The study, published Thursday in the science journal Nature, also reveals that the orangutan -- "the man of the forest" -- has hardly evolved over the last 15 million years, in sharp contrast to Homo sapiens and his closest cousin, the chimpanzee.

Once widely distributed across Southeast Asia, only two populations of the intelligent, tree-dwelling ape remain in the wild, both on islands in Indonesia.

Some 40,000 to 50,000 individuals live in Borneo, while in Sumatra deforestation and hunting has reduced a once robust community to about 7,000 individuals, according to the International Union for the Conservation of Nature (IUCN).

These two groups split genetically about 400,000 years ago, considerably later than once thought, and today constitute separate albeit closely related species, Pongo abelii (Sumatra) and Pongo pygmaeus (Borneo), the study showed.

An international consortium of more than 30 scientists decoded the full genomic sequence of a female Sumatran orangutan, nicknamed Susie.

They then completed summary sequences of 10 more adults, five from each population.

"We found that the average orangutan is more diverse -- genetically speaking -- than the average human," said lead author Devin Locke, an evolutionary geneticist at Washington University in Missouri.

Human and orangutan genomes overlap by about 97 percent, compared to 99 percent for humans and chimps, he said.

But the big surprise was that the far smaller Sumatran population showed more variation in its DNA than its close cousin in Borneo.

While perplexing, scientists said this could help boost the species' chances of survival.

"Their genetic variation is good news because, in the long run, it enables them to maintain a healthy population" and will help shape conservation efforts, said co-author Jeffrey Rogers, a professor at Baylor College of Medicine.

Ultimately, however, the fate of this great ape -- whose behaviour and languid expressions can be eerily human at times -- will depend on our stewardship of the environment, he said.

"If the forest disappears, then the genetic variation won't matter -- habitat is absolutely essential," he said. "If things continue as they have for the next 30 years, we won't have orangutans in the wild."

The researchers were also struck by the persistent stability of the orangutan genome, which appears to have changed very little since branching off on a separate evolutionary path.

This means the species is genetically closer to the common ancestor from which all the great apes are presumed to have originated, some 14 to 16 million years ago.

One possible clue to the lack of structural changes in the orangutan's DNA is the relative absence, compared to humans, of telltale bits of genetic code known as an "Alu".

These short stretches of DNA make up about 10 percent of the human genome -- numbering about 5,000 -- and can pop up in unpredictable places to create new mutations, some of which persist.

"In the orangutan genome, we found only 250 new Alu copies over a 15-million year time span," Locke said.

Orangutans are the only great apes to dwell primarily in trees. In the wild, they can live 35 to 45 years, and in captivity an additional 10 years. Females give birth, on average, every eight years, the longest interbirth interval among mammals.

Earlier research has shown that the great apes are not only adept at making and using tools, but are capable of cultural learning, long thought to be an exclusively human trait.

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