Ape's fossilised teeth help fill evolutionary gap

An empty chapter in the long story of human origins can now be written with the discovery of a few fossilised teeth of a gorilla-like ape that lived more than 10 million years ago. The teeth are among the first key clues that could allow scientists to begin filling in the picture of how and when gorillas split from the human lineage of ancestral apes.

Scientists discovered the teeth in the Afar rift valley of Ethiopia, a region that has provided some of the best-known and most important fossils of the ape-like hominids leading to the evolutionary descent of Man. A joint team from Ethiopia and Japan found nine teeth of members of the large ape which they have called Chororapithecus abyssinicus. The similarity of the teeth to those of present-day gorillas suggest that the ape was an early ancestor - or near-ancestor - of modern gorillas.

The first specimen, a canine tooth, was found in 2006 and the eight molars in 2007. They are described today in the journal Nature.

"It was our last day of field survey in February 2006, when our sharp-eyed field assistant, Kampiro Kairente, found the first ape tooth," said Berhane Asfaw of the Rift Valley Service in Addis Ababa.

The scientists analysed the molars using three-dimensional microscopic techniques which suggested the apes ate fibrous food such as stems and leaves, as do modern-day gorillas.

Another team member, Dr Gen Suwa of the University of Tokyo, said: "It does show some telling signs of gorilla-like molar structure. If it's not a gorilla relative, then it's something very similar to what an early gorilla must have looked like."

Gorillas are unique among apes in that their molar teeth are adapted to grinding tough plant material which they have to be able to eat and digest because of their large size; chimps can rely on softer fruit and leaves.

The teeth were unearthed in soil sediments that have been dated as being between 10 million and 11 million years old. With an age of more 10 million years, Chororapithecus must have predated the origin of the human lineage, which is believed to have split off from the apes at least eight million years ago.

But the discovery of such an old gorilla-like ape suggests that the split between humans and other apes may have been even earlier, the scientists say.

"Most molecular and DNA studies have concluded that humans and gorillas had split by at least eight million years ago, and humans and chimps by five to six million years ago," they said. "Chororapithecus indicates a reconsideration of this assumption is needed."

Other ancient bones, those of monkeys, horses and early hippos, have also been unearthed at the site. "The big ape is the most common species here, and we don't get the three-toed horses abundant elsewhere," Dr Suwa said. "So this was probably a forest close to water."