African fossils hailed as the 'Rosetta Stone of humanity'

Skeletons reveal ancestor that walked on two legs but could also live in the trees

A remarkably well preserved series of fossilised skeletons have been unearthed from a cave in South Africa and identified as a new ancestral species of ape-like hominid that could have been the direct ancestor of humans.

The species lived about 1.9 million years ago, walked on two legs and shared many other human features, but it also retained clear ape-like traits such as very long arms which showed that it had not yet made the complete transition from a life in the trees.

Researchers claimed yesterday that the species, named Australopithecus sediba, possesses such a mosaic combination of ape-like and human-like traits that it might belong to the group of "apemen" who evolved into the Homo genus – the human family.

The Homo genus came into existence around two million years ago, possibly evolving from the Australopithecines, the "southern apes" that lived in sub-Saharan Africa before this period. However, the paucity of the fossil record for this critical period in human history has cast doubt on exactly how the ape-to-human transition occurred. Now, however, with the discovery of four skeletons belonging to Australopithecus sediba scientists believe they may have found the immediate ancestor of all subsequent species of Homo, from the primitive species such as Homo erectus and Homo habilis, to the anatomically modern Homo sapiens.

"We do feel that possibly sediba might be a Rosetta Stone for defining for the first time just what the genus Homo is," said Professor Lee Berger of the University of Witwatersand in Johannesburg, the leader of the study.

"In our team's opinion they fill a critical gap, lying between the Australopithecines and, most probably, the early members of the genus Homo ... The skeletons are remarkable in their proportions and unseen before in the early hominid record," Professor Berger said.

Two of the partial skeletons, an adult female about 30 years old and an adolescent boy with a complete skull, are described in the latest issue of the journal Science. Studies on two further partial skeletons, another adult female and an infant, have yet to be published.

"These fossils give us an extraordinary detailed look into a new chapter of human evolution, and provide a window into a critical period when hominids made the committed change from dependency on life in the trees to life on the ground. Australopithecus sediba appears to present a mosaic of features demonstrating an animal comfortable in both worlds," Mr Berger explained.

The scientists discovered the first skeleton in 2008 from a pit-like excavation at Malapa, near Johannesburg, which was once part of an ancient underground cave system some 50m deep. Professor Berger's nine-year-old son, Matthew, was the first to spot a human fossil sticking out of a lump of rock strewn on the ground nearby.

"I thought it was an antelope fossil but as I walked over to him, five metres away I realised he had a hominid clavicle [collarbone] sticking out of the rock. When I got to the rock and turned it over there was a hominid mandible and canine sticking out of the back," Professor Berger recalled.

Although the species has very long forearms like tree-dwelling apes, its hands and fingers are short and powerful, more like those of humans. They stood about 1.3m tall and their pelvis indicates they could walk and run easily on two legs. They had small heads and their brain was less than a third of the size of modern human brains. Their legs were long, but the anatomy of the foot was "primitive", the scientists said.

The remarkable preservation of the fossils occurred because the creatures had probably died after falling or climbing down a vertical "death shaft" leading to the underground cave system. Within days of their death, a sudden rainstorm and mudflow probably washed their bodies – along with the bones of sabre-toothed cats, hyenas, wild dogs and horses – into a deeper, waterlogged recess where they were quickly encased in calcified rock.

"We think there was some sort of calamity that caused all these fossils to be dragged down into the cave where they became trapped," said Paul Dirks of James Cook University in Australia, who dated the fossils at between 1.95 and 1.78 million years old.

1.9m years

Scientists at James Cook University dated the skeletons of Australopithecus sediba at 1.78-1.9m years old.