Fossil Jaw Grows Orangutan Family Tree, Scientists Say

Stefan Lovgren
for National Geographic News

January 28, 2004

Researchers believe a jawbone found in Khorat, Thailand, and dating to the Late Miocene era between seven and nine million years ago belongs to a newly discovered relative of orangutans.

The jawbone from the new hominoid, named Khoratpithecus piriyai, is similar to the lower jaw, or mandible, of modern orangutans. And like today's orangutans, the ancient jawbone shows no evidence of anterior digastric muscles, the tell-tale muscles used to lower the jaw in most other primates.

The ancestry of orangutans is highly disputed. While one hypothesis maintains that orangutans originated from Lufengpithecus, a South Chinese and Thai hominoid, another theory says they originated from Sivapithecus, a Miocene hominoid from Indo-Pakistan.

The new discovery, however, suggests that the orangutan's most recent ancestors evolved in equatorial forests similar to those in Southeast Asia that the orangutan inhabits today.

"The [discovery] challenges the place of all the other miocene fossil hominoids as close orangutan relatives," said Jean-Jacques Jaeger, a professor of paleontology at Montpellier II University in France, who led the study.

The research is described in this week's issue of the science journal Nature.

Selling Fossils

Modern orangutans hail from the Pleistocene period, two million to 100,000 years ago. While their geographic distribution once included much of Southeast Asia, they became extinct from many areas through hunting and deforestation. Today, the orangutan is found only in Borneo and Sumatra.

The jawbone, along with some fossil elephant teeth, were found in 2002 in Khorat, in northeastern Thailand, by a sandpit worker. He sold the fossils to a private Thai collector. A group of Thai scientists, informed about the discovery, convinced the collector to give it to a public museum in Thailand so it could be studied by Thai hominoid expert Yaowalak Chaimanee.

A first look at the jaw shows that it's different from all other fossil hominoids and more similar to large, existing apes. The jaw has a wide incisor-canine area. The symphysis, or fusion of the two halves of the lower jaw, is also similar to modern apes.

The newly discovered jawbone also shares some important characteristics with the jaw of the orangutan. Neither shows evidence of anterior digastric muscles, which act to lower the jaw in many primates.

"[That] one specialized character is uniquely shared with orangutans … indicating very close affinities," said Jaeger.

Redesigning Apes

The fossil record of the living great apes is poor. The orangutan is actually the only great ape that has a fossil record. No African fossil has ever been found that is related to chimpanzees or gorillas.

But determining the ancestry of the orangutan has proven extremely difficult. Reconstructing phylogeny—lines of descent—of very rare fossils is hard because researchers lack knowledge of how these characters evolved.

"Some evolved as a result of adaptation to special ways of feeding and diets … and may have evolved independently in different lineages," said Jaeger. "We call that parallel evolution.

"Some other specialized characters are shared because they have been inherited from a common ancestor," he continued. "We can hardly separate these two kinds of characters. Only a probabilistic approach is possible."

Evolution virtually redesigned apes. Most features of the living apes—their torso, internal organs, ligaments and joints—are different from their more primitive kin. No known fossil ape related to the orangutan is adapted for life in the trees, leading researchers to believe orangutans descended from a ground-dweller.

However, the post-cranial of the new fossil is not known. Its discovery may help researchers understand the locomotion evolution of large apes.

Two main competing hypotheses have been proposed for the orangutan's origins. While dental similarities support an origin from Lufengpithecus, a South Chinese and Thai Middle Miocene hominoid, facial and palatal similarities suggest an origin from Sivapithecus, an Indo-Pakistan hominoid.

However, there is now strong evidence that suggests neither of those species was the ancestor of the orangutan.

In a discovery described last year in the journal Nature, Jaeger and his team unearthed a fossil ape (Lufengpithecus chiangmuanensis) in Thailand dating back 10 to 13.5 million years ago. They now consider it to be an ancestor of the new form.

But those fossils only consisted of 22 isolated teeth from the upper and lower jaws of several apes. Some experts warn against establishing ancestry by comparing teeth because animals may have similar dental structures and still be very different.

Rewriting Evolution

The new discovery, meanwhile, includes both teeth and jaw. It suggests that ancestors of the orangutan evolved in Thailand under tropical conditions similar to those of today, in contrast with Southern China and Pakistan, where temperate or more seasonal climates appeared during the Late Miocene.

But Jaeger admits that many more fossils are needed to understand how the new species developed the same characteristics as the modern orangutan.

"At least we have shown that the ancestors of the orangutan, which were closely related to the Africans, were present in Thailand," said Jaeger. "Maybe it had a sister species further south, in Malaysia or Indonesia, that looked even more similar to the extant orangutan."

Jaeger has been working in Southeast Asia for the past 20 years, and his Thai-French team has been studying the site in Thailand for more than eight years. He predicts that future discoveries in Southeast Asia will re-write the story of how apes—and humans—evolved.

"I am convinced that Southeast Asia played a most critical role in the evolution of anthropoids and hominoids, much more important than what is commonly believed," he said.