Recently discovered fossils show evolutionary changes in an extinct human species

The fossil skull suggests that environmental conditions brought about rapid changes.

Males of the extinct human species Paranthropus robustus they 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 suggests it instead P. robustus it evolved rapidly during a turbulent period of local climate change about 2 million years ago, resulting in 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 today (November 9, 2020).

“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 what the appearance of P. robustus in South Africa more or less coincided with the disappearance of Australopithecus, a slightly more primitive primitive human and the emergence in the region of the first representatives of Homo, the genre 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 has created stress in the populations of Australopithecus eventually leading to their disappearance, but for which environmental conditions were more favorable Homo is Paranthropus, which may have dispersed into the region somewhere else, ”Strait said. “Now we see what the environmental conditions were probably stressful for Paranthropus and that they needed to adapt to survive. “

The new specimen discovered in Drimolen, identified as DNH 155, is clearly a male but differs significantly from the others 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 at 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 it. P. robustus it 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 drawn from the same geographic region but slightly different times showing subtle anatomical differences, and this is consistent with 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 the species, which raises the question of why. There are reasons to believe that environmental changes have placed these populations under food stress, and this points to future research that will allow us to test this possibility. “

Drimolen project co-director Andy Herries of the University of La Trobe said: “Like all other creatures on earth, our ancestors adapted and evolved based on the landscape and environment around them. . For the first time in South Africa, we have the resolution of dating 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 the skull, jaws and teeth that indicate that it was adapted to eat a diet consisting of very hard or very hard foods, “Strait said.” We believe these adaptations have enabled him 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 samples show skeletal features that suggest that their chewing muscles were positioned in such a way that they were less able to bite and chew with the same force as the next. P. robustus Swartkrans population, “he said.” Over the course of 200,000 years, a dry climate has likely led to natural selection favoring the evolution of a more efficient and powerful food system in the species.

Leece said it was remarkable P. robustus it appeared around the same time as our direct ancestor Standing man, as documented by a newborn H. erectus skull 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, e P. robustus with their relatively large teeth and small brains, they represent divergent evolutionary experiments, “Leece said.” While we were the progeny that ultimately won, the fossil record suggests that P. robustus it 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 think paleoanthropology needs to be a little more critical of interpreting 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 del Johannesburg University added: “Drimolen is fast becoming a hot spot for early hominid discoveries, testifying to the current team’s dedication to holistic excavation and post-field analysis. The DNH 155 skull is one of the best preserved. P. robustus specimens known to science. This is an example of what careful large-scale research can tell us about our distant ancestors. “

Reference: “Drimolen cranium DNH 155 documents microevolution in one of the earliest hominid species” by Jesse M. Martin, AB Leece, Simon Neubauer, Stephanie E. Baker, Carrie S. Mongle, Giovanni Boschian, Gary T. Schwartz, Amanda L . Smith, Justin A. Ledogar, David S. Strait and Andy IR Herries, November 9, 2020, Nature, ecology and evolution.
DOI: 10.1038 / s41559-020-01319-6