'Cradle of Humankind' Fossils May Be a Million Years Older Than Previously Thought

The age of several ancient hominid bones discovered in South African caves may be much, much older than previously thought.

The Sterkfontein limestone cave system, located not far from Johannesburg, has produced so many prehistoric bones of the hominin genus Australopithecus over the past century that its location has been dubbed the Cradle of Humankind. This designation emphasizes the significance of this location for the study of human evolution.

The remains may now be approximately 4 million years old, making them older than the well-known Australopithecus afarensis individual Dinkinesh, often known as Lucy. This information comes from modern dating techniques.

"Sterkfontein has more Australopithecus fossils than anywhere else in the world," stated Darryl Granger, a geologist and geophysicist at Purdue University.

"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."

It's not precisely simple to date old relics, especially in caverns. Based on radiometric dating of the volcanic ash in the silt where Dinkinesh was discovered, her age was determined to be 3.2 million years ago; nevertheless, caves are more pure environments as volcanic ash doesn't fall there.

Based on the age of the calcite flowstone found inside the cave fill, previous estimations for the intricate Sterkfontein system. About two to five million years ago, it began to develop.

Flowstone can, however, develop over earlier material, as appears to have happened in Sterkfontein.

A cave infill known as Member 4 has yielded the majority of the Sterkfontein Australopithecus fossils. It is precisely what it sounds like: debris that filled in a hollow to form a sedimentary deposit, which in this case obscured but preserved ancient hominid bones. The most complete specimen of its kind ever uncovered, Member 4 earlier produced the well-known Mrs. Ples skull.

An age of 3.67 million years was previously determined for the Little Foot skeleton, an iconic Australopithecus skeleton from the Sterkfontein site that was unearthed from the infill Member 2. That dating benefited much from Granger's techniques. He and his colleagues focused their techniques on Member 4 since the ages of other deposits are still a contentious issue.

The scientists looked at the rock in which the Australopithecus remains were lodged rather than the flowstone or other bones discovered nearby (which might not be contemporaneous with the remains in issue). They specifically investigated the radioactive decay of the rare isotopes beryllium-10 and aluminum-26 in 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," Granger stated.

The study determined that the sediments containing Australopithecus all date from between 3.4 and 3.7 million years ago based on these isotopes. Therefore, rather than from the end of the Australopithecus period as was previously believed, the bones found in the deposit date all to around its inception.

According to the experts, this has significant consequences for how we interpret human evolution and how Sterkfontein fits within it.

"Younger hominins, including Paranthropus and our genus Homo, appear between about 2.8 and 2 million years ago," according to Dominic Stratford, an archaeologist and coordinator of the Sterkfontein research project at the University of the Witwatersrand in South Africa.

"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 new finding, which is in line with the dating of Little Foot, suggests that Homo and Paranthropus, which were also discovered in the Cradle of Humankind, emerged nearly a million years after the Member 4 people lived. This allows for a revision to the timeline's events and the locations where they took place.

"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," Granger explains.

The team's research has been published in PNAS.