The direct ancestors of orang-utans were once widely distributed in south-east Asia but the two modern orang-utan species are confined to the islands of Sumatra and Borneo. Both species are endangered, largely as a consequence of destruction of their rainforest habitat. The orang-utans are the only great apes that spend most of their time in trees. Nevertheless, the species share a number of features with other apes: they are adept in their use of tools and live in complex social groups that show evidence of cultural learning.
A wide-ranging international consortium has now presented the full genomic sequence of a female Sumatran orang-utan named Susie. An analysis of the sequence reveals the orang-utan genome to be remarkably stable: it has suffered far fewer duplication events than the human or chimpanzee genome. As a result of the slow rate of genomic evolution, the orang-utan is genetically closest to a putative ancestral great ape. Comparing the new sequence with that of the human genome and other mammalian genomes thus provides unique insights into the evolution of man. Kosiol has examined a total of nearly 14,000 human genes that are also found in the orang-utan, chimpanzee, macaque and dog genomes. She was able to show that genes involved in two processes have been particularly subject to natural selection in primate evolution: visual perception and the metabolism of glycolipids. Intriguingly, defects in glycolipid metabolism are known to be associated with a number of neurodegenerative diseases in humans. "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," Kosiol believes. "Apes, especially orang-utans, 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."
In fact, the scientists have accomplished far more than merely sequencing a single genome. Taking advantage of the new next generation sequencing technologies, they also sequenced the genomes of ten additional orang-utans, five from Sumatra and five from Borneo. Examining all the sequences together permits an estimate of when the two species separated from one another – about 400,000 years ago, which is considerably more recent than previously believed – and enables an assessment of the diversity among them. The surprising finding is that the Sumatran orang-utan is genetically much more variable than its close cousin on Borneo despite now having a much smaller population. There are thought to be about 40,000-50,000 Bornean orang-utans left in the wild but the Sumatran orang-utan is believed to number only 7,000-7,500 individuals and the International Union for Conservation of Nature lists the species as critically endangered. As Kosiol says, "the greater diversity of Sumatran orang-utans compared with Bornean orang-utans could be very important for conservation efforts. We need to do all we can to maintain the diversity of both species."