Fossils reveal the evolutionary split between monkeys and apes

Scientists have discovered fossils of two newfound primate species, dating back to 25 million years ago. One fossil belongs to the group that contains great apes (hominids), while the other is from the group that includes Old World monkeys. The discovery may mark the moment when our primate ancestors first diverged into the two lineages we have today.

As humans, we are naturally curious about our own evolutionary history. So researchers have long sought to pinpoint the exact moment when primates split into the two distinct groups, monkeys and apes. Evidence from DNA studies suggests that the divergence from a common ancestor happened between 25 and 30 million years ago. But the oldest known fossils of both groups has only been dated as far back as 20 million years ago.

For the past 10 years, researchers from the U.S. and Tanzania have been digging for fossils in Tanzania's Rukwa Rift Basin. In the 2011-2012 season, a team lead by Ohio University vertebrate paleontologist Nancy Stevens found two new primate species: ape ancestor Rukwapithecus fleaglei and the Old World monkey Nsungwepithecus gunnelli. ScienceNOW explains:

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 Rukwa Rift in Tanzania is part of a tectonic-plate boundary called the East African Rift. Here the Earth's crust is being pulled apart, causing changes to the landscape. These tectonic changes, along with environmental and climatic changes, probably helped facilitate the evolutionary split, though the "how" of it all is still a bit of a mystery, researchers say.

More work is needed to figure out why the groups diverged. And further research is also apparently needed to even prove the two new species are what they seem to be. At least one scientist, anthropologist Terry Harrison, is unconvinced that Nsungwepithecus is a primate and thinks that Rukwapithecus could actually be a species that predates the ape-monkey split.

Read more about the new discovery at ScienceNOW and LiveScience. You can check out the accompanying study in Nature.

Top image: Artist's concept of Rukwapithecus fleaglei (front) and Nsungwepithecus gunnelli (back), via Mauricio Anton.