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Originally published September 1, 2005 at 12:00 AM | Page modified September 1, 2005 at 9:43 PM

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Researchers: Chimp is man's closest living relative

Scientists have come up with the most exact answer yet to a question philosophers and academics have pondered for centuries: What separates...

Seattle Times staff reporter

Scientists have come up with the most exact answer yet to a question philosophers and academics have pondered for centuries: What separates man from apes?

Genetically speaking, not much.

The first comprehensive comparison of human and chimpanzee genetic codes found the two primates are 98.8 percent identical in the most important areas of their DNA.

Researchers say the work provides powerful confirmation of Charles Darwin's ideas about evolution — including his speculation that the chimp is man's closest living relative.

A better understanding of chimp genetics will help reveal the history of the human species and shed light on human diseases, said Robert Waterston, chair of Genome Sciences at the University of Washington.

"Chimpanzees are especially suited to teach us about ourselves," said Waterston, who led the study published today in the journal Nature. "Comparing the genomes will help us address the question of the specific things that make us human."

Though the initial analysis didn't find clear genetic markers that set humans apart, it identified several candidates for follow-up study, including genes related to speech, immunity and diseases such as Alzheimer's, which does not afflict chimps.

"There's no smoking gun in this study," said UW geneticist Evan Eichler, part of the consortium of 67 scientists and 24 organizations that participated in the $25 million project. "The answers are all in there — but we're just not smart enough to figure them out yet."

Chimpanzees and humans have been evolving separately since they split from a common ancestor 5 million to 6 million years ago. Their genes will tell the story of how each species changed during that time.

"I couldn't imagine Darwin hoping for a stronger confirmation of his ideas than what we see when we compare the human and chimp genomes," Waterston said.

The genetic similarity between human and chimps is about 60 times greater than between humans and mice, and 10 times higher than between mice and rats. Nearly 30 percent of chimp and human proteins are identical, Waterston said. Most others differ by only one or two basic building blocks, called amino acids.

But given the chasm between a species that builds skyscrapers and writes sonnets and one whose most sophisticated tool is a stick for fishing termites from mounds, the genetic differences are clearly profound.

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The new analysis identified 40 million separate sites where the two genomes vary — either by a single DNA unit or big hunks. When all variations are counted, including those that occur in DNA regions without genes, the differences between the two species add up to about 4 percent.

Among the most intriguing are seven regions of the human genome where scientists found evidence of rapid evolution over the past 250,000 years — about the time modern Homo sapiens emerged. One gene is associated with speech, which is crucial for language development. Chimps carry a defective version of the gene, which appears identical to the version carried by a small number of people who are unable to speak intelligibly.

Chimps also appear to have a mutated version of an immunity gene that leaves them vulnerable to a sleeping sickness humans don't contract. Conversely, Chimps don't suffer from Alzheimer's disease, possibly because they have a gene that produces a protective enzyme humans lack.

The most rapidly evolving human genes are those that act like conductors, controlling the action of other genes. Differences in these master genes may explain why humans and chimps look and behave so differently, despite their close genetic kinship, Waterston said.

In a separate study, Eichler found chimps lack many of the repetitive DNA sections found in humans. Crammed with multiple copies of genes, these sections are believed to be evolutionary hot spots.

The DNA for the genome study came from a chimp called Clint, who lived at the Yerkes National Primate Research Center in Atlanta until his death last year at the age of 24.

Sandi Doughton: 206-464-2491

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