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 phylogenylines of descentof 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 apestheir torso, internal organs, ligaments and jointsare 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.
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 apesand humansevolved.
"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.
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