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New Fossil of Extinct Human Relative Suggests Climate Change Led to Rapid Evolution

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A fossil skull of an extinct human relative is shown against the background of a grasslands. The discovery of a fossil from the extinct human species Paranthropus robustus suggests rapid evolution during a turbulent period of local climate change.
Courtesy of Jesse Martin and David Strait

Evolution within a species can be difficult to see in the fossil record: changes may be subtle, and the fossil record is notoriously incomplete. Usually, the fossil record reveals larger-scale patterns, such as when species or groups of species either appear in the fossil record or go extinct.

A new fossil discovery in South Africa suggests that Paranthropus robustus, an extinct species that co-existed with early members of our own genus, Homo, may have evolved rapidly during a turbulent period of local climate change about 2 million years ago.

“This is an incredibly well-preserved fossil that adds to the evolutionary story of this small-brained, large-toothed hominin from South Africa. The ability to document this level of anatomical detail in one of our extinct relatives is a rare and exciting opportunity for understanding human evolution,” said Carrie Mongle, a Gerstner Scholar and Postdoctoral Fellow at the Museum who was part of an international research team that discovered and described the specimen, one of the most complete skulls of P. robustus ever found, in a study published this week in the journal Nature Ecology & Evolution.

The discovery was made in the Drimolen cave system, which is northwest of Johannesburg. The specimen is a male but differs in important ways from other P. robustus fossils at a nearby site called Swartkrans — where most of the fossils of this species have been found, and which dates about 200,000 years later than the Drimolen site. The new skull is larger than a well-studied member of the species previously discovered at Drimolen—an individual presumed to be female—but is smaller than males from Swartkrans.

A map indicating the locations of several fossil sites in South Africa, with Swartkrans indicated as southwest of Drimolen.
The Drimolen site and nearby Swartkrans, lower left, in South Africa.
Courtesy of Andy Herries

“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 Jesse Martin, a doctoral student at La Trobe University and a co-author of the study. 

Researchers already knew that the appearance of P. robustus in South Africa roughly coincided with the disappearance of Australopithecus, a somewhat more primitive early human, and the emergence in the region of early representatives of Homo, the genus to which modern people belong. 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 David Strait, professor of biological anthropology at Washington University and the corresponding author of the study. “We now see that environmental conditions were probably stressful for Paranthropus as well, and that they needed to adapt to survive.” 

Some of those adaptations occurred in its cranium, jaws, and teeth and indicate that P. robustus likely ate a diet consisting of either very hard or tough foods, which would have been more prevalent as the environment got cooler and drier. Comparisons between the Drimolen and Swartkrans skulls suggest that over the course of 200,000 years, these chewing muscles became more powerful as a result of natural selection.