Image caption Evolution of the orangutan genome has proceeded more slowly than in other great apes

Scientists have released a draft sequence of the orangutan genome, revealing intriguing clues to the evolution of great apes and humans.

The work suggests orangutans may be genetically closer to the proposed ancestral great ape than are chimps, gorillas and humans.

Details of the research are outlined in the journal Nature.

Two modern species of orang-utan live on the islands of Borneo and Sumatra respectively; both are under threat.

Of the great apes, the orangutan is the most genetically distant from humans.

Fossil finds show that it once had a wider range across South-East Asia; modern populations are threatened by the destruction of their forest habitat and other human activities such as trapping and selling the juvenile apes as pets.

An international team led by Devin Locke, from the Washington University School of Medicine in St Louis, US, sequenced the full genome of a female Sumatran orangutan named Susie.

With Susie's data as a reference, the researchers took advantage of next-generation sequencing technology to obtain lower resolution data on the genomes of 10 additional orangutans - five from Sumatra and five from Borneo.

The team's analysis reveals that the orangutan genome has experienced a slower rate of evolution than those of other great apes, with fewer rearrangements, duplications and repeats in the sequence.

This suggests their genomes are closer to that of the putative ancestral great ape, researchers say.

The researchers also compared 14,000 human genes with their equivalents in the orangutan, chimpanzee, macaque and dog.

The results suggest that genes involved in visual perception and the metabolism of molecules known as glycolipids have been particularly exposed to natural selection in primates.

Species split

"Changes in lipid metabolism may have played a big part in neurological evolution in primates, as well as being involved in the diversity of diets and life history strategies," said co-author Dr Carolin Kosiol, from the University of Veterinary Medicine, Vienna.

"Apes, especially orangutans, have slower rates of reproduction and much lower energy usage than other mammals. It would be very valuable to sequence more primate genomes to enable more comparative analysis of this kind and thus help us understand the evolution of primates and our own species."

The results also provide an estimate of when the Sumatran species split from the Bornean species: 400,000 years ago. This is more recent than other studies have suggested.

The data show that the Sumatran orangutan is more genetically diverse than the Bornean species, despite the fact that the Sumatran apes are now fewer in number than their Bornean cousins.

Genetic diversity could be important for conservation efforts, because it can be related to the ability of those populations to stay healthy and adapt to changes in their environment.

There are thought to be some 40,000-50,000 Bornean orangutans left in the wild; the Sumatran orangutan is believed to number only 7,000-7,500 individuals.

Paul.Rincon-INTERNET@bbc.co.uk