Learning about the history of Australopithecus, the ancient ancestors of modern man, is no small challenge. Few remains have ever been found and even when they are, they are usually too old for carbon dating to be very accurate.

For a long time, the oldest known Australopithecus was ‘Lucy’. Lucy was found in Ethiopia in 1974 and the remains were dated at 3.2 million years old. Now a skeleton named Little Foot, found in South Africa 21 years ago, appear to be older than Lucy by almost half a million years.

Little Foot’s bones were dated, using a sophisticated new dating method, to 3.67 million years ago. Additionally, stone tools found in the same area have been dated to 2.18 million years ago, making them some of the oldest in the world.

Little Foot was discovered by Ronald Clarke, professor in the Evolutionary Studies Institute at the University of the Witwatersrand. According to Clarke, the skeleton represents Australopithecus prometheus, which is very different from Australopithecus afarensis.

“It demonstrates that the later hominids, for example, Australopithecus africanus and Paranthropus did not all have to have derived from Australopithecus afarensis. 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,” said Clarke in a statement.

Previous dates for the Little Foot remains have ranged from 2 to 4 million years, which is roughly the time that Australopithecus is thought to have lived on Earth. The estimate of 3 million years was the preferred date among palaeontologists who had worked on the site, according to Darryl Granger of the university of Purdue.

The dating technique used for Little Foot was pioneered by Granger using technology originally designed for the analysis of solar wind samples by NASA’s genesis mission.

The technique, known as isochron burial dating uses radioisotopes within rock samples found with the remains to determine how long the rocks have been underground, or sheltered from sunlight.

When the rocks are exposed to cosmic rays, the radioactive isotopes aluminum-26 and beryllium-10 are created in quartz within the rock. After they are buried or hidden within a cave, the isotopes decay at known rates and can be measured. The margin of error for the testing is 160,000 years for Little Foot and 210,000 years for the stone tools which is relatively small given the timelines involved.

“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 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,” said Granger.

This was Grangers second attempt at dating the remains. The first attempt was not well received because it could not account for the possibility of earlier excavation and reburial within the cave.

“The original date we published was considered to be too old, and it wasn’t well received. However, dating the 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,” he said.

The full results of the work are expected to be published in the journal Nature.