The Sterkfontein caves, South Africa are a UNESCO World Heritage site often referred to as the “Cradle of Humankind”. Scientists have studied there for many decades as this location is filled with the richest deposits of early hominin Australopithecus fossils worldwide. Now researchers have discovered that the fossils may be older than we once thought. One million years older than previously estimated in fact. 

The age of Member 4, an ancient cave infill where the majority of Sterkfontein's Australopithecus have been found, had previously been contested, with dates ranging from 2-3 million years ago. 

Now, scientists developed a new dating method and in a new study in PNAS, reveal that Australopithecus actually lived at the Sterkfontein cave almost a million years before the appearance of genera Homo and Paranthropus. This indicates that these fossils are actually older than Lucy, the world’s most famous Australopithecus.

“The new ages range from 3.4-3.6 million years for Member 4, indicating that the Sterkfontein hominins were contemporaries of other early Australopithecus species, like Australopithecus afarensis, in east Africa,” said Professor Dominic Stratford, director of research at the caves, in a statement. 

Purdue University geologist and lead author Professor Darryl Granger had previously developed a method for dating buried cave sediments and helped date Little Foot, an incredibly rare nearly complete Australopithecus, to 3.7 million years old. He and a team of scientists analyzed the rest of the Australopithecus-bearing cave sediments and have now dated them to 3.4 to 3.7 million years old rather than the previous 2-2.5 million years. This new information indicates that these fossils are actually from the beginning of the Australopithecus era. 

“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 Stratford.

So how did previous scientists get the date so wrong? 

Well, the cave system itself is very complex and deep. Getting accurate dates on the fossils found in this area can be very tricky, as bones and rocks tumbled to the bottom of a deep hole in the ground. As such, scientists normally use other animal fossils around the bones and calcite flowstone deposits for dating found fossils. Unfortunately, young flowstone can be deposited in old sediment when bones shift in the caves. This is what seems to have happened in this case and the reason why the dating was so underestimated. 

“Sterkfontein has more Australopithecus fossils than anywhere else in the world,” Granger said in another statement. “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.”

In a newer technique, the science team used an accelerator mass spectrometer to measure radioactive nuclides in the rocks and conducted a geologic mapping to get a detailed understanding of how cave sediments accumulated. All this data helped determine the age of the Australopithecus-bearing sediments.

The team looked at the cosmogenic nuclides – these are extremely rare isotopes produced by cosmic rays. Cosmic rays can cause nuclear reactions inside the rocks at the ground surface, which can cause new radioactive isotopes in the mineral crystals. These radioactive isotopes can then be used to date the cave sediments. 

This data along with the cave maps can help uncover the decades-long confusion that occurred when the excavations in the 1930s and '40s mixed all the animal fossils together. 

"What I hope is that this convinces people that this dating method gives reliable results,” Granger said. “Using this method, we can more accurately place ancient humans and their relatives in the correct time periods, in Africa, and elsewhere across the world.”

“This re-assessment of the age of Sterkfontein Member 4 Australopithecus fossils has important implications for the role of South Africa on the hominin evolution stage," added Stratford. 

"Younger hominins, including Paranthropus and our genus Homo appear between about 2.8 and 2 million years ago. 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."

This work is incredibly exciting, as it will help increase the scientists’ understanding of the living landscape of the time.

“The redating of the Australopithecus-bearing infills at the Sterkfontein Caves will undoubtedly 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 Granger.