Fossil find stirs human debate

100s of hominids have been found at Sterkfontein

The fossil of an early human-like creature (hominid) from southern Africa is raising fresh questions about our origins.

Remains from the Sterkfontein Caves near Johannesburg suggest our ancestors were less chimp-like than we thought.

The revelation follows the discovery of missing bones from a 3.5 million-year-old skeleton found in 1998.

This Sterkfontein individual was a climber in the trees and bipedal on the ground

Dr Ron Clarke

Fragments of pelvis, upper leg, ribs and backbone have recently been dug out of the rock, allowing scientists to piece together its gait.

The anatomy of the hominid, a member of the genus Australopithecus, raises some interesting questions.

Its bone structure shows it did not walk like modern chimps, using the knuckles of its hands.

It probably walked on two legs when it was on the ground but spent much of the time climbing trees, says Dr Ron Clarke, of the University of the Witwatersrand, who discovered the fossil.

Tool-making

Dr Clarke goes further. He argues that the fact the hominid was not a knuckle-walker suggests chimps and humans are not as closely related as we thought.

Knuckle-walking and vertical climbing - up and down tree trunks - are a specialisation of chimps and gorillas after humans split off from them

Dr Robin Crompton

It pushes the last common ancestor of chimps and humans much further back in history, he says.

Dr Clarke sets out his position in the South African Journal of Science, which publishes the latest data.

"My conclusion from the limb proportions and the morphology of the foot and of the hand is that this Sterkfontein individual was a climber in the trees (using its powerful thumb in a vice-like grip) and bipedal on the ground," he says.

"It would appear, therefore, that the strong opposable thumb evolved in the human ancestral stock for grasping branches. Then, in the mainly terrestrial subsequent descendants in the form of Homo, it was to prove useful for tool-making and manipulation.

"The suggestion in reconstructions and in the scientific literature that human ancestors were transformed into an upright position from a knuckle-walking ancestor is not supported by this new and important addition to the fossil record."

Fresh debate

Other experts in human evolution are more circumspect. Professor Chris Stringer of London's Natural History Museum says the idea that humans and chimps derive from a knuckle-walking common ancestor is "not a majority view".

The peculiar gait of chimps and gorillas could have developed after the three lines diverged, he says.

Dr Robin Crompton, of the University of Liverpool, agrees. He says there is "very strong" genetic evidence that we are closely related to chimps (and bonobos).

"It is likely that the common ancestor of the African apes, including ourselves, was arboreal," he told BBC News Online.

"In my view, knuckle-walking and vertical climbing - up and down tree trunks - are a specialisation of chimps and gorillas after humans split off from them."

Sterkfontein is probably the richest site on Earth for the fossils of early hominids, and the ancient cave system is now part of a World Heritage Site.

Some 600 hominid fossils from the Sterkfontein Caves have now been collected and classified.

The early humans they represent are thought to have fallen to their deaths in the caves when the limestone complex first broke the surface.