Primitive Primate Makes The Case For Asian Anthropoid Origins

Researchers from the Myanmar-French Pondaung Expedition Project, led by Dr. Jean-Jacques Jaeger of the Université Montpellier-II in France, discovered the Bahinia fossil in a layer of red clay, along with a complete lower jaw from a more advanced primate called Amphipithecus. According to Jaeger and his colleagues, both Amphipithecus and Bahinia belong to the primate group called the anthropoids, which includes all fossil and living monkeys and apes. Anthropoids are distinguished from more primitive-looking types of primates like lemurs (and their extinct relatives) by a number of anatomical features. The research group hopes that their scientific detective work involving Bahinia will help them solve a long-standing mystery in primate paleontology: where, when and from whom did the earliest anthropoids come from?

Many early anthropoids have been found in Africa, most of them in Egypt where a site called the Fayum has yielded a dizzyingly complex variety of primate species. This fueled scientific speculation that Africa, already known as the cradle of human origins, was the ancestral home of anthropoids as well. In recent years however, Asian anthropoids have begun to join the fossil ranks, with discoveries from Thailand, China, and Myanmar. The Asian fossils range in age from the Middle to Late Eocene (49-33 million years ago), and include some of the most primitive-looking anthropoids ever found. One species in particular, a tiny Chinese primate named Eosimias, is so primitive-looking that some scientists doubt whether it was even advanced enough to be called an anthropoid.

The Pondaung Expedition team re-opened the investigation of anthropoid origins after taking a closer look at the details of Bahinia's dentition. To begin with, they found a strong resemblance between Bahinia's teeth and the teeth of Eosimias, so much so that they decided to place Bahinia in the same family as Eosimias.

"Bahinia is important because it is a much more complete fossil than Eosimias," said Jaeger. "With the additional fossil material, we were able to look at more dental characters, and these extra characters in the Bahinia fossil helped us to confirm that Eosimias is definitely an anthropoid."

Together, Bahinia and Eosimias are the most primitive anthropoids ever uncovered, and both of these species are only found in Asia. Jaeger and his colleagues think that this is good evidence, along with some of the other Asian fossil finds, to support the claim that anthropoids may have originated on that continent. This could mean that anthropoids migrated from Asia to Africa at some point, but so far there isn't much concrete evidence to support this part of the tale, Jaeger said.

Another intriguing aspect of Bahinia's telltale teeth suggests that anthropoids may have originated much earlier than scientists have thought. The new fossil's teeth display a combination of anthropoid features and traits seen in older, less advanced primates. This makes Bahinia look like a living fossil. "Bahinia is a 40 million year old fossil that looks like a 57 million year old fossil in its teeth," said Jaeger. "What makes this interesting is that it appears in the fossil record at the same time as a much more advanced anthropoid, Amphipithecus. It's like having your ancestors live along side you." The mix of old and new in Bahinia could mean that the anthropoid group has its roots stretching far back into time, possibly even into the late Paleocene (58-55 million years ago).

Pinpointing the anthropoid ancestor's arrival on the evolutionary scene is important, but it doesn't quite solve the mystery of the ancestor's identity, lurking somewhere in the shadowy forests of the time. Jaeger and his colleagues feel that the Bahinia fossil offers clues to answer this question as well. Primate fossils from the Paleocene and the Eocene generally fall into three main families: a group of animals called adapids, another group called omomyids, and a third group called the tarsiids. Adapids and omomyids may have been similar to today's lemurs and lorises. The tarsiids are represented today by one genus, Tarsius, a nocturnal primate from Southeast Asia that appears to be intermediate between primates like omomyids and primates like monkeys and apes, including humans. At one time or another, scientists have suggested each of these primate groups as a likely candidate for giving rise to anthropoids. Adapids, omomyids and tarsiids are still vying for the title of closest relative, as the debate continues with the addition of new fossils and even genetic data from possible descendents of the three groups. After comparing Bahinia's dental anatomy with fossils from each of the three groups, the researchers believe that Bahinia is most similar to tarsiers. They consider this strong support for a tarsiid origin for anthropoids. "But more fossils, including the skull and ear bones, will be necessary to definitely solve the problem," Jaeger said. "That's why we're going back to Myanmar at the end of the year."

By primate standards, Bahinia was a small animal. The researchers estimated its size to be about 400 grams, somewhere around the size of the smallest South American monkeys. These earliest anthropoids were tree-dwellers, and the shape of their teeth, along with their small size, suggest that Bahinia probably ate mostly insects, like its sister species Eosimias.

"The Bahinia find is important because it tells us that there was a complex community of primates living in Asia, a tremendous anthropoid radiation much earlier than anyone thought," said Jaeger. "We didn't have that kind of information about Asia before, only Africa. Now we may have to change our whole story about anthropoid origins and evolution."