A common ancestor of higher primates?

For years anthropologists assumed that the higher primates, or anthropoids, originated in Africa because the oldest known anthropoid remains were found there. But now a group of researcher believes that the oldest anthropoid, and possibly the common ancestor of monkeys, apes and humans, came from southern Asia. After carefully scrutinizing fossil jawbones and teeth found in Burma in 1978 (SN: 5/12/79, p. 310), as well as a newly discovered incisor tooth from the same site, they conclude that the primate Amphipithecus mongaungensis, or some closely related animal, may be the evolutionary link between anthropoids and the lower primates, or prosimians.

Other researchers, while applauding the increased information these specimens bring, caution that the fossil evidence is still scanty. Some remain uncertain that Amphipithecus is indeed a higher primate. What is certain is that the new finds have stirred up an old debate over which prosimian branch gave rise to the anthropoids.

Fragments of Amphipithecus jawbones were first discovered in the 1920s. At that time Amphipithecus and Pondaungia, another primate found at the same site, were tentatively called anthropoids. But because the jaws were incomplete and worn, their classification remained cloudy. A relative wealth of Aegyptopithecus fossils in Egypt (SN: 4/1/78, p. 196) convinced scientists that this primate -- thought to live 30 million to 35 million years ago -- was the earliest known anthropoid on the road to humans.

Now, after reconstructing an entire side of a lower jaw with the help of the new Burma specimens, paleoanthropologist Russell Ciochon of the State University of New York in Stony Brook, paleontologist Donald Savage of the University of California at Berkeley and Thaw Tint and Ba Maw, two Burmese researchers, believe there is enough evidence to make Amphipithecus, whose Burma fossils are dated at 40 million to 44 million years old, the earliest known anthropoid that might have given rise to the more advanced forms. Ciochon's group and other researchers had already argued that Pondaungia, dated the same age as Amphipithecus, also falls in the oldest anthropoid category, but Ciochon thinks Pondaungia is much less likely to be on the ancestral line to humans, apes and monkeys.

In the Aug. 23 SCIENCE, Ciochon and his colleagues outline the features that they believe Amphipithecus shares with other anthropoids. Like higher primates, it has a deep and thick jaw that is fused in front. In contrast, the jaws of most prosimians are thin and they taper toward the front, where they are jointed rather than fused. Two of Amphipithecus's molars, the researchers say, are anthropoidlike in that the front and back parts are of equal width. The researchers note that the newly found incisor, which Ciochon is "95 percent" certain belongs to Amphipithecus, is very straight and smooth and would be implanted vertically -- all very much like human incisors.

"This is neighter a monkey nor an ape nor a human, but the common link between them," says Ciochon. "Conceptually we thought something like this had to exist. We think now we've found the fossil evidence of it."

At the same time, Amphipithecus shares a number of features with prosimians which hark back to the animal's ancestry. The first molar, for example, narrows toward the front, and all the molars have a cusp characteristic of lower primates.

In particular, Ciochon's group stresses the similarities between Amphipithecus, and hence anthropoids, and a family of lower primates called adapids, which led to modern-day lemurs and lorises. And this puts Amphipithecus smack in the midst of a long-standing debate over which lower primate gave rise to the anthropoids: adapids or another prosimian group called the omomyids, which are related to modern tarsiers. Scientists' responses to Ciochon's group's findings appear to depend largely on where they stand on this other issue.

Philip Gingerich, director of the Museum of Paleontology at the University of Michigan in Ann Arbor and a strong proponent of adapid origins, is greatly encouraged by the new paper. "I basically agree with the interpretation of the authors," he says.

Eric Delson, a paleoanthropologist at Lehman College in New York City and a member of the omomyid-origin camp, doesn't think there is enough fossil evidence to determine that Amphipithecus is an anthropoid, although he admits the possibility. "But [even] if it is, I don't think it shows that adapids are related to anthropoids," he says.

If Ciochon's interpretation of Amphipithecus's place on the primate tree is correct, then the Asian origin of anthropoids complicates the geographic picture of evolution. Ciochon has proposed one scenario in which anthropoids evolved from lower primates in Asia around 40 million years ago and spread, over thousands of generations, into Africa by crossing a narrow, swamplike sea. Meanwhile, he suggests, some early anthropoid forms spread to South America to become the ancestors of New World monkeys, for which there are 27-million-year-old fossils. Ciochon suggests that the anthropoids got to South America by crossing a series of volcanic islands in the Atlantic Ocean when it was much narrower than it is today.

Delson counters that the higher primates didn't necessarily evolve in Asia but that they, or their ancestors, could have been in a band north from Burma around the Bering Strait and down the west coast of North America. In Delson's scenario, the primates in South America originated in North America and are not descendants of African primates.

However hotly debated, says Gingerich, the recent findings "give us a rare view of the stage of evolution of primate 40 million years ago."