Drimolen cranium DNH 155 documents microevolution in an early hominin species

The discovery of an adult male Paranthropus robustus cranium from Drimolen Cave in South Africa shows that variation in the species previously thought to represent sexual dimorphism is instead due to the Drimolen material representing an earlier part of the lineage of this early human side branch
Published in Ecology & Evolution
Drimolen cranium DNH 155 documents microevolution in an early hominin species

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Dimolen Cave in South Africa was first discovered in 1992 by Andre Keyser, a time when most of the current team were still at school. In 1994, Andre found the most complete skull (DNH 7; Drimolen Hominid 7) of an early human side branch in our evolution, Paranthropus robustus. The skull was also unique because it was considered to represent a female. A presumed male mandible, DNH 8, was also found at the same time. Very few complete hominin remains would subsequently come from Drimolen until the discovery in 2015 of a juvenile cranium (DNH 134) – the world’s oldest Homo erectus. In 2018, we discovered two P. robustus crania (DNH 152 and DNH 155), the latter of which is the focus of this paper. The DNH 134 cranium was named for our long term collaborator Simon Mokobane, who sadly did not live to see the discovery of the DNH 155 cranium, but whose memory lives on in everyone who has been involved at Drimolen and the experience he imparted to all of us. 

The Drimolen fossil during excavation. The site represents a deroofed cave system.

DNH 155 is an adult male P. robustus cranium, and thus provides a useful sex based comparison to the female DNH 7 cranium. While P. robustus is not directly ancestral to Homo sapiens, it provides a wonderful example of diversity in the human fossil record. This diversity, between the earliest origins of our own genus Homo and the paranthropines, demonstrates how diverging lineages adapted to changes in climate and environments, as well as potential competition with each other, in very different ways. Paranthropus jaws and teeth are massive when compared against our own, leading some researchers to conclude that the genus practiced vegetarianism, and zeroed in on particularly hard or tough foods. However, while this may have been more the case for eastern African Paranthropus, isotopic analysis shows that this South African species had a more diverse diet than the species' morphology might first suggest. The earliest discoveries of P. robustus fossils were made at Kromdraai - around 6 km from Drimolen - in 1938, but the material mostly consisted of isolated teeth and one partial skull. Nothing is known of P. robustus outside of a small area of limestone (10 km by 25 km) located outside Johannesburg, now known as the Cradle of Humankind, a UNESCO World Heritage Area. Fragmentary material has also been recovered from the sites of Gondolin, Sterkfontein, and Coopers, but most of what is known about the species has come from the site of Swartkrans, where several more complete crania have been recovered. However, many of these are found in solid breccia and have been crushed or distorted. The discovery of the male DNH 155 cranium outlined in our paper, combined with the female DNH 7 cranium allows, for the first time, a detailed comparison of relatively complete and undistorted male and female crania from Drimolen.

Andy showing the 2019 field school students the male DNH 155 (left) and female DNH 7 (right) crania (left) and the faces of the same two fossils.  

Andy first visited Drimolen as a PhD student in 2000 when he and another University of Liverpool PhD student Emma Nelson were shown the site by Andre. Both being cavers Andy and Andre got on extremely well and spent the whole day chatting, also visiting the nearby site of Sterkfontein to chat about the work Andy was doing their for his PhD. Sadly Andre passed away in 2010, the same year Andy revisited Drimolen  to undertake palaeomagnetic analysis. The opportunity for Andy to undertake research at Drimolen came about because he had just worked with some of the Drimolen excavators from the University of Johannesburg (including the current Drimolen lead permit holder Steph) at the famous Australopithecus site of Taung. Steph had been working on the Taung project as part of her honours and Masters research at the University of the Witwatersrand, but would go on to join the Drimolen field schools in the coming years as a student tutor until she joined the permit in 2014. She is now completing her PhD at the University of Johannesburg.

Andy returned to Drimolen for excavations in 2012 with a University of Johannesburg and Italian team that included Giovanni, who has remained a key part of the current Drimolen team. It was on this season that Andy found his first early hominin fossil at the site, an isolated Paranthropus robustus molar (DNH 122; Drimolen Hominid number 122). In 2013, at the request of the University of Johannesburg team, Andy started running a joint field school at Drimolen and the research there became a focus of his then Australian Research Council Future Fellowship (2012-2016) and current Discovery Grant (2018-present). Because of this collaboration the field school students have mostly been a mix of South Africans and Australians. Angeline came on the first field school in 2013 and had just finished her undergraduate degree in the United States with David, who later joined the project in 2016. Angeline has become a permanent fixture at Drimolen, moving to Australia, leading the study of the hominin teeth and recently completing her PhD at La Trobe. Jesse, the paper’s lead author, first came on the field school in 2015 and is now finishing his PhD at La Trobe working on the hominin cranial remains from Drimolen. Despite their early career status both Angeline and Jesse have been instrumental in the success of the Drimolen project, helping to direct excavations and lead the hominin research. This paper is primarily the publication of their PhD research.

