Sterkfontein caves still yield clues to our family tree
Dr Dominic Stratford’s job is to put the fossils back. He has to place them exactly where they lay before the lime miners and the academics lifted them from the Sterkfontein caves during a century of excavation.
There are thousands and thousands of fossils. Some of them belong to our ancestors, while others are the bones of animals that once lived in this area and died out millions of years ago.
Stratford, who is the research co-ordinator at the Sterkfontein Caves, doesn’t really have to put those fossils back but what he does have to do is place them where they were originally found, on a 3D digitalised map of the cave. It is a hard job because large parts of the cave that once held some of those fossils are long gone, dynamited away by limestone miners.
So it is a job that requires detective work, of poring over excavation diaries and digging into the historical archive left by three generations of academia who worked the caves. Much of this work is done in what is known as the top shed, a laboratory close to the cave filled with casts and fossils.
It is here under the gaze of early ape men busts that preparators work on deciphering the academic scrawl felt on faded labels. Each label identifies the fossil fragment held in a bag, its location often recorded in empirical feet and inches.
Andrew Phaswana, who for a long time has worked as a preparator at the cave, has already found bone fragments that were missed or misidentified. There was a piece of a hominid Australopithecus skull that was mislabelled and needs to be identified.
"We are systematically moving through the cave, trying to understand where everything is. We are building up more information about the old collection and we are building it up strata by strata." says Stratford.
For decades Sterkfontein was the shining star of paleoanthropological research. This cave, which lies within sight of the Johannesburg skyline, has produced some of the science’s most important finds.
In 1936, Prof Robert Broom discovered the first Paranthropus robustus fossils at Sterkfontein; a decade later the almost complete Mrs Ples skull was found. In the 1990s, paleoanthropologist Prof Ron Clarke discovered Little Foot.
These early discoveries were made, explains Clarke, thanks to those men who initially tore chunks out of the cave.
"If it wasn’t for the lime miners, we wouldn’t even know what was there," he says.
"The lime miners did us a great service by exposing the fossils. Now, however, it is better to do things in a slower, more methodical manner."
But in the eyes of some, Sterkfontein has lost its lustre. There are those who believe the cave has become little more than a tourist attraction.
"Is there anything left to find at Sterkfontein?" is a question Stratford says he is continually asked by both members of the public and even researchers.
Other sites like the Rising Star Cave system, where Homo naledi was recently found, are more in the public eye.
But Sterkfontein still has secrets to reveal.
Beyond the well-worn paths the tourists follow lies a labyrinth of tunnels and chambers, which scientists say are 90% unexplored. Getting there requires crawling through passage ways and climbing ladders, but it is here where Stratford and colleagues have made some interesting discoveries.
It was in the Milner chamber that an unusual fossilised finger bone was discovered.
The large finger is a mystery.
"The finger looks like nothing else we have seen before. It is bigger than our finger and more robust than our finger bone and much more heavily muscled. It really was quite a strong hominid that was using this," says Stratford, who adds that it is the largest hand bone of any hominin yet found in SA.
The suspicion is that the finger might have belonged to a Homo ergaster, a hominin considered to be on our branch of the family tree. Ergaster is believed to have lived between 1.4-million and 1.9-million years ago and is suspected to have been a sophisticated tool maker.
Also found in the same pit was a hominid tooth that might have belonged to an older species of hominid. The hope is that the pit will reveal more fossils that might help identify the species of those creatures.
What also excites the scientists about these finds is that they are associated with early stone tools. "We used to dig where other people have dug," says Stratford, "but now in every chamber we sampled we have found fossils and hominids."
How these fossils ended up in Sterkfontein probably had to do with flash floods that washed bodies into the entrances of the cave.
It has been argued that our earlier ancestors took advantage of the denser vegetation that grew around the opening of the caves. Besides using this thicker bush for shelter, it was here where they probably napped stone tools, which also ended up washed into the cave.
"We have no evidence of any of these hominids living in these caves," says Stratford.
But with excavating new places comes new challenges. In the Jacovec Cavern, the team is digging into the roof of the cave.
"This area is packed full of bones," says Stratford.
"So instead of focusing around the hominids, we are trying to build up the sequencing before and after the hominids, so we can place them in a changing environment," he says.
In the Silberberg grotto, where Clarke found Little Foot, is a section of the cave scientists are itching to start excavating.
It is known as the "very mysterious member three".
"Member three hasn’t been sampled yet but it is full of fossils," says Clarke.
This documents the changes between Little Foot and Australopithecus africanus. We have this huge amount of time, just over half a million years, that we haven’t yet sampled," Stratford says. "If it has hominids in it, it will give Sterkfontein a sequence of hominid evolution all the way from 4-million to homo egaster
The section is 8m thick and forms part of the roof of the Silberberg grotto. Opening it up could give science a snap shot of a very important time in human evolution.
"This documents the changes between Little Foot and Australopithecus africanus. We have this huge amount of time, just over half a million years, that we haven’t yet sampled," Stratford says. "If it has hominids in it, it will give Sterkfontein a sequence of hominid evolution all the way from 4-million to homo egaster."
Buried somewhere in this 4-million-year-long sequence could be clues to paleoanthropology’s most recent star discoveries. Australopithecus sediba and the more recent Homo naledi have been found at sites close to Sterkfontein.
"If Sediba is living just 15km away, there should be no reason why it is not found here," says Stratford.
Some paleoanthropologists believe Sediba might not be new species but a later form of Australopithecus africanus.
Homo naledi was discovered at the Rising Star Cave, which is just over 2km from Sterkfontein. If naledi is found in Sterkfontein, Stratford believes it would come from a younger part of the cave. "We have sediments that are 150,000 years old and it is here where we might find something much more recent," says Stratford.
In 2017, the naledi fossils were dated to between 335,000 and 236,000 years ago, at a time when humans were roaming the cradle.
In fact, at least seven hominid species are believed to have walked the cradle over a span of 4-million years.
At about 2-million years ago paleoanthropologists believe that there were as many as five hominid species sharing the landscape, including possibly an early relative, Homo habilis.
"You might have all those things on the landscape and each one needs to survive and use slightly different resources, maybe moving in slightly different ways.
"Homo habilis might be the first one to start using stone tools," says Stratford.
"There is a very interesting dynamic and it is our job to see how they relate".
But recently far older hominid species have been pulled out of Sterkfontein, which the team are still trying to put names to.
There are some deposits in the cave that Stratford believes could date to 5-million years.
"The morphology of these hominids looks really old, they fit into Australopithecus. We have this really old clavicle and what looks like two different species," he says.
It is fossils like these that could one day reveal a new species to add to the hominin family line or perhaps just give a better understanding of that time millions of years ago when we huddled around the opening of a cave that far in the future would reveal so much about the journey we walked.