Chimp brains show humanlike tilt to the left.

Human language abilities depend on tissue located mostly on the left side of the brain, or left hemisphere. A new study finds that the common chimpanzee, despite its inability to speak, shares with people one feature of this anatomical pattern--a structure called the planum temporale is larger on the left side of the brain than on the right.

In humans, a swath of neural tissue, known as Wernicke's area, encompasses the entire planum temporale and helps to orchestrate language comprehension.

A larger planum temporale in the left hemisphere also characterized the common ancestor of chimps and humans, a creature that lived around 8 million years ago, contends a scientific team headed by neurobiologist Patrick J. Gannon of Mount Sinai School of Medicine in New York. Whether the planum temporale fostered species-specific forms of communication or assumed other responsibilities during the course of evolution remains unclear, the group notes.

"This is a great contribution to the field [of comparative brain studies]," says neuroscientist Katerina Semendeferi of the University of California, San Diego. "Now we need to look more closely at brain organization in all apes." Semendeferi suspects that the larger left-brain planum temporale exists in gorillas and orangutans, as well as in chimps. She directed a related study, published in the April 1997 Journal of HUMAN EVOLUTION, indicating that the brain's frontal lobe--often assumed to have expanded greatly in humans to support complex thought--is actually about the same size, relative to overall brain volume, in all apes.

Gannon's team conducted a microscopic analysis of the surface of 18 preserved chimp brains. The group identified anatomical landmarks delineating the planum temporale and then calculated its surface area in each hemisphere. A pronounced left-side size advantage appeared in 17 of the brains, the researchers report in the Jan. 9 Science.

An investigation reported 20 years ago found few signs of the planum temporale in chimps. As a result, some scientists thought this brain area was poorly developed in nonhuman primates.

"If Gannon's group is able to see planum temporale landmarks in chimps, they're to be congratulated," comments anthropologist Dean Falk of the State University of New York at Albany. "Their finding fits with some previous research on asymmetry in chimp brains."

The greatest left-right asymmetry of the planum temporale has been reported in people with perfect pitch, Falk notes. In chimps, a similar left-hemisphere emphasis may help in processing melodic aspects of their vocal communication, she theorizes.