January 13, 1998

Brain of Chimpanzee Sheds Light on Mystery of Language


A surprising new study reveals that chimpanzees have a structure in their brains that is similar to a so-called "language center" in human brains, challenging cherished notions of how language evolved in humans and why apes cannot talk.

In most people, the structure, a slender inch-long piece of tissue called the planum temporale, is larger in the left side of the brain than the right. Since this area is involved in the processing and comprehension of speech sounds and sign language, scientists concluded 30 years ago that an enlarged plenum temporale in the left hemisphere was required for language and may have evolved for this purpose. Until now, no other animal was shown to have the same asymmetry in this brain region, located at the side of the head and connected to the ears.

The study, published in the current issue of the journal Science, was carried out by three anthropologists -- Dr. Patrick Gannon, of the Mount Sinai School of Medicine in New York, Dr. Ralph Holloway, of Columbia University, Dr. Douglas Broadfield, of the City University of New York -- and Dr. Allen Braun, a neurologist at the National Institute on Deafness and Other Communication Disorders, in Bethesda, Md.

"This is an interesting and useful finding," said Dr. Antonio Damasio, an expert on brain and language at the University of Iowa College of Medicine, in Iowa City. It shows the dangers of concentrating on brain centers and areas, he said, and supports recent research showing that language is widely distributed in the brain and probably evolved from novel connections rather than from new structures.

Language evolution remains a profound mystery. Sometime within the last 2 million years, two-legged primates, or hominids, developed the ability to talk with words, a dazzlingly difficult skill, said Dr. Terrence Deacon, a biological anthropologist at Boston University and McLean Hospital at Harvard Medical School.

Language requires lightning-fast processing of speech and understanding of abstract symbols -- traits that other animals seem not to possess to the degree that humans do, he said. One can imagine the brain undergoing important reorganizations that underlie the ability to argue, cajole, complain, and pontificate.

In the late 1960s, scientists were strongly influenced by the idea that bigger is better and that evolutionary pressures would lead to obvious structural changes in the human brain, Deacon said. If they could find unique bits of human brain architecture, they concluded, it might explain language. A study conducted in 1968 seemed to confirm this view, Deacon said. Of 100 human brains examined, 68 had an enlarged left planum temporale, 24 had structures of equal size, and 11 had larger plana temporale on the right side.

Many people took this to mean that the planum temporale might be a "control center" for language, Gannon said. It is part of the auditory association cortex where sounds come in from the ear, are processed, and then sent to other parts of the brain. Further evidence stemmed from links between the planum temporale and "a melange of behaviors and disorders including musical talent, handedness, and schizophrenia," Gannon said.

A few years ago, Gannon and his colleagues were examining preserved chimpanzee brains with the same methods used in the 1968 study. "We were simply exploring, looking for asymmetries, when one day our eyes popped out," he said. Of 18 chimpanzee brains examined, 17 had enlarged plana temporale on the left side of the brain. "This was more pronounced than in humans," Gannon said in a telephone interview.

Because chimpanzees cannot talk or play the violin, what does the finding mean? Aside from the obvious fact that the common ancestor of chimpanzees and humans had this brain asymmetry 8 million years ago, Gannon said there are three possibilities.

First, the asymmetry in the common ancestor is unrelated to language or communication. But later on, humans built on it and evolved the unique capacity for language. The planum temporale in chimpanzees did not evolve along the same path and plays an unknown role.

Second, the ancestral planum temporale is involved with communication but followed different trajectories in the two species. In humans, it laid the basis for spoken and sign language and in chimps it laid the basis for a more gesture-based language.

"Chimps may have their own sophisticated form of language that we fail to recognize," Gannon said. "They have sense of self, can deceive one another, and show many complex communicative behaviors. Our language is vocal and auditory. Their language is gestural and visual."

The third possibility is that the planum temporale is not directly related to language or communication but has tangential functions, and its role in language has been vastly overrated.

"I think this study provides a strong demonstration that this particular brain asymmetry is not likely to be crucial for language," said Deacon, whose book "The Symbolic Species" (Norton, 1997) lays out modern theories of language evolution. It supports the idea that humans did not evolve new brain structures for language but used structures that were present in other animals, he said.

To find out how language really evolved, researchers are looking more at microcircuitry than at gross anatomy, Deacon said. After all, 30 of every 100 people on average do not show the asymmetry, yet they appear to use language just like everyone else.

Studies show that there is tremendous variability in where language ends up in each person's brain, he said, and it can even move around in young adulthood after injury or, as one study showed, in learning how to do simultaneous translations. In that study, one language stayed on the left and the second language literally shifted to the right side of brain.

Better answers about language evolution lie in the way regions are connected in the brain, said Dr. Jeffrey Hutsler, a research assistant-professor at Dartmouth College, in Hanover, N.H., who dissects human brains to look for such clues. Patches of connected cells in so-called language areas are laid out differently in the left and right sides of the brain, he said. Such structural variations could lead to different firing rates among cells, making some better at processing fast speech sounds.

In the meantime, no one has a clue about the function of the large left planum temporale in chimpanzees. They may use it for hearing calls, hoots, and other sounds, Gannon said, or, they may have traits that are on the threshold of human abilities.

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