About two million years ago a species of hominid, a descendant of Australophitecus , called Paranthropus robustus lived in South Africa. Like their ancestors, the Paranthropus had a small skull and protruding jaws, and were noted for the development of a very powerful chewing apparatus.
Thanks to the fossil record, paleoanthropologists have been able to get a more or less clear idea of the main features that characterize each of the extinct hominid species that we know of. However, studying the evolution produced within each species is much more complicated, since we are talking about very subtle changes that must be reflected in an incomplete fossil record.
However, sometimes luck accompanies and there are findings such as the one that has just been published in the journal Nature Ecology & Evolution and in which an international team of researchers has participated. The work describes a male individual, named DNH 155 and found at the Drimolen site in South Africa, very close to Swartkrans, a locality where almost all the remains of P. robustus have been found so far.
Differences not explained by sexual dimorphism
P. robustus is considered to be a sexually dimorphic species: males were much larger than females. The finding of DNH 155 agrees with this hypothesis, since its size is larger than that of another specimen previously found in the site and which is thought to correspond to a female.
However, the protagonist of this study is much smaller than the males found in Swartkrans, the neighboring site : “These findings cannot be explained only by the dimorphism between males and females, but rather appear differences at the population level between the two locations ”, explains Jesse Martin, a doctoral student at the University of La Trobe (Australia) and first author of the work. “Our investigations have shown that the Drimolen site is about 200,000 years older than Swartkrans, so we believe that P. robustus evolved over time: in Drimolen we found an earlier population and in Swartkrans a later more anatomically evolved.”
A local climate change altered everything
Why did this change occur? It is known that environmental changes occurred in these locations that also altered the feeding patterns of the species that inhabited there. Evidence for rapid but significant climate change during this period in South Africa comes from a variety of sources. The fossil record, for example, reveals that certain mammals associated with forest or scrub environments became extinct or became less frequent, while other species associated with drier and more open environments began to emerge.
“One of the most significant characteristics of P. robustus is its adaptation to a diet consisting of hard or very hard foods,” explains David Strait, a researcher at the University of Washington and a participant in the study. ” As the environment became colder and drier, the local vegetation also changed, and the anatomical adaptations of P. robustus allowed it to survive in this environment with foods that were more difficult to chew .”
“But the Drimolen specimens exhibit skeletal features that suggest that their chewing muscles were positioned in such a way that they were less able to bite and chew as hard as the later population of P. robustus from Swartkrans,” he adds. “Over the course of 200,000 years, an increasingly drier climate probably led natural selection to favor the evolution of a more efficient and powerful feeding apparatus in the species.”
Our ancestors were already swarming around
It is known that P. robustus appeared around the same time as our direct ancestor Homo erectus . “We are talking about two very different species: H. erectus with its relatively large brains and small teeth, and P. robustus with large teeth and small brains, which represent divergent evolutionary experiments ,” explains Angeline Leece, researcher at the University of La Trobe. . “While we were the surviving lineage, the fossil record suggests that two million years ago P. robustus was much more common than H. erectus.”
On the other hand, the authors consider that this discovery should serve as a warning to be more cautious when extrapolating general conclusions from a few fossil remains. In recent years, a large number of fossil human species have been discovered, and many of their descriptions are based on a very small number of fossils from one or a few sites located in very close geographic areas and in very small time ranges.
“We think paleoanthropology needs to be a little more critical in interpreting variation in anatomy as evidence of the presence of multiple species,” Strait said. “Depending on the ages of the fossil samples, differences in bone anatomy could represent changes within lineages rather than evidence for multiple species.”
