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When a rock is on the surface, it builds up these isotopes.
When it is buried or deposited in a cave, the isotopes decay at known rates.
"If we had only one sample and that rock happened to have been buried, then re-exposed and buried again, the date would be off because the amount of radioisotopes would have increased during its second exposure," he said.
"With this method we can tell if that has happened or if the sample has remained undisturbed since burial with the fossil.
It turns out it was a good idea after all." Granger's original attempt was the first time aluminum-26 and beryllium-10 radioisotopic dating had been used to determine the age of a fossil.
He developed the method in 1997 and first used it to study changes in mountains, rivers and other geological formations.
The burial dating relies on measuring radioactive isotopes aluminum-26 and beryllium-10 in quartz within the rock.
These isotopes are only created when the rock is exposed to cosmic rays.
It is thought that Australopithecus is an evolutionary ancestor to humans that lived between 2 million and 4 million years ago.
"We have only a small number of sites and we tend to base our evolutionary scenarios on the few fossils we have from those sites.
This new date is a reminder that there could well have been many species of Australopithecus extending over a much wider area of Africa." There had not been a consensus on the age of the Little Foot skeleton, named for four small foot bones found in a box of animal fossils that led to the skeleton's discovery.
— A skeleton named Little Foot is among the oldest hominid skeletons ever dated at 3.67 million years old, according to an advanced dating method.
Little Foot is a rare, nearly complete skeleton of Australopithecus first discovered 21 years ago in a cave at Sterkfontein, in central South Africa.