The recently discovered fossil shows small-scale evolutionary changes in an extinct human species

Males of the extinct human species Paranthropus robustus were thought to be substantially larger than females, just like the size differences observed in modern primates such as gorillas, orangutans, and baboons. But a new fossil discovery in South Africa instead suggests that P. robustus evolved rapidly during a turbulent period of local climate change about 2 million years ago, causing anatomical changes that were previously attributed to sex.

An international research team that includes anthropologists from Washington University in St. Louis reported their discovery from the fossil-rich Drimolen cave system northwest of Johannesburg in the journal Nature, ecology and evolution on November 9th.

“This is the kind of phenomenon that can be difficult to document in the fossil record, especially with respect to early human evolution,” said David Strait, professor of biological anthropology in Arts & Sciences at Washington University.

The remarkably well-preserved fossil described in the document was discovered by a student, Samantha Good, who attended the Drimolen Cave Field School co-led by Strait.

Researchers already knew that the appearance of P. robustus in South Africa roughly coincided with the disappearance of Australopithecus, a somewhat more primitive human being, and the emergence in the region of the first representatives of Homo, the genus to which modern people belong. This transition happened very quickly, perhaps within a few tens of thousands of years.

“The working hypothesis was that climate change created stress in Australopithecus populations which ultimately led to their demise, but that environmental conditions were more favorable for Homo and Paranthropus, who may have dispersed into the region from elsewhere.” Strait said. “Now we see that the environmental conditions were probably stressful for Paranthropus as well, and that they needed to adapt to survive.”

The new specimen discovered at Drimolen, identified as DNH 155, is clearly a male but differs significantly from other P. robustus previously discovered at the nearby Swartkrans site – where most of the fossils of this species have been found.

Evolution within a species can be difficult to see in the fossil record. The changes may be subtle and the fossil record is notoriously incomplete.

Usually, the fossil record reveals patterns on a larger scale, such as when species or groups of species appear in the fossil record or become extinct. So this Drimolen discovery provides a rarely seen window into early human evolution.

The new specimen is larger than a well-studied member of the species previously discovered in Drimolen – an individual known as DNH 7 and presumably female – but is measurably smaller than the alleged Swartkrans males.

“It now appears that the difference between the two sites cannot be explained simply as differences between males and females, but rather as population-level differences between the sites,” said Jesse Martin, a PhD student at La Trobe University and the co-first author of the study. “Our recent work has shown that Drimolen predates Swartkrans by about 200,000 years, so we believe that P. robustus evolved over time, with Drimolen representing a primitive population and Swartkrans representing a later, anatomically derived population.”

“The fossil record can be used to help reconstruct evolutionary relationships between species, and that model can provide all kinds of insights into the processes that have shaped the evolution of particular groups,” said Martin. “But in the case of P. robustus, we can see discrete specimens of the species from the same geographic region but slightly different times showing subtle anatomical differences, and this is consistent with the change within a species.”

“It is very important to be able to document evolutionary change within a lineage,” said Angeline Leece of La Trobe University, the other first author of the study. “It allows us to ask very focused questions about evolutionary processes. For example, we now know that the size of teeth changes over time in species, which raises the question of why. There are reasons to believe that environmental changes have placed these populations under. food stress, and this indicates future research that will allow us to test this possibility “.

Co-director of the Drimolen project, Andy Herries of the University of La Trobe, said: “Like all other creatures on earth, our ancestors adapted and evolved according to the landscape and environment around them. . For the first time in South Africa, we have dating resolution and morphological evidence that allows us to see such changes in an ancient hominid lineage over a short span of time. “

Evidence of rapid but significant climate change during this period in South Africa comes from a variety of sources. Critically, fossils indicate that some mammals associated with woodland or scrub environments have become extinct or become less widespread, while other species associated with more arid and open environments have appeared locally for the first time.

“P. robustus is notable in that it possesses a number of features in its skull, jaws and teeth that indicate it has been adapted to eat a diet consisting of either very hard or very hard foods,” Strait said. “We believe these adaptations allowed them 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 Drimolen specimens show skeletal features that suggest their masticatory muscles were positioned in such a way that they were less able to bite and chew with the same force as Swartkrans’s later P. robustus population,” he said. “Over the course of 200,000 years, a dry climate has probably led to natural selection which has favored the evolution of a more efficient and powerful food system in the species.”

Leece said it was noteworthy that P. robustus appeared around the same time as our direct ancestor Homo erectus, as documented by a newborn H. erectus cranium that the team discovered at the same Drimolen site in 2015.

“These two very 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 ultimately won, the fossil record suggests that P. robustus was much more common than H. erectus on the landscape two million years ago.”

More generally, the researchers think this discovery serves as a warning to recognize species in the fossil record.

A large number of fossil human species have been discovered over the past quarter century, and many of these new species designations are based on a small number of fossils from one or a few sites in small geographic areas and narrow time frames.

“We believe paleoanthropology needs to be a little more critical in interpreting the variation in anatomy as evidence of the presence of multiple species,” Strait said. “Depending on the age of the fossil specimens, the differences in bone anatomy could represent changes within lineages rather than evidence of multiple species.”

Project co-director Stephanie Baker of the University of Johannesburg added: ‘Drimolen is fast becoming a hot spot for early hominid discoveries, testifying to the current team’s commitment to holistic excavation and post-analysis. field. DNH 155 skull is one of the best preserved specimens of P. robustus known to science. This is an example of what careful large-scale research can tell us about our distant ancestors. ”

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