KEY POINTS:
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 one of the first key clues that could allow scientists to begin filling in the picture of how and when gorillas split off from the human lineage of ancestral apes.
Scientists discovered the teeth in the Afar rift valley of Ethiopia, where some of the best known and most important fossils of the ape-like hominids leading to the evolutionary descent of man have been found.
A joint team from Ethiopia and Japan found nine teeth of several 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 February 2006 and the eight molars were found in March 2007. They are described in a study published in the journal Nature.
"It was our last day of field survey in February 2006, our sharp-eyed field assistant, Kampiro Kairente, found the first ape tooth, a canine," said Berhane Asfaw of the Rift Valley Service in Addis Ababa.
"He picked it up and showed it to me, and I knew this was something new - Ethiopia's first fossil great ape," Dr Asfaw said.
Another member of the team, Gen Suwa of the University of Tokyo, described how desolate this particular region of the Rift Valley is in terms of fossilised remains. The team surveyed it on foot over a distance of more than 100 kilometres.
The scientists analysed the molar teeth using sophisticated three-dimensional microscopic techniques which suggested that the apes ate fibrous food such as stems and leaves, just like modern-day gorillas.
"It's a subtle distinction, but we've compared it with everything we could think of and it does show some telling signs of gorilla-like molar structure," Dr Suwa said.
"If it's not a gorilla relative, then it's something very similar to what an early gorilla must have looked like," he said. "The molars were as big as those of gorillas. Their diet probably included hard fruits and some fibrous plant materials such as stems."
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 of Chororapithecus were unearthed in a layer of soil sediments that have been dated as being between 10 million and 11 million years old.
At more than 10 million years, Chororapithecus must have predated the origin of the human lineage, which is believed to have split from the apes at least eight million years ago.
However, the discovery of such an old gorilla-like ape suggests that the split between humans and other apes may well have occurred even earlier, the scientists suggest.
"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 that a reconsideration of this assumption is needed."
Several other animal bones, such as monkeys, ancient horses and early hippos, have also been unearthed at the site, which was once a wetter, more verdant place than it is today.
"The big ape is the most common species here, and we don't get the three-toed horses which are abundant elsewhere. So it was probably a forest close to water," Dr Suwa said.
- Independent