The 2018 Drimolen Field School excavation team: (from top left: David Strait, Reibeck Nsibande, Daniel Toomey, Giovanni Boschian, Jesse Martin, Angeline Leece, Banele Dosi, Khethi Nkosi, Andy Herries, Amber Jaeger, Alana Sutton, Anastasia Cajigal, Samantha Good, Rachel Sender, Nthabeleng Rantso, Stephanie Baker, Eyad Maleb, Julia Praeger, Eunice Lalunio, Victoria Hain, Rachel Brough, Manuel Guerra)

After an extremely difficult period for the project, and primarily through the support of the Drimolen landowner, Khethi Nkosi, and the University of Johannesburg, Steph became the lead permit holder at Drimolen in 2017 and invited Andy to be a joint permit holder. The same year Andy was successful with an Australian Research Council Discovery Grant for work at the site. The 2018 field season thus represented a fresh start at the site and we began running a joint field school with David at Washington University in St Louis that year. Critically, this also involved the landowner, Khethi, who is a prominent businessman in South Africa with a focus on digital infrastructures, so this work on ancient heritage was new to him. Our work would not be possible without the local support of the landowners, and Khethi coming to excavate with us at Drimolen was a really important moment. It is quite rare for a landowner to be so directly involved with research on their land in South Africa.. Khethi even went on to discover another partial cranium of P. robustus (DNH 152), recently published in Science. The new Drimolen team has always included African students and local communities, an objective assisted by the field school scholarships for African students that are funded from the participation of international students, as well as students from the recently formed Palaeo-Research Institute at the University of Johannesburg. In 2018 we have students from the University of Johannesburg, University of Cape Town and Lesotho, but we hope to provide even more places in the future. Moreover, as the lead permit holder Steph is currently the only female South African director of an early human fossil site in the Cradle of Humankind World Heritage Area. She was also recently named one of the 'Top 200 young South Africans of 2020'.

Stephanie and Angeline excavating the DNH 155 cranium

In 2018, this new team began a new excavation strategy at the site. A number of years of excavation at the nearby, chronologically older site of Drimolen Makondo had taught us which specific areas in these palaeocave deposits tended to best preserve fossils. Instead of concentrating on the collapsed ex-situ deposits from which previous excavators had recovered DNH 7, they targeted key areas of the in situ deposits. This strategy would turn out to be very fruitful. DNH 155 itself was discovered on South African Father’s Day June 17th 2018 by a Canadian field school student, Samantha Good. When the rest of us first saw the fossil, Sam had already done a beautiful job exposing most of one side of the maxilla, the cranium being upside down. This was the point at which the shout of “"I've found something you're going to want to look at!” rang around the cave walls. The air became electric as Angeline climbed to the back of the cave where Samantha was excavating and everyone waited for the identification of what that something might be. Obviously all the fossils we find at Drimolen are interesting, with species such as Dinofelis (a false sabre tooth cat) having also been discovered, but obviously the finding of a hominin fossil, be it a single tooth or a whole cranium, is always exciting as it maybe a little bit of our direct ancestry, that everyone on the excavation, no matter their nationality, maybe a descendant from. We have found hominin fossils every season I have excavated at Drimolen and so finding the first for the season is always a weight lifted that we keep that record of discovery. 

Samantha Good next to the DNH 155 cranium while in situ (in red square; left) and holding the newly reconstructed face of DNH 155 back at camp (right)

Paranthropus teeth are immediately recognisable and the fact there was a row of them together was highly unusual. Several days of pain-staking excavation by Angeline, Steph, Sam and myself indicated that the whole cranium might be preserved. Fossils coming from newly decalcified breccia at Drimolen are extremely fragile and very easy to break until they have been consolidated with paraloid (a conservator’s glue). The advantage, though, is that fossils can be collected in pieces and reconstructed with little distortion, whereas those recovered from solid breccia cannot as easily be reconstructed and are often deformed. The cranium was eventually removed as a whole block of lightly consolidated sediment. This was then transported back to the field camp and the cranium was further cleaned and reconstructed over a two-week period by Jesse, the current reconstruction being further finished back in the hominin fossil vault at the University of the Witwatersrand by Angeline and Jesse over another few weeks. But even as it was being cleaned and pieced together in camp, it was clear that the DNH 155 cranium was going to represent the most complete and undistorted cranium of an adult male P. robustus yet discovered, and that it possessed anatomy that was going to force a change in how we think about P. robustus.  David was as giddy as a schoolchild (although if you knew David, you’d understand that this is not far off of his baseline mood with dancing a regular feature of hominin finds at Drimolen). Then the hard work started in figuring out precisely what we had found and how it compared to all the other Paranthropus material.

Jesse excavating DNH 155 out of the block of sediment removed from the site back at the field camp. The right hand side of the maxilla that Sam first found is on the lower right of the block and Jesse can be seen excavating out the left side of the maxilla, showing how the two were displaced relative to each other before reconstruction. 

The nearby sites of Swartkrans and Kromdraai (within 7 km) have yielded a number of what are considered to be male specimens of Paranthropus, however many of the fossils from these sites were recovered from hard breccia and the skulls have been crushed or significantly deformed. Nonetheless, the anatomical differences between DNH 7 and the fossils from Swartkrans were originally assumed to be due to sexual dimorphism (the phenomenon in which males and females of the same species have different forms).  However, Angeline and Jesse’s PhD research and analysis of DNH 155 now tells us is that males and females at Drimolen strongly resemble each other, and both differ from specimens at Swartkrans. Thus, this variability is not simply due to sexual dimorphism. Instead, these differences are likely due to changes in P. robustus over time, with Drimolen representing an early population dating to around 2 million years and the Swartkrans Hanging Remnant specimens likely dating closer to 1.8 million years. While these differences may be considered by some researchers to represent species level differences we have attributed the Drimolen crania to an early part of the P. robustus lineage, and not to yet another new species (many of which have been named by paleoanthropologists over the past quarter century). Indeed, we believe that our discovery should force paleoanthropologists to rethink the conventions governing how we recognize new hominin species. 

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