‘Little Foot’ fossils found to be older than ‘Lucy’

A study by researchers from South Africa, the US, Canada and France has found that the famous Australopithecus fossil skeleton known as ‘Little Foot’ is around 1.47 million years older than was initially estimated and might therefore potentially represent a direct human ancestor.

The finding, made possible by using an advanced dating method called isochron burial dating, has dated the Little Foot skeleton to being 3.67 million years old (± 0.16 million years) – making it one of the oldest hominid skeletons that has so far been dated.

It suggests that Little Foot, of the species Australopithecus prometheus, must have lived in Africa at roughly the same time as the famous Australopithecus afarensis skeleton ‘Lucy’ that was excavated in Ethiopia and dated as being 3.2 million years old.

This stunning announcement was made on 2 April at the University of the Witwatersrand, where two of the authors of the study, Prof Darryl Granger from Purdue University in the US and Profs Ronald Clarke and Kathy Kuman from the University of the Witwatersrand (who discovered the first Little Foot bones in 1994), briefed media on the implications of the finding.

“The dating of Little Foot has always been controversial and we really wanted to know the true age of the fossil,” said Granger.

The cave infills at the well-known Sterkfontein caves in the Cradle of Humankind World Heritage Site in South Africa, where Little Foot was excavated, contain one of the richest assemblages of Australopithecus fossils in the world, but the chronology of the site has remained controversial owing to the complex history of the cave infilling.

“Initial cosmogenic nuclide burial dating of the sediment suggested an age near 4 million years old, while uranium-lead dating of the calcite flowstones indicated a much younger age of 2.2 million years. Although Clarke had recognized as early as 2002 that the flowstones are younger than the skeleton, the discrepancy left the age of Little Foot in doubt,” said Granger. “Subsequently it was determined beyond doubt that the dated flowstones had indeed formed within voids opened by collapse of the cave sediments and must be younger than the skeleton – which opened the possibility that the sediment and the skeleton within it could be far older than the flowstones.”

Granger and Dr Ryan Gibbon from Wits worked extensively on cosmogenic dating at the Sterkfontein caves and with the method of isochron burial dating (which analyses the radioactive decay of Aluminium-26 and Berilium-10), 11 rock and sediment samples from the excavation site were very accurately dated thanks to the newly developed gas-filled magnet at the Purdue University’s PRIME lab, which determined Little Foot is actually 3.67 million years old.

The resulting collaborative study, titled “New cosmogenic burial ages for Sterkfontein Member 2 Australopithecus and Member 5 Oldowan”, which was published this week in Nature, could have important implications about the evolutionary relationships between mankind’s earliest ancestors.

Clarke explained the implications by stating that up until the new finding it was assumed that all hominids must have had a common ancestor in Australopithecus afarensis (Lucy) or possibly Kenyanthropus platyops (excavated in 1999 in the Rift Valley in Kenya), which had similar characteristics and was also dated to around 3.2 million years old.

“These hominids were the oldest that had been found and it was therefore assumed they represent potential common ancestors for later hominid species, including humans,” said Clarke. “What this finding means, is that Little Foot (A. prometheus) is even older than, and contemporaneous with A. afarensis and K. platyops. In other words, it cannot be assumed anymore that all later hominids must be descendant from these two species and A. prometheus is also a potential common ancestor for later hominids and humans.”

For the same study, isochron burial dating was also done on the earliest stone tools found at Sterkfontein, which dated the tools to 2.18 ± 0.21 million years old – placing them in the Oldowan at a time similar to that found elsewhere in South Africa at Swartkans and Wonderwerk. This proves that hominids in South Africa were using stone tools at least as far back as 2 million years ago.

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