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Smell, Hearing Genes Differ between Chimps and Humans

By Cheryl Simon Silver

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Drawing on their newfound ability to compare the genetic make-up of two closely related species, scientists have asked a question that never fails to fascinate: Genetically speaking, what separates humans from chimpanzees? The answer, based on a statistical comparison of thousands of human and chimp genes, is that some of our defining differences stem from genes involved in hearing and smell.

The human and chimp genomes have about 99.2 percent of their genes in common, and it’s the other fraction of a percent that may hold the secrets to what makes people human.

Using a chimp genome sequence recently completed at Celera Genomics in Rockville, Maryland, researchers compared how more than 7,600 chimpanzee gene sequences matched up with their counterparts in humans. Michelle Cargill of Celera Diagnostics in Alameda, California, led the research.

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For an added evolutionary perspective, the researchers included mouse genes. This revealed some of the genes that have changed the most since the three species diverged from a common ancestor perhaps five million years ago.

The analysis shows that some critical changes happened relatively quickly, over the last 100,000 years. The researchers cite several changes, possibly related to differing life styles, as being especially significant in tipping us toward the “human” end of the primate spectrum.

The study highlights differences related to one of humankind’s defining qualities—the ability to understand language and to communicate through speech. Interestingly, the locus for hearing seems to lie not only in the brain, but in the hearing apparatus itself, notably a gene, alpha tectorin, in a membrane of the inner ear. Mutations in this gene are known to cause congenital deafness in humans.

The alpha tectorin gene differs greatly in humans and chimps. Researchers can now explore whether these differences have biological significance, and whether inadequate hearing may explain the difficulties chimpanzees experience in learning language.

The recent analysis also shows striking adaptations in genes for smell. The current findings build on work done by scientists at the Weizmann Institute in Rehovot, Israel, and show that as our sense of smell became less important, many genes involved in smell were turned off permanently, joining a large group of inactive “pseudogenes” found in the human genome.

At the same time, other genes for smell evolved in humans at an accelerated pace. These genes include those needed for finding food and distinguishing its nature and freshness, and possibly for selecting a mate.

Diet figures into the evolutionary story as well. The genome comparison also reveals changes and differences in genes that enable humans and chimps to metabolize amino acids and in genes that may affect the abilities of the two species to digest protein. The researchers speculate that this divergence could stem from the time humans began eating more meat, a dietary change anthropologists say occurred possibly in last two million years or so.

Becoming human, however, required a detailed constellation of evolutionary changes, and the current analysis, published in Science, is only a start.

“There’s plenty of room for speculation in this analysis,” says Andrew Clark of Cornell University in Ithaca, New York, and a leader of the study. “It’s meant to be a big hypothesis-generating exercise.”

More answers are likely to come this summer when researchers funded by the U.S. National Human Genome Research Institute are expected to publish an analysis of another chimpanzee genome sequence. This sequence was made available online to researchers in December 2003.

Clark, A. et al. Inferring Nonneutral Evolution From Human-Chimp-Mouse Orthologous Gene Trios. Science. 302, 1960-1963. (December 12, 2003).

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