Washington ? As different as chimps and humans look, they share 98.7 percent of the same genes, researchers say.

Most of the genetic differences seem to be in the brain, according to a new study appearing today in the journal Science.

A team of European and American researchers compared gene activity in the brains, liver and blood of chimpanzees and humans. They found that the body tissues of the two are very similar, but the human brain has about five times more genetic activity.

“There were many genetic changes that occurred on the way to developing humans,” said Dr. Ajit Varki, a professor of medicine at the University of California, San Diego, and co-author of the study. “This study suggests that changes in the brain were one of the main ways that humans evolved away from chimps.”

“There are many people who have spoken out about the differences, but they have really oversimplified things,” said Dr. Elaine V. Muchmore, a researcher at the VA San Diego Healthcare System and a UCSD professor of medicine.

“The human brain is a very, very complicated organ and this study validates that,” said Muchmore, a co-author of the study.

Chimpanzees are more closely related to humans than any other primate. The two are thought to have shared a common ancestor 5 million to 7 million years ago. Since then, chimps and humans have evolved separately, with humans developing a brain about twice the size of a chimp’s brain, Varki said.

The study also examined gene activity in the liver, brain and blood of the orangutan, another type of ape like the chimp, and the rhesus macaque monkey. The researchers found that although the chimp’s tissue and blood resembled that of humans, the gene activity in the animal’s brain was more like that of the monkey.

“If you look at the blood of the chimp and the human, it is very hard to tell them apart,” said Varki, a blood expert. “Humans are expressing more gene differences in the brain, and that is what enables us to do what we can do.”

The researcher said the genetic comparison of chimps and humans might lead to better therapies for human disease.

He noted that chimps can become infected with HIV, the AIDS virus, and never get sick.

Varki said that by searching for an explanation for this different response to disease, researchers may find genes that protect the chimp but are missing in humans. This could lead, perhaps, to new cures, he said.

But Varki cautioned that “research on chimps should follow the same principles as research on humans.”