Newfound Species Pushes Back Human-Ape Split

Recently unearthed fossils belonging to a new ape species suggest the lineages leading to humans and gorillas split several million years earlier than previously thought.

Found in Ethiopia, the 10 million-year-old fossilized teeth resemble those of modern gorillas and appear specialized for eating fibrous foods such as stems and leaves.

"If it's not a gorilla relative, then it's something very similar to what an early gorilla must have looked like," said study leader Gen Suwa, a paleoanthropologist at the University of Tokyo.

Dubbed Chororapithecus abyssinicus, the new species is the oldest primate known to be directly related to living African gorillas.

The finding, detailed in the Aug. 23 issue of the journal Nature, suggests humans and gorillas last shared a common ancestor at least 10 million years ago. It could also push back the time when the lineages of humans and chimpanzees split.

Recalibrating the molecular clock

Most molecular studies have concluded that humans and gorillas diverged by about 8 million years ago, and that humans and chimpanzees split some 5 to 6 million years ago. However, these conclusions were based on the assumption that the lineages leading to humans and orangutans split about 15 million years ago.

Since each "tree branch" is placed on the evolutionary tree in relation to the other branches, the time scale is relative, making the human-orangutan split critical to the timing of other changes.

"The molecular data themselves do not provide ages on the branches of the lineages. You have to calibrate the molecular distances, so it's kind of like a relative scale," said Tim White, an anthropologist at the University of California, Berkeley, who was not involved in the study.

The new fossils could potentially serve as even better calibrators for the molecular scale than the current orangutan ancestor specimen.

"If it is accepted by paleontologists that this is indeed a fossil which is very close to the split of humans and gorillas, then it would become a very useful calibration point to [look] backwards in time, towards orangutans, and also forward in time" toward the human and chimpanzee split, said Sudhir Kumar, a researcher at Arizona State University whose genetic analyses have helped determine the time of the human-chimp split.

However, scientists will first have to determine if Chororapithecus lived after the gorilla lineage split from that of humans and chimps, or if it lived right before that point.

"Unless that question is answered, it is very hard to place whether these fossils are telling us about the gorilla-human divergence, or about the divergence of the ancestors of humans and gorillas from orangutans," said Kumar, who was involved in the study. "That question is not answered yet."

The original motherland

The new discovery also supports the idea that, like humans, gorillas and chimpanzees have primary roots in Africa, and not in Europe or Asia, as others have suggested.

"Chororapithecus suggests, once again, that Africa was the place of origin of both humans and modern African apes," the authors said in a prepared statement.

The new fossils also help anthropologists with a data problem, White said.

"So many people have been saying there's a gap in the African fossil record [from that time], and these fossils begin to fill that gap," White told LiveScience.

Even though the fossil record of human evolution is still patchy, it is much better than that of the great apes. Very few fossils have surfaced for gorilla evolution for the past 6 million years, and the first ever chimpanzee fossil was found only in 2005.

"The human fossil line is really quite well-known between 6 million years ago and today," White said. "It's basically a black hole when it comes to fossils of the African apes themselves."