The fossil remains found in a cave in South Africa could represent an evolutionary link between tree-dwelling apes and our earliest human ancestors to walk upright

Fossilised skeletons recovered from a deep underground cave in South Africa belong to a previously unknown species of human ancestor, scientists claim.

The partial skeletons of an adult female and a young male, aged 11 or 12, were found lying side by side in sediments that first covered their remains an estimated 1.9m years ago.

The individuals are thought to have fallen into the cave network through a fissure before being carried a few metres by mud or water into a subterranean pool, where they were gradually encased in rock.

The extraordinary remains are thought to represent a period of evolutionary transition between tree-dwelling apes and the earliest human ancestors, or hominids, to take their first tentative steps on two feet. Their position at the very root of our family tree has led scientists to claim that the skeletons will help define what it means to be human.

The remains were recovered alongside the fossilised bones of at least 25 other animals, including sabre-toothed cats, a hyena, a wild dog, several antelope and a horse, according to two reports in the journal Science. At the time the creatures died, the region was dominated by a grassy plain crossed by wooded valleys.

The discovery of the mass grave has led researchers to suggest that the ancient animals and the hominids fell into the cave network through "death trap" holes in the surface and were unable to escape. The skeletons were so well preserved that palaeontologists believe the two individuals fell into the cave together and were dead and buried within days or weeks.

The remains, found in the Malapa cave network at the Cradle of Humankind World Heritage Site 40km outside Johannesburg, have already triggered a row over their identity, because they share anatomical features with both early humans from the genus, Homo, and their ancient predecessors, the Australopithecines, or southern apes.

The skeletons have long arms similar to those of orang-utans, a trait shared with Australopithecines, which suggests they were adept at living in trees. But unlike other Australopithecines, they have long legs and a pelvis that is well adapted to walking upright. Analysis of the male's skull revealed small teeth and facial characteristics seen in early members of the genus Homo. Their brains were exceptionally small, around a third the size of a modern human's.

Given their long arm bones and other physical characteristics, lead scientist Lee Berger, a palaeontologist at the University of Witwatersrand in Johannesburg, assigned the remains to a new species, Australopithecus sediba. The word "sediba" means fountain or wellspring in Sotho, one of the official languages of South Africa, and was chosen because the species might be a direct ancestor of the genus Homo, and the point at which the story of modern humans begins.

According to Berger, Australopithecus sediba may be descended from the more primitive South African ape men, Australopithecus africanus, which lived more than 2m years ago and are known from the skeletons of the Taung child and Mrs Ples, recovered in South Africa in 1924 and 1947 respectively.

"These fossils give us an extraordinarily 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," said Prof Berger. "Sediba may very well be the Rosetta stone that unlocks our understanding of the genus Homo."

Other experts argue that the remains have been misclassified and belong to the genus Homo. "The transition to Homo continues to be almost totally confusing," Donald Johanson at Arizona State University in Tempe told Science magazine. Based on the thinness of the lower jaw and other bone features, Johanson is convinced Berger has it wrong: "It's Homo," he said.

Teams of palaeontologists will spend the next weeks and months poring over the fossils in the hope of building up a more complete picture of their postition in the human story. Berger said he hoped to retrieve DNA from the remains, which could add enormously to scientists' understanding.

"Any time we find remarkable fossils like this, in remarkable completeness, and of this quality, they are going to answer a great many questions in a very fragmented fossil record," he added. "As more fossils are recovered from the species, it is undoubtable that it is going to contribute enormously to our understanding of what was going on at that moment when the early members of the genus, Homo, emerged."

The fossils were discovered by Berger's nine-year-old son, Matthew, in a visit to the Malapa site in 2008. Since their recovery, researchers have carefully removed the fossils from the concrete-like sediments in which they were encased.

"These new fossils from Malapa return the spotlight to South Africa as a possible location for the presumed transition from Australopithecus to the genus Homo," said Chris Stringer, head of human origins at the Natural History Museum in London. "The fact that experts differ over whether to classify these specimens as Australopithecine or human indicates the mixed features that they display, and the fossils provide valuable clues to the evolutionary changes that led to the first members of the human genus."