Island orangs descend from small group

Science News: Susan Milius: 23 November 2010

Bornean apes went through a genetic bottleneck during ancient glaciation.

Orangutans on the island of Borneo descend from a relatively small number of ancestors who apparently squeezed through a rough patch about 176,000 years ago, according to the broadest genetic analysis to date of their species.

The genetic data suggest an ancient population bottleneck , says anthropological geneticist Natasha Arora of the University of Zurich, in which animal numbers shrink but eventually expand again when conditions improve.

A serious chill gripped the planet roughly 190,000 to 130,000 years ago, Arora and her colleagues point out in a paper posted online November 22 in Proceedings of the National Academy of Sciences. Borneo itself wasn’t iced over, but rain forests where orangutans live might have shrunk during this time, constraining the orangutan population within it. Such work, she says, “is important to better understand one of our close relatives, the only Asian great ape.”

Orangutans today live only on Borneo and Sumatra, in two species, both endangered, that diverged several million years ago. The new genetic findings are “very surprising” in light of the ancient split, says Lounès Chikhi , a population geneticist in Toulouse, France, with the CNRS research agency.

“Something really important happened” roughly 170,000 years ago, Chikhi says. That something, however, does not seem to have bottlenecked a gibbon and a macaque species which shared ancient Borneo with the orangutan Pongo pygmaeus.

At least one other primate may have had a similar history to Borneo’s orangutans. “There is a strange parallel with human evolution,” Chikhi says. Geneticists have calculated that for modern humans, the time to a most recent common ancestor is 170,000 years ago. “What happened between 150,000 and 200,000 years ago that influenced both Homo sapiens and Pongo pygmaeus?” he asks.

Evidence for the orangutan bottleneck turned up thanks to a collaboration of 17 researchers who collected fecal and hair samples from 211 wild orangutans at 12 study sites. A previous, smaller analysis set the date for common ancestry of orangutans in Borneo much further back, some 860,000 years ago. That date doesn’t hold up, Arora says, now that researchers have updated sequencing for the old samples and added new ones.

Orangutan history has been complex, says conservation geneticist Erik Meijaard,  of the Jakarta-based firm People & Nature Consulting International.  A simple bottleneck may be the most straightforward explanation for the recent shared ancestry, he says. But some more complex scenario involving extinctions and repopulations might also fit the data.

The study also fuels some conservationists’ worries. The genetic evidence confirmed that female orangutans don’t move around much, with rivers in particular being a big barrier. So what may look to a human like a fine stretch of rain forest, just waiting for orangutans to move in, may in reality be blocked off to animals. 

“Once you’ve lost orangutans from a watershed, they’re gone,” Meijaard says.

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