding theory that hand preferences first appeard as evolutionary pressures forced the ancestors of modern humans to manufacture and use stone tools. In the November BEHAVIORAL AND BRAIN SCIENCES, they assert that new evidence from a number of researchers "makes it no longer possible to deny the existence of handedness in nonhuman primates."

In the same journal, for example, psychologist Jeannette P. Ward of Memphis (Tenn.) State University describes a left-hand bias in food-reaching among members of several prosimian species housed in zoos, including black-and-white ruffed lemurs, black lemurs, ring-tailed lemurs and lesser bushbabies. …