By Dr David Whitehouse
BBC News Online science editor
Australopithecus fossils from caves in South Africa may have been buried up to four and a half million years ago - as much as one million years earlier than previously thought.
Now a million years older
Australopithecus is an important hominid - human ancestor - that demonstrates the transition from ape-like features to human ones. Its kind were first discovered in East Africa.
Researchers used a technique that measured the decay of radioactive elements formed in sediments when the original specimen would have been on the surface but which would then have declined as the fossil was buried.
The new dates make the South African hominids as old as similar specimens found in the famous rift valley system of eastern Africa, forcing a revision of how far scientists believe Australopithecus ranged.
The South African fossils are from the caves and quarries at Sterkfontein, 50 kilometres northwest of Johannesburg; they are some of the richest hominid fossil sites in the world.
About 500 specimens have been recovered there since the 1936 discovery of the first adult Australopithecus.
The fossils are encased in a calcified accumulation of rock and surface soil that formed as debris dropped into the cave from the roof.
Finding a date for the age of the fossils has been problematical as different techniques have yielded conflicting results.
The latest dating technique involves measuring the radioactive decay of different forms, or isotopes, of aluminium and beryllium found in the cave sediments.
Writing in the journal Science, the researchers say they looked for so-called cosmogenic isotopes that are created when quartz minerals are bombarded by cosmic rays at the Earth's surface.
Later, when the rocks are buried, production of the isotopes ceases. Measuring their slow decay then indicates how much time has passed since the rock lay near the surface.
This method makes it possible to date specimens found in caves, which usually lack easily dated volcanic or sedimentary deposits.
The results indicate that the Sterkfontein fossils are of a similar age to similar specimens found in East Africa, and are consequently some of the earliest examples of hominids.
"We found that the [Sterkfontein fossil] was between 3.5 and 4.5 million years old," said Darryl Granger, from Purdue University, West Lafayette, US.
"That's admittedly quite a large window of possibility, but even if it's on the young side, it still puts Australopithecus in southern Africa far earlier than expected."