The extinct human species Paranthropus robustus has long been believed to exhibit a significant sexual dimorphism, where males were substantially larger than females, a pattern commonly seen in modern primates like gorillas, orangutans, and baboons. However, recent fossil discoveries from South Africa have called this assumption into question. Researchers now suggest that P. robustus may have evolved rapidly during a period of dramatic climate change around 2 million years ago, leading to anatomical changes that were previously attributed to sex differences. These new findings offer an important window into early human evolution, showing how environmental pressures might have shaped physical traits and behaviors within a species.
In a groundbreaking study published in Nature Ecology & Evolution on November 9, an international team of researchers, including anthropologists from Washington University in St. Louis, reported a significant discovery at the Drimolen cave system, located northwest of Johannesburg. This discovery challenges the prevailing narrative about the sexual dimorphism in P. robustus and sheds light on how the species adapted to changing environments during a time of climatic instability.
David Strait, a professor of biological anthropology at Washington University in St. Louis, who co-led the study, explained the complexity of documenting evolutionary change in the fossil record. “This is the type of phenomenon that can be hard to document, especially in early human evolution,” Strait remarked. The fossil in question, identified as DNH 155, was discovered by Samantha Good, a student participating in the Drimolen Cave Field School co-led by Strait.
Before this discovery, researchers believed that the appearance of P. robustus in South Africa coincided with the disappearance of Australopithecus, a more primitive early human species, and the arrival of early members of the genus Homo, to which modern humans belong. This transition was thought to have occurred rapidly, likely within just a few tens of thousands of years. The prevailing hypothesis had suggested that climate change led to environmental stress that contributed to the extinction of Australopithecus, while Homo and Paranthropus thrived in more favorable conditions. However, this new study indicates that P. robustus might have also faced environmental challenges that drove its rapid evolution.
The newly discovered specimen, DNH 155, was identified as a male, but its physical characteristics challenge previous assumptions about size differences between sexes in P. robustus. While it is larger than another well-studied specimen, DNH 7, presumed to be female, it is still smaller compared to other presumed male specimens found at Swartkrans, a site where many P. robustus fossils have been discovered. This finding led the researchers to conclude that the size difference between the two sites cannot be attributed solely to sexual dimorphism but rather may reflect population-level variations.
Jesse Martin, a doctoral student at La Trobe University and co-first author of the study, explained, “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.” Martin also noted that the Drimolen site predates Swartkrans by about 200,000 years, suggesting that P. robustus evolved over time, with Drimolen representing an earlier population and Swartkrans representing a later, more anatomically derived one. This finding provides valuable insight into the evolutionary processes that shaped the species.
Angeline Leece, another first author of the study, emphasized the importance of documenting evolutionary change within a species. She noted that understanding the subtle anatomical changes in P. robustus over time can help answer critical questions about the species’ adaptation to its environment. One key observation from the study was that tooth size in P. robustus appears to have changed over time, possibly in response to dietary stress caused by environmental shifts. This opens up new avenues for future research into how climate change may have influenced the species’ ability to survive in increasingly challenging conditions.
The Drimolen site has become a focal point for understanding early human evolution, offering a rare opportunity to study the evolution of P. robustus in the context of a rapidly changing environment. Andy Herries, co-director of the Drimolen project, explained, “Like all other creatures on earth, our ancestors adapted and evolved in accordance with the landscape and environment around them. For the first time in South Africa, we have the dating resolution and morphological evidence that allows us to see such changes in an ancient hominin lineage through a short window of time.”
The evidence supporting rapid climate change during this period in South Africa comes from various sources. Fossils indicate that mammals associated with more forested or bushland environments became less prevalent, while species adapted to drier, more open environments began to appear locally. This shift in fauna suggests that the climate was undergoing a significant transformation, which likely exerted pressure on species like P. robustus to adapt.
Strait pointed out that P. robustus possessed several unique adaptations in its cranium, jaws, and teeth that allowed it to consume a diet consisting of tough, hard-to-chew foods. These adaptations would have been crucial for survival as the environment became cooler and drier, leading to changes in local vegetation. However, the specimens found at Drimolen exhibited skeletal features suggesting that their chewing muscles were positioned differently from those in the later P. robustus population from Swartkrans. This indicates that over the course of 200,000 years, natural selection favored the evolution of a more powerful and efficient feeding apparatus in response to environmental pressures.
The study also underscores the divergent evolutionary paths taken by P. robustus and Homo erectus, our direct ancestors. Both species appeared around the same time, with Homo erectus possessing relatively large brains and small teeth, and P. robustus having large teeth and smaller brains. These two species represent different evolutionary experiments, with Homo erectus ultimately emerging as the dominant lineage. Leece noted, “While we were the lineage that won out in the end, the fossil record suggests that P. robustus was much more common than H. erectus on the landscape two million years ago.”
More broadly, the researchers suggest that this discovery highlights the challenges of interpreting anatomical variation in the fossil record. With the growing number of new fossil species discovered in recent decades, many paleoanthropologists are increasingly cautious about designating new species based on small numbers of fossils from limited geographic areas and narrow time ranges. Strait commented, “We think that paleoanthropology needs to be a bit more critical about interpreting variation in anatomy as evidence for the presence of multiple species. Differences in bony anatomy might represent changes within lineages rather than evidence of multiple species.”
Stephanie Baker, co-director of the Drimolen project at the University of Johannesburg, added, “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. The DNH 155 cranium is one of the best-preserved P. robustus specimens known to science. This is an example of what careful, fine-scale research can tell us about our distant ancestors.”
More information: Drimolen cranium DNH 155 documents microevolution in an early hominin species, Nature Ecology & Evolution (2020). DOI: 10.1038/s41559-020-01319-6 , www.nature.com/articles/s41559-020-01319-6