Scientists have unearthed the fossil of an ancient creature, half-human, half-ape, a discovery that could give paleoanthropologists a missing link in the evolutionary history of the human being.

An analysis of the fossil, which was discovered in South Africa, has revealed to paleontologists that there could be a hitherto unknown member in the human family tree.

According to Lee Berger of the University of Witwatersrand in Johannesburg, who analysed the skull, pelvis, hands and feet of the new species, Australopithecus sediba may be a leading candidate for an ancient human ancestor.

If this theory is accepted, it could the fossil of Australopithecus sediba will get a pride of place in the history of human evolution.

Berger says the new species, whose fossils were unearthed three years ago, is the most plausible ancestor of archaic and modern humans. The researchers have published five papers in journal Science analysing the nearly 2 million years old fossils.

The papers disclosed a curious set of attributes. While some of them were found in Apes and earlier Australopithecus fossils, others were thought to be belonged to Homo erectus and its descendants, including modern humans. Homo erectus was the tall, thin-boned hominin that emerged around 2 million years ago in eastern Africa and colonized Europe and Asia.

Palaeoanthropologists were left unsure by this mix of features about how Australopithecus sediba relates to other ancient human relatives. Berger and his team proposed that the species may have evolved into Homo erectus. However, many other researchers are doubtful of that claim.

Berger believes that Australopithecus sediba could replace Homo habilis, the famous tool-making fossil found by previously Louis and Mary Leakey, as the most plausible link between the australopithecenes and the human species.

Fossils discovered in 2008 from the Malapa site in South Africa, a series of exposed ancient caves, establish that Australopithecus sediba were about 4 feet tall.

According to the researchers, the mosaic of features indicates that Australopithecus sediba was at an evolutionary transition point between the tree-climbing australopithecines and upright-walking, often tool-using members of the genus Homo, which includes Homo sapiens.

Their brains were less than one-third the size of a modern human being's, but features preserved on their inner skull, the team suggests, resemble precursors to specialized brain areas found in modern humans.

They also had hands capable of using tools, but still strong enough to grasp tree branches. They had a more curved pelvis that would have theoretically accommodated a bigger brain on its way through the birth canal, even though they still had small heads.

The unearthed fossils suggest that the ancient creatures had fallen into a cave around 1.97 million years ago and got swept into a sediment that fossilized their bones. Berger's 9-year-old son who had accompanied his father to a dig site stumbled upon the fossils when he was chasing his dog. He took the fossil to his father, who recognized a collarbone in the block of stone.

Though the mix of primitive and modern features led Berger and colleagues to suggest that A. sediba could be an ancestor to the genus Homo, the age of the fossils presented a problem.

Isotopic and magnetic dating showed the fossils were 1.977 million years old, about 300,000 years younger than a Homo habilis fossil that should have been their junior.

According to scientists, the discovery is of importance even if Australopithecus sediba is not a direct ancestor to humans. They are evidence that a ferment of evolutionary experimentation was going on at the time, out of which the human lineage somehow emerged, said Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York, according to the New York Times.

“If you take sediba as a metaphor for evolutionary change, it is a whole lot more powerful than the claim for direct ancestry,” Tattersall said.