The part of Sterkfontein Caves that lies 40km northwest of Johannesburg is known as Member 4 and it contains the richest deposit of Australopithecine fossils in the world. 

More than 500 of these ancestors that belong to a family of hominids from an ancient branch of the human family tree have been found in the cave, but their being there is a decades-old mystery. The Australopithecus fossils found here were younger than their counterparts in East Africa, and this didn’t fit into our understanding of hominid evolution.

Scientists have for decades puzzled over the age of Member 4. Initially, it was thought that Member 4 was just over two million years old, making all those fossils found there that age too.

“It just didn’t make sense,” says Professor Dominic Stratford, director of research at the caves, and one of the authors of the paper that appeared in the journal Proceedings of the National Academy of Sciences.

Fossils now tie up with those in East Africa

Now, through the use of a state-of-the-art dating technique, Member 4 has been found to be a lot older and those Australopithecus fossils now tie up in age with those found in East Africa. These new dates were obtained thanks to the work of Darryl Granger, a professor of Earth, atmospheric, and planetary sciences at Purdue University’s College of Science, in the US. 

Granger and the team used accelerator mass spectrometry to measure the radioactive decay of the rare isotopes aluminium-26 and beryllium-10 in the mineral quartz. 

Granger explains: “These radioactive isotopes, known as cosmogenic nuclides, are produced by high-energy cosmic ray reactions near the ground surface, and their radioactive decay dates [from] when the rocks were buried in the cave when they fell in the entrance together with the fossils.”

The same technique was used previously to date perhaps the most famous Australopithecus fossil to come out of Sterkfontein, that of Little Foot. This near-complete skeleton was found to be 3.67 million years old. 

Mixed fossils

The research also involved mapping cave deposits and it was found that animal fossils from different ages became mixed during excavations in the 1930s and 1940s. This, said Granger, added to the confusion over dates during the later decades.   

For Stratford, the new dates for Member 4 mean the pieces of a palaeoanthropological puzzle are falling into place. 

“So if I was to talk to some random palaeoanthropologist they would have said, well hang on a second, you’ve got really young Australopithecus in southern Africa. So they can’t be a contender for the origin of Paranthropus or early Homo [a direct ancestor of humans]” explains Stratford. “Now what we have is kind of old and they had the opportunity to evolve into Paranthropus or early Homo.”

The older hypothesis was that South African Australopithecines were in a kind of backwater where they continued to evolve on their own. Their branch on the human family tree eventually withered away and they became extinct.  

SA back in origin of species contention

This research now puts South Africa back in contention for being the place where our species originated. One of Sterkfontein’s most famous fossils is that of Mrs Ples. This most complete skull of an Australopithecus africanus was discovered in 1947. For years, scientists have debated everything from her sex to how old she is. 

The most recent conclusion is that she was in fact a middle-aged female Australopithecus africanus. But, like the other fossils, Mrs Ples has got a whole lot older. She was initially dated to just over two million years and now is between 3.4 and 3.6 million years old. 

Retired palaeoanthropologist Professor Francis Thackeray spent much of his career studying Mrs Ples and has a soft spot for the hominid. In fact, his  email address starts with mrsples@. 

“The age of Mrs Ples has always been disputed because we haven’t always been exactly sure where she comes from. One needs to understand the history of the discovery of Mrs Ples. It was found on April the 18th 1947, and Robert Broom was with miners and allegedly pointed to a spot and asked them to blast here. So they blasted with dynamite and Mrs Ples was broken into two pieces,” says Thackeray, who wasn’t involved in the study.

“And now that we have this new date, it is wonderful to hear.” 

Gene flow

Thackeray explained that over millions of years there would have been a gene flow between Australopithecus populations in southern Africa and East Africa, through contact.

“There has been in Africa, episodic expansion, and contraction, of habitats. So during some periods, there’s been the opportunity, for example, for blue wildebeest to range from East Africa to South Africa when habitats opened up, and we could well imagine hominins responding in the same kind of way.” 

For close to 90 years, Sterkfontein Caves has been the grande dame of South African paleo sites and this discovery means its influence on the science is set to continue.

“We are hoping that this work stimulates people to come back and look at Australopithecus in southern Africa again, and look at it with some new eyes and realise they are really old and wonder what else is going on here,” says Stratford. DM