The 20 million-year-old evolutionary family tree that includes humans has a new member. The deserts of Ethiopia have yielded up evidence of a new species of great ape.
The fossil remains were meagre, just nine teeth, but they are reliably dated to 10-10.5 million years ago (mya) and suggest that the creature was very like a modern gorilla.
Details of the exciting find are published this morning in the journal Nature, and their discovery is likely to demand changes to the hominoid family tree.
The report's lead author Gen Suwa from the University of Tokyo, Berhane Asfaw from the Rift Valley Research Service in Addis Ababa and colleagues discovered a canine tooth and eight partial molars from at least three and perhaps six or more individuals.
They were buried in the Chorora Formation at the southeastern end of the Afar rift in Ethiopia. Weather erosion exposed patches of sediment dating back to between 10-11mya, which allows the teeth to be dated with some confidence.
"These teeth are collectively indistinguishable from modern gorilla subspecies in dental size and represented proportions," the authors say.
Scientists search for ancient fossils so they can put dates on when the various members of the "great ape" family that includes orangutans, gorillas, chimpanzees, bonobos and modern humans last had common ancestors.
The current assumption is that the last common ancestor for all great ape species dates to between 18-20mya, and the best estimate for when gorillas formed a unique species is 6-8mya.
That assumption has now been turned on its head with the discovery of a new great ape that its discoverers from Japan and Ethiopia have called chororapithecus abyssinicus.
Acceptance of chororapithecus as a member of the gorilla family "would push back the gorilla species split to 10.5mya", the authors say. And this, they add, would be a minimum, with the probable split dating to 12mya.
Accurate dating of when various hominoid species arose is straightforward when fossils are available, with an example being the famous 3.2 million-year-old skeleton of "Lucy", a member of the australopithecus afarensis family. However, as the authors point out, the hominoid fossil record goes very thin between 7-12mya.
Comparative studies of DNA from the great ape family have been used to fill in the gaps. Differences in the DNA record allow scientists to spin the genetic clock backwards, backtracking in time to when the differences in DNA disappeared, thus indicating a common ancestor.
The research team was able to show that the 10.5mya teeth were virtually the same as those from a modern gorilla, and designed to chew down a "comparatively fibrous diet".