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.


An orangutan mother and her one-year-old infant hurtle through the
forest canopy in Gunung Palung National Park, Borneo. Researchers
believe an ancient jawbone found in Thailand comes from an orangutan
ancestor previously unknown to science. Scientists say clues on the
fossil suggest the new hominoid species, Khoratpithecus piriyai,
evolved in equatorial forests similar to those in Southeast Asia—the
habitat of modern orangutans.


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.