2-Million-Year-Old Skull of Paranthropus robustus Suggests Climate Change Drove Rapid Changes
Paranthropus robustus is a small-brained extinct hominin that lived between 2 million and 1.2 million years ago in what is now South Africa. Discovered in 1938, it was among the first early hominins described and the first discovered robust australopithecine. Paranthropus robustus males were thought to be substantially larger than females — much like the size differences seen in modern-day primates. But the well-preserved adult male skull of Paranthropus robustus from the Drimolen cave system northwest of Johannesburg in South Africa, dated from approximately 2.04-1.95 million years ago, instead suggests that this hominin evolved rapidly during a period of local climate change about 2 million years ago, resulting in anatomical changes that previously were attributed to sex.
Paleoanthropologists already knew that the appearance of Paranthropus robustus in South Africa roughly coincided with the disappearance of Australopithecus and the emergence in the region of early Homo.
This transition took place very 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 favorable for Homo and Paranthropus, who may have dispersed into the region from elsewhere,” said Professor David Strait a researcher at Washington University in St. Louis.
“We now see that environmental conditions were probably stressful for Paranthropus as well, and that they needed to adapt to survive.”
The newly-discovered specimen from the Drimolen site, designated DNH 155, is clearly a Paranthropus robustus male.
It is larger than a well-studied member of the species previously discovered at the site — an individual known as DNH 7, and presumed to be female — but is measurably smaller than presumed males from the nearby site of Swartkrans.
“It now looks as if the difference between the two sites cannot simply be explained as differences between males and females, but rather as population-level differences between the sites,” said first author Jesse Martin, a doctoral student at La Trobe University.
“Our recent work has shown that Drimolen predates Swartkrans by about 200,000 years, so we believe that Paranthropus robustus evolved over time, with Drimolen representing an early population and Swartkrans representing a later, more anatomically derived population.”
“It’s very important to be able to document evolutionary change within a lineage. It allows us to ask very focused questions about evolutionary processes,” added co-author Dr. Angeline Leece, also from La Trobe University.
The evidence of rapid but significant climate change during this period in South Africa comes from a variety of sources.
Critically, fossils indicate that certain mammals associated with woodland or bushland environments went extinct or became less prevalent — while other species associated with drier, more open environments appeared locally for the first time.
“As is the case today, shifting climates alter local environments, and the rapid appearance of a more open, arid and cooler environment in South Africa at this time resulted in a change to the local ecology and thus, to the foods available on the landscape,” said co-author Dr. Gary Schwartz, a researcher at Arizona State University.
“We already knew that Paranthropus robustus had evolved a host of anatomical specializations of the skull that allowed it to consume and survive on the sorts of hard, tough, difficult-to-process foods.”
“But when compared to geologically younger specimens from the nearby site of Swartkrans, the Drimolen cranium very clearly shows that it was less well adapted to eating these challenging menu items.”
“What we have is a remarkable snapshot of how a dry climate led to natural selection altering the anatomy of this one species over only 200,000 years from the less-efficient form at Drimolen to the evolution of a more powerful feeding apparatus present in Swartkrans Paranthropus robustus.”
“It was notable that Paranthropus robustus appeared at roughly the same time as our direct ancestor Homo erectus, as documented by an infant Homo erectus cranium that the team discovered at the same Drimolen site in 2015,” Dr. Leece said.
“These two vastly different species, Homo erectus with their relatively large brains and small teeth, and Paranthropus robustus with their relatively large teeth and small brains, represent divergent evolutionary experiments.”
“While we were the lineage that won out in the end, the fossil record suggests that Paranthropus robustus was much more common than Homo erectus on the landscape two million years ago.”
“These two genera lived alongside a third genus of early human ancestor, Australopithecus, also present from sites within the same small valley,” Dr. Schwartz said.
“How these hominins divided up the landscape, especially during this period of radically shifting environments, is not yet clear and that is a major focus of our future research.”
“Drimolen is fast becoming a hotspot for early hominin discoveries, which is a testament to the current team’s dedication to holistic excavation and post-field analysis,” said co-author Dr. Stephanie Baker, a researcher at the University of Johannesburg.
“The DNH 155 cranium is one of the best-preserved Paranthropus robustus specimens known to science.”
“This is an example of what careful, fine-scale research can tell us about our distant ancestors.”
The research is described in a paper published online in the journal Nature Ecology & Evolution.
J.M. Martin et al. Drimolen cranium DNH 155 documents microevolution in an early hominin species. Nat Ecol Evol, published online November 9, 2020; doi: 10.1038/s41559-020-01319-6