The oldest known fossils of an ape and a monkey have been uncovered, providing an intriguing glimpse of a crucial time in primate evolution.
The discoveries suggest that by 25 million years ago, two major groups of primates were distinct: one that today includes apes and humans and another that encompasses Old World monkeys such as baboons and macaques. Previous studies using living primates’ DNA suggested that ancient apes and Old World monkeys parted from a common ancestor between 25 million and 30 million years ago.
The new ape and monkey fossils, from Tanzania’s Rukwa Rift Basin, suggest that the evolutionary split between these primate lines must have occurred close to 30 million years ago, or perhaps even earlier, anthropologist Nancy Stevens of Ohio University in Athens and her colleagues conclude in the May 15 Nature.
Fossil finds since the 1800s have revealed that dozens of ape species inhabited Africa, Asia and Europe between 22 million and 5.5 million years ago. Fewer fossils of Old World monkeys have been found, but a handful of monkey species are known to have inhabited Africa around 20 million years ago.
“The period from 25 million to 30 million years ago is the least sampled interval in primate evolutionary history, with only three fossil primates known before our discoveries and five known now,” Stevens says.
Her team assigns a tooth-bearing lower right jaw to a new ape genus and species, Rukwapithecus fleaglei. The scientists classify a second find, a jaw fragment containing a tooth, as a new monkey genus and species, Nsungwepithecus gunnelli.
These animals lived 25.2 million years ago, based on age estimates of volcanic ash layers that sandwiched the Tanzanian fossils.
Rukwapithecus may not be a new ape genus, though. The newly discovered jaw appears to belong to Rangwapithecus, an ape genus previously known from East African fossils dating to around 17 million years ago, says anthropologist K. Christopher Beard of the Carnegie Museum of Natural History in Pittsburgh.
Even so, Beard says, the new report “makes a strong case that Old World monkeys and apes had already diverged 25 million years ago.”
But New York University anthropologist Terry Harrison isn’t so sure. The new Rukwapithecus jaw joins a cluster of fossils, including those categorized as Rangwapithecus, from ancient African primates that probably did not evolve into apes despite having some apelike jaw and tooth traits, Harrison says.
The tooth-and-jaw piece that Stevens’ group attributes to a monkey may instead come from an ancient form of pig or peccary, Harrison adds. In his view, the researchers need more fossils from the animal to tell whether it’s a primate.