Split Between Apes and Monkeys Linked by New Fossil Records

Newly discovered fossils in Tanzania may help explain what led to the split between hominoids (large tailless primates, including today's gorillas, chimpanzees and humans) and cercopithecoids, or Old World monkeys, which include today's baboons and macaques, but the announcement, which is based on dental fragments, does not have all experts convinced.

Paleontologists recovered a lone tooth and jaw fragment with three teeth from a site in the East African Rift at the Rukwa Basin in southwestern Tanzania. After geologically dating the surrounding rocks, the paleontologists determined the fossil fragments were 25.2 million years old, several million years older than any other known specimen in either primate group.

"These discoveries are important because they offer the earliest fossil evidence for either of these primate groups," said lead study author Nancy Stevens, an anthropologist at Ohio University in Athens, Ohio, according to a report by LiveScience.

"These finds can help us to further refine hypotheses about the timing of diversification of major primate groups."

The fossil fragments belong to two previously unknown primates that lived in the Oligocene epoch, which lasted from 34 to 23 million years ago. Researchers report that the two lineages were already evolving separately during this geological period, something previously undocumented.

Prior to these finds, the oldest fossil representatives of the hominoid and cercopithecoid lineages were recorded from the early Miocene, at sites dating millions of years younger. 

"The late Oligocene is among the least sampled intervals in primate evolutionary history, and the Rukwa field area provides a first glimpse of the animals that were alive at that time from Africa south of the equator," said Stevens, in a press statement.

The early hominid, represented by a mandible preserving several teeth, has been named Rukwapithecus fleaglei. The early cercopithecoid, represented by a tooth and jaw fragment, is called Nsungwepithecus gunnelli. (The species were named after two notable primatologists, John Fleagle of Stony Brook University in New York, and Gregg Gunnell, at Duke University.)

Fleagle, after whom one of the species was named, calls the discoveries "a wonderful story of perseverance" over many years of research, according to Science Now. He said that the researcher's claim to have found the earliest known ape and Old World monkey ancestors is "as good as it can be with the material in hand. But he said the "real split" could have taken place even earlier.

Terry Harrison, an anthropologist at New York University, expressed more hesitation when speaking to Science Now.

 "Despite the seductiveness of the conclusions, I'm skeptical about the interpretations," he said.  He postulated that the dental features used to identify the species are "questionable" and that one of the specimens could be from a suiform artiodactyl, a piglike, hoofed animal.

"Similar cases of mistaken identity have occurred in the past," Harrison said to Science Now, including claims for Miocene primates in East Africa that turned out to be artiodactyls and other nonprimates.