Famous Sterkfontein Caves deposit 1 million years older than previously thought

The majority of Sterkfontein's wealth ofAustralopithecusfossils has been excavated from an ancient cave infill called 'Member 4' -- the richest deposit ofAustralopithecusfossils in the world. Over the last 56 years of Wits-led research at Sterkfontein, the age of Member 4 at Sterkfontein have remained contested, with age estimates ranging from as young as about 2 million years ago, younger than the appearance of our genusHomo, back to about 3 million years.

New research presented in a paper published in the journalPNASre-evaluates the age ofAustralopithecusfrom Member 4 at Sterkfontein together with the Jacovec Cavern, which contains a few additional hominin fossils in a deeper chamber in the cave.

"The new ages range from 3.4-3.6 million years for Member 4, indicating that the Sterkfontein hominins were contemporaries of other earlyAustralopithecusspecies, likeAustralopithecus afarensis, in east Africa," says Professor Dominic Stratford, director of research at the caves, and one of the authors on the paper.

The new ages are based on the radioactive decay of the rare isotopes aluminum-26 and beryllium-10 in the mineral quartz. "These radioactive isotopes, known as cosmogenic nuclides, are produced by high-energy cosmic ray reactions near the ground surface, and their radioactive decay dates when the rocks were buried in the cave when they fell in the entrance together with the fossils," says Professor Darryl Granger of Purdue University in the United States and lead author on the paper.

Previous dating of Member 4 has been based on dating calcite flowstone deposits found within the cave fill, but careful observations show that the flowstone is actually younger than the cave fill and so it underestimates the age of the fossils.

"This re-assessment of the age of SterkfonteinMember 4 Australopithecus化石的作用有重要意义South Africa on the hominin evolution stage. Younger hominins, includingParanthropus和我们的属Homoappear between about 2.8 and 2 million years ago. Based on previously suggested dates, the South AfricanAustralopithecus物种太年轻是他们的国民大会tors, so it has been considered more likely thatHomoandParanthropusevolved in East Africa," says Stratford.

The new dates show thatAustralopithecusexisted at Sterkfontein almost a million years prior to the appearance ofParanthropusandHomo, providing more time for them to evolve here, in the Cradle of Humankind, and placing the hominins from this site front and center in the history early human evolution.

"This important new dating work pushes the age of some of the most interesting fossils in human evolution research, and one of South Africa's most iconic fossils, Mrs Ples, back a million years to a time when, in east Africa, we find other iconic early hominins like Lucy," says Stratford.

"The redating of theAustralopithecus-bearing infills at the Sterkfontein Caves will undoubtably re-ignite the debate over the diverse characteristics ofAustralopithecusat Sterkfontein, and whether there could have been South African ancestors to later hominins," says Granger.