From the human perspective, few events in evolution were more momentous than the split among primates that led to apes (large, tailless primates such as today's gorillas, chimpanzees, and humans) and Old World monkeys (which today include baboons and macaques). DNA studies of living primates have estimated that the rift took place between 25 million and 30 million years ago, but the earliest known fossils of both groups date no earlier than 20 million years ago. Now, a team working in Tanzania has found teeth and partial jaws from what it thinks are 25-million-year-old ancestors of both groups. If the interpretations hold up, the finds would reconcile the molecular and fossil evidence and possibly provide insights into what led to the split in the first place.

Researchers have long been frustrated by a paucity of fossils from this key period in evolution, which sits at the borderline between two major geological epochs: the Miocene (about 23 million to 5 million years ago) and the Oligocene (about 34 million to 23 million years ago). The earliest known fossils of early apes and Old World monkeys date from the early Miocene and have been found in just a handful of sites in Kenya, Uganda, and North Africa. Meanwhile, molecular studies of existing primates consistently suggest that these two groups arose during the Oligocene, leading scientists to wonder whether the molecular dates are wrong or if paleontologists have been looking in the wrong places.

For more than a decade, researchers from the United States and Tanzania have been combing Tanzania's Rukwa Rift Basin, searching for fossils of all kinds. During the 2011 and 2012 seasons, a team led by Nancy Stevens, a vertebrate paleontologist at Ohio University in Athens, discovered fossils that it identified as belonging to two previously unknown species of primates: one, an apparent ape ancestor the team has named Rukwapithecus fleaglei; the other, a claimed Old World monkey ancestor dubbed Nsungwepithecus gunnelli. (The species were named after two notable primatologists, John Fleagle of Stony Brook University in New York, and Gregg Gunnell, who is now at Duke University in Durham, North Carolina.)

Both specimens, consisting of teeth and partial jaws, were found in Rukwa Rift sediments dated by several techniques, including the often used argon-argon method, to 25.2 million years ago. The team identified them as ape and Old World monkey ancestors from the features of their molars, which paleontologists routinely use to tell primates apart. For example, Stevens says, Nsungwepithecus "has a much more triangular outline" of its last lower molar than Rukwapithecus, and there are "a number of other major differences in the shape and position of the cusps and crests that run along the chewing surface of the teeth." The two species also show other dental features that group them with later Old World monkeys and apes, but are still different enough to be classified as separate—and more ancient—species.

The team, which reports its discoveries online today in Nature, points out that the split between early apes and Old World monkeys took place during a time of dramatic environmental, climatic, and tectonic changes in East Africa. Indeed, the Oligocene-Miocene boundary was marked by numerous tumultuous geological events, including a collision between the land masses that make up Africa and Arabia on one side and Europe and other parts of Asia on the other. However, just why those changes led the two groups to split "is one of the mysteries we would like to solve" by further research, Stevens says, although she thinks that it could be related to changes in the kinds of food resources available to the primates.

Michael Steiper, an anthropological geneticist at Hunter College in New York City who has conducted numerous molecular studies of primate evolution, welcomes the new findings. "At long last they reconcile the fossil and molecular records of early apes and monkeys," he says.

And Fleagle, after whom one of the species was named, calls the discoveries "a wonderful story of perseverance" over many years of research. As for the team's claims to have found the earliest known ape and Old World monkey ancestors, he says that "their identifications are as good as they can be with the material in hand." The "real split" between the groups could have taken place even earlier, he says.

But Terry Harrison, an anthropologist at New York University in New York City, cautions that the team's claims could be wrong. "Despite the seductiveness of the conclusions, I'm skeptical about the interpretations," he says. Harrison thinks that Rukwapithecus, the claimed ape ancestor, could represent a primate that actually predates the ape-monkey split and that the dental features used to identify it as an ape are "questionable"; and he argues that Nsungwepithecus might not even be a primate, but rather a suiform artiodactyl, a piglike, hoofed animal. "Similar cases of mistaken identity have occurred in the past," Harrison says, including claims for Miocene primates in East Africa that turned out to be artiodactyls and other nonprimates.