Well-preserved addition to the evolution story

A two-million-year-old hominin skull has been uncovered in a South African cave, providing fresh insight into the micro evolution of our ancient cousins.

The skull is the earliest-known and best-preserved specimen of Paranthropus robustus: a short, robust, upright hominin that is thought to have gone extinct around a million years ago. The species possessed distinctive large molars and powerful jaws that would have been useful for eating tough vegetation, seeds and roots.

This new specimen was discovered by a team of paleoanthropologists – led by Australia’s La Trobe University – at the Drimolen caves near Johannesburg. The findings are published in the journal Nature Ecology and Evolution.

“The DNH 155 cranium shows the beginning of a very successful lineage that existed in South Africa for a million years,” says La Trobe’s Andy Herries, a co-author.

“Like all other creatures on earth, to remain successful 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.”

Since their discovery in 1992, the Drimolen caves have acted as a window into early hominin evolution. In 2018 they revealed some of the world’s oldest bone tools, and earlier in 2020, a research team led by Herries uncovered the earliest-known skull of Homo erectus, a much closer relative of modern humans.

The species existed from around two million to 100,000 years ago, arising at around the same time as Paranthropus robustus. The Drimolen caves also previously yielded several other P. robustus skulls, providing evidence for their co-existence with H. erectus.

But the two species were vastly different. While H. erectus had relatively large brains and small teeth, P. robustus were small-brained and large-toothed. According to co-author Angeline Leece, also from La Trobe, the two represent “divergent evolutionary experiments”.

This new study presents another P. robustus cranium – dubbed DNH 155 – that dates back further, to approximately 2.04-1.95 million years ago.

“The Drimolen fossils represent the earliest known, very first step in the long evolutionary story of Paranthropus robustus,” says Jesse Martin, co-lead author.

The skull belongs to an adult male and is similar in size to female specimens found at the same site, which is at odds with our previous understanding of the species.

Until now, explains Martin, P. robustus was thought to exist “in social structures similar to gorillas, with large dominant males living in a group of smaller females”.

“The DNH 155 male fossil from Drimolen is most similar to female specimens from the same site, whereas Paranthropus robustus specimens from other sites are appreciably different,” he says.

The specimen also suggests that the species evolved their distinctive chewing adaptations in incremental steps over hundreds of thousands of years, leading the team to argue that this is the first high-resolution evidence of microevolution in an early hominin species.

Leece explains that, over time, “Paranthropus robustus likely evolved to generate and withstand higher forces produced during biting and chewing food that was hard or mechanically challenging to process with their jaws and teeth – such as tubers.”

These adaptations are believed to have taken place during a period of environmental change, when climate records indicate that the region was drying out. The increasingly arid conditions led to the extinction of several mammal species and may have placed hominins under dietary stress.

According to the team, further research will give us a more detailed understanding of how different species of ancient humans competed for resources – and how this shaped our own evolution.

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