Bloomberg — Rhesus monkeys and humans, primates who diverged from a common ancestor 25 million years ago, still carry 97.5 percent of the same genes, scientists said.The finding, announced online yesterday by the journal Science, could enable more effective use of the monkey to test drugs and vaccines for humans, and may help geneticists answer questions about how early humans split from our ancestors, shed body hair and grew bigger brains, researchers said.

Teams at three US universities, using powerful computers and $22 million in government funds, mapped a complete genome for the rhesus macaque after extracting DNA from the liver of a female monkey from India. The macaque is the third primate, after humans and chimpanzees, to have its genome published.

“It’s exciting partly because of what it can tell us about the biological basis of being human,” said Francis Collins, one of the leaders in the earlier mapping of the human genome and director of the National Human Genome Research Institute, which helped fund the macaque work.

“Before, with just the chimp and human genomes, you could look at differences between them but you couldn’t decide which are responsible for chimpness and which are responsible for humanness,” Collins said in a phone interview yesterday. “Now being able to compare all three, you can find areas where rhesus and chimp are the same, but the human is different.”

Chimps and humans split off from each other more recently, about five million years ago, and share a greater number of common genes than humans and rhesus macaques. A three-way comparison, or “triangulation,” as Collins called it, can allow researchers to identify genes that are the same in macaques and chimps and changed in the human genome.

Initial comparisons of the human, chimpanzee and monkey suggest that only about 200 of the 23,000 genes that each species has include evolutionary changes, said Richard Gibbs, the overall co-ordinator of the effort and a director of the Human Genome Sequencing Center at Baylor College of Medicine.

“The grail here is to say what is it that makes us specifically human and why don’t we grow up and become chimps or macaques,” Gibbs said. “We’ve long give up the idea that just a few genes cause all the difference but still expect that a fairly short number” are the key drivers.

Many of these genes affect reproduction and the immune systems, Collins said.

“It does look as if the immune system and reproductive systems have gone through more rapid change in recent evolution,” said Collins. “That makes sense since evolution cares a lot about reproduction and since the immune system is constantly being asked to respond to new threats and if it doesn’t evolve, a species could be wiped out.”

One of the reasons the rhesus macaque was chosen as the next primate to have its genes mapped is because its abundance and similarity to humans has made it widely used in drug testing. Pharmaceutical companies are “extremely interested” in the rhesus macaque genome and have been steadily downloading the pieces of its genome that the teams have been placing on publicly accessible websites, Collins said.

“For drug companies that have been assessing whether a particular drug is safe and effective the rhesus macaque has been a good model,” Collins said.

By being able to see how particular genes and proteins differ between humans and macaques, drug companies can gain a better sense as to whether drugs that are safe in macaques will also be safe in humans, he said.

Geneticists, it seems, are not easily satisfied. To really understand which genes changed to drive human evolution, the genomes of other primates must also be mapped, Collins says.