Fossils of early human ancestors in the so-called ‘Cradle of Humankind’ in South Africa may be more than a million years older than previously thought, according to a new study.
The Sterkfontein Caves in Johannesburg contain more than a third of the world’s early hominid fossils – crucial links in the evolutionary chain to modern humans.
Sterkfontein was made famous by the discovery of the first adult Australopithecus, an ancient hominin, by Robert Broom in 1936.
It had previously been theorised that the Australopithecus-bearing cave sediments were between 2 million and 2.5 million years old.
However, new analysis has revealed that the sediments date from about 3.4 to 3.7 million years old, placing these fossils toward the beginning of the Australopithecus era, rather than near the end.
This would make them older than the world’s most famous Australopithecus fossil called Lucy, also known as Dinkinesh, which is 3.2 million years old.
The ‘Cradle of Humankind’ is a UNESCO World Heritage Site comprising a variety of fossil-bearing cave deposits, including at Sterkfontein Caves.
Since the first Australopithecus discovery in 1936, hundreds of Australopithecus fossils have been found there, including a famous pre-human skull, known as ‘Mrs Ples’, and a nearly complete skeleton known as Little Foot.
Paleoanthropologists and other scientists have studied Sterkfontein and other cave sites in the Cradle of Humankind for decades, to try to shed light on human and environmental evolution over the past 4 million years.
The majority of Sterkfontein’s Australopithecus fossils have been excavated from an ancient cave infill called ‘Member 4’ – the richest deposit of Australopithecus fossils in the world.
The age of Member 4 has been disputed for more than half a century, with estimates ranging from two million years – younger than our genus Homo – back to about three million years.
The new study was led by Darryl Granger, a professor of earth, atmospheric, and planetary sciences in Purdue University’s College of Science, who specialises in dating geologic deposits.
As a doctoral student, he devised a method for dating buried cave sediments that is now used by researchers all over the world.
His previous work at Sterkfontein dated the Little Foot skeleton to about 3.7 million years old, but scientists are still debating the age of other fossils at the site.
Previous dating of Member 4 has been based on dating calcite flowstone deposited in the cave, but observations show that the flowstone is actually younger than the cave fill and so it underestimates the age of the fossils.
A more accurate method is to date the actual rocks in which the fossils were found.
Granger and his team used a method called accelerator mass spectrometry to measure so-called cosmogenic nuclides in the rocks.
Cosmogenic nuclides are extremely rare isotopes produced by cosmic rays —high-energy particles that constantly bombard the earth.
These incoming cosmic rays have enough energy to cause nuclear reactions inside rocks at the ground surface, creating new, radioactive isotopes within the mineral crystals.
An example is aluminum-26 – aluminum that is missing a neutron and slowly decays to turn into magnesium over a period of millions of years.
Since aluminum-26 is formed when a rock is exposed at the surface, but not after it has been deeply buried in a cave, researchers can date cave sediments (and the fossils within them) by measuring levels of aluminum-26 in tandem with another cosmogenic nuclide, beryllium-10.
They used this method, together with maps of the cave deposits and knowledge of how cave sediments accumulate to determine the age of the Australopithecus-bearing sediments at Sterkfontein.
‘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,’ said Professor Dominic Stratford, director of research at the caves and one of the co-authors of the paper.
The study also overturns the long-held idea that South African Australopithecus is a younger offshoot of East African Australopithecus Afarensis.
‘Younger hominins, including Paranthropus and our genus Homo appear between about 2.8 and 2 million years ago,’ Stratford explained.
‘Based on previously suggested dates, the South African Australopithecus species were too young to be their ancestors, so it has been considered more likely that Homo and Paranthropus evolved in East Africa.’
The study, published in the journal PNAS, place Australopithecus ‘front and centre’ in the history of early human evolution, according to the researchers.
Most species of Australopithecus were diminutive – usually standing 3 ft 11 in to 4 ft 7 in tall. The most famous Australopithecus, ‘Lucy’, who was discovered in 1974 in Ethiopia – was even smaller at 3.5 feet.
Lucy was an Australopithecus afarensis and lived between 3.85 and 2.95 million years ago.
‘The redating of the Australopithecus-bearing infills at the Sterkfontein Caves will undoubtably re-ignite the debate over the diverse characteristics of Australopithecus at Sterkfontein – and whether there could have been South African ancestors to later hominins,’ said Prof Granger.
‘Sterkfontein has more Australopithecus fossils than anywhere else in the world. But it’s hard to get a good date on them.
‘People have looked at the animal fossils found near them and compared the ages of cave features like flowstones and gotten a range of different dates.
‘What our data does is resolve these controversies. It shows that these fossils are old – much older than we originally thought.’