It was Father’s Day in 2018 when a student helping to excavate a quarry in the Cradle of Humankind unearthed what has since been dubbed the “big daddy” of SA human fossils.
Samantha Good, a Canadian anthropology student, was on her hands and knees in the Drimolen main quarry on Sunday, June 17, when she unearthed a tooth.
Two more teeth and a partial palate later, Good shouted to her instructors at Drimolen Palaeoanthropology Fieldschool: “There’s something interesting happening.”
This week, when her discovery was revealed as a two million-year-old skull that belonged to a human ancestor, she told the New York Times: “It was in fact something very interesting.”
The Paranthropus robustus skull, now known as DNH 155, is the earliest and best specimen of the species yet found.
Not only that, researchers from the University of Johannesburg and elsewhere who revealed the discovery in the journal Nature Ecology & Evolution said it provided the best evidence yet of a human ancestor evolving as it adapted to a changing climate.
The pace of discoveries at Drimolen has been earth-shattering in comparison to the time its fossils lay hidden.
The site 40km northwest of Johannesburg was discovered in 1992 by palaeontologist and geologist Andre Keyser, and two years later he found a female Paranthropus robustus skull, nicknamed Eurydice, and a male mandible.
In 2015, the partial cranium of a Homo erectus child was unearthed and nicknamed Simon after a member of the excavation team, Simon Mokobane, who died in 2018.
Homo erectus, thought to be the direct ancestor of modern humans, is a “cousin species” of Paranthropus robustus, and they lived at the same time.
Good’s find, during her undergraduate anthropology studies at Vancouver Island University in British Columbia, happened just a few metres from Simon’s resting place.
The excitement was recalled this week by Angeline Leece, a palaeoanthropologist at La Trobe University in Melbourne, Australia, who walked across to see what Good had called “interesting”.
Leece told the New York Times: “I think my breath stopped for a second. I looked up at her, and I hadn’t said anything. But she saw my face, and she goes, ‘Yeah, that’s what I thought’.”
Leece and geo-archaeologist Andy Herries, from UJ’s Palaeo-Research Institute, lifted the specimen out of the ground in a block of dirt and bone, using conservator’s glue to keep the fossils and sediment together.
La Trobe doctoral student Jesse Martin, who was asked to reassemble the skull, told the BBC handling the fossil pieces was like working with “wet cardboard”.
But after weeks spent sucking off dirt through plastic straws, he revealed a skull that was so well preserved that another UJ palaeoanthropologist, David Strait, could see its tear ducts. “This Paranthropus could have cried,” he told Martin.
Paranthropus robustus had large teeth and a small brain, but the scientists were intrigued when they realised DNH 155 had chewing muscles that were weaker than those in fossils of the same species from the nearby Swartkrans cave, which are 200,000 years younger.
“Paranthropus robustus is remarkable in that it possesses a number of features in its cranium, jaws and teeth indicating that it was adapted to eat a diet consisting of either very hard or very tough foods,” said Strait.
“We think that these adaptations allowed it to survive on foods that were mechanically difficult to eat as the environment changed to be cooler and drier, leading to changes in local vegetation.
“The specimens from Drimolen exhibit skeletal features suggesting that their chewing muscles were positioned in such a way as to make them less able to bite and chew with as much force as the later Paranthropus robustus population from Swartkrans.
“Over the course of 200,000 years, a dry climate likely led to natural selection favouring the evolution of a more efficient and powerful feeding apparatus in the species.”
Meanwhile Homo erectus, with its smaller teeth, was more likely to have eaten both plants and meat, and “these two vastly different species ... represent divergent evolutionary experiments”, Leece told the BBC.
“While we were the lineage that won out in the end, two-million years ago the fossil record suggests Paranthropus robustus was much more common than Homo erectus on the landscape.”
The appearance of Paranthropusanthropus in SA roughly coincided with the disappearance of Australopithecus, a more primitive early human, and the emergence of early representatives of Homo, the genus to which modern people belong. This transition took place rapidly, perhaps within only a few tens of thousands of years.
“The working hypothesis has been that climate change created stress in populations of Australopithecus leading eventually to their demise, but that environmental conditions were more favourable for Homo and Paranthropus, who may have dispersed into the region from elsewhere,” said Strait.
“We now see that environmental conditions were probably stressful for Paranthropus as well, and that they needed to adapt to survive.”
The co-director of the Drimolen project, UJ’s Stephanie Baker, said the site was “fast becoming a hot spot for early hominin discoveries, which is a testament to the current team’s dedication to holistic excavation and post-field analysis”.
