Researchers have long believed that the distant human ancestor species called Paranthropus robustus were somewhat like modern gorillas, orangutans, and baboons because they thought the males of this species were considerably larger than the females. However, a newly discovered Paranthropus skull suggests that the difference in body size was due to rapid evolution instead of substantial sex differences in the anatomical build of the members of this ancient species.
A press release by Washington University in St. Louis reports that a student discovered the paradigm-shifting Paranthropus robustus skull during a field school dig at “the fossil-rich Drimolen cave system northwest of Johannesburg” in South Africa. Previous excavations at the Drimolen Cave Complex have provided hominid fossils belonging to the Australopithecus , Homo erectus , and Paranthropus species.
The researchers write in the journal Nature Ecology & Evolution that the recently uncovered skull shows that Paranthropus rapidly evolved during a time of dramatic climate change in the region about 2 million years ago, and this evolutionary event accounts for the differences in fossil size which had previously been attributed to sexual dimorphism (color, size, shape, and/or structural differences between sexes in the same species).
Paranthropus robustus had relatively large teeth and a small brain. ( Washington University in St. Louis )
Adapt or Say Goodbye!
According to EurekAlert!, researchers have known for some time that the rise of Paranthropus robustus “roughly coincided” with the disappearance of the more primitive Australopithecus species. The decline of the latter was linked to climate change. Homo erectus also appears on the scene around the same time – in a transition which has been described as “very rapid” (in evolutionary terms), “within only a few tens of thousands of years.”
Study co-author David Strait, a professor of biological anthropology in Arts & Sciences at Washington University, discussed the transition in climate and hominid species, saying:
“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. We now see that environmental conditions were probably stressful for Paranthropus as well, and that they needed to adapt to survive.”
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Angeline Leece of La Trobe University in Australia and another of the new study’s authors thinks that the differences in the hominids, most notably the Paranthropus and Homo specimens found at the Drimolen cave system, provide significant clues on what was happening in the region two million years ago. “These two vastly different species, H. erectus with their relatively large brains and small teeth, and P. robustus with their relatively large teeth and small brains, represent divergent evolutionary experiments,” Leece said . “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.”
Researchers also believe that the disappearance of some woodland or bushland mammals from the fossil record in the region at that time period provide more evidence for a quick and significant climate change. Species which are generally linked to drier and more open environments also begin to appear in the area around the same time.
The Paranthropus Skull with Evolutionary Clues
The researchers have determined that the new Paranthropus skull belonged to a male of the species. However, they also state that it differs from other P. robustus fossils which have been found at the Swartkrans site – a location which has provided most of the remains belonging to this species. Although the new skull is larger than previous examples found at Drimolen, it is smaller than most of the remains of presumed Paranthropus males found at Swartkrans.
These differences in skull size could have been attributed to sex differences in the past, but the new study asserts that this is a lucky example of researchers witnessing evolution at work – something that can be difficult to ascertain in the fossil record, especially for hominids with fewer samples to compare. Jesse Martin, a doctoral student at La Trobe University and the co-first author of the study explains that “It now looks as if the difference between the two sites [Drimolen and Swartkrans] cannot simply be explained as differences between males and females, but rather as population-level differences between the sites.”
The face of the Paranthropus robustus skull is put back together for the first time. ( Washington University in St. Louis )
Martin also said that the study shows the difference between the ages of the two sites. “Drimolen predates Swartkrans by about 200,000 years, so we believe that P. robustus evolved over time, with Drimolen representing an early population and Swartkrans representing a later, more anatomically derived population.” Strait said that studying the Paranthropus skull also provides signs about the environmental differences during the two time periods:
“P. 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. 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. But 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 P. robustus population from Swartkrans. Over the course of 200,000 years, a dry climate likely led to natural selection favoring the evolution of a more efficient and powerful feeding apparatus in the species.”
Sunrise at the Drimolen field site, South Africa. ( David Strait )
Generally, researchers find it easier to identify big changes, such as when a species appears or goes extinct, by using the fossil record. Smaller adaptations are not so easy to see. As Martin said , “One can use the fossil record to help reconstruct the evolutionary relationships between species, and that pattern can provide all sorts of insights into the processes that shaped the evolution of particular groups. But in the case of P. robustus , we can see discrete samples of the species drawn from the same geographic region but slightly different times exhibiting subtle anatomical differences, and that is consistent with change within a species.”
Finally, the researchers believe that their study can serve as a cautionary tale for others identifying hominid species in the fossil record. Project Co-Director Stephanie Baker of the University of Johannesburg said the analysis of the new Paranthropus skull “is an example of what careful, fine-scale research can tell us about our distant ancestors.” On the other hand, Strait warns, “Depending on the ages of fossil samples, differences in bony anatomy might represent changes within lineages rather than evidence of multiple species.”
Top Image: The newly discovered Paranthropus skull. Source: Jesse Martin and David Strait