The new date places an almost complete skeleton of Australopithecus prometheus from the Sterkfontein cave in South Africa as an older relative of famous Lucy – a 3.18-million-year-old specimen of Australopithecus afarensis that was found in Ethiopia.
Little Foot (Australopithecus prometheus) in the Sterkfontein cave, central South Africa. Image credit: Purdue University.
Little Foot, also known as StW 573, was discovered in 1994 in a cave at Sterkfontein in central South Africa.
The specimen was named for four small foot bones found in a box of animal fossils that led to the skeleton’s discovery.
There has been much confusion surrounding the dating of the Little Foot. Previous dates ranged from 2 million to 4 million years old, with an estimate of 3 million years old preferred by anthropologists familiar with the Sterkfontein cave.
A new dating method has found that Little Foot is 3.67 million years old (± 160,000 years).
The method, called isochron burial dating, uses radioisotopes within several rock samples surrounding a fossil to date when the rocks and the fossil were first buried underground.
It 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. 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. The ratio of the remaining aluminum-26 and beryllium-10 can be used to determine how long the rock has been underground.
Forensic facial reconstruction of another Australopithecus species – A. afarensis. Image credit: Cicero Moraes / CC BY-SA 3.0.
“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,” said Prof Darryl Granger of Purdue University, who is the lead author of the paper published in the journal Nature.
“With this method we can tell if that has happened or if the sample has remained undisturbed since burial with the fossil. It is expensive and a lot of work to take and run multiple samples, but I think this is the future of burial dating because of the confidence one can have in the results.”
This was Prof Granger’s second attempt at dating the fossil through the burial dating technique and a chance to prove its abilities.
In 2003 he estimated the fossil to be around 4 million years old, give or take a few hundred thousand years. The dates were questioned because this earlier work could not show whether the burial dates were compromised by earlier burial elsewhere in the cave.
“The original date we published was considered to be too old, and it wasn’t well received,” Prof Granger said.
“However, dating the Australopithecus prometheus ‘Little Foot’ fossil as 3.67 million years old actually falls within the margin of error we had for our original work. It turns out it was a good idea after all.”
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Darryl E. Granger et al. New cosmogenic burial ages for Sterkfontein Member 2 Australopithecus and Member 5 Oldowan. Nature, published online April 01, 2015; doi: 10.1038/nature14268
