Science1 publisher2 min readPublished
Adult skull fragment from Drimolen backs a 2-million-year date for Homo erectus in South Africa
La Trobe researchers describe DNH 127, the first adult Homo erectus fossil from South Africa's Drimolen cave, dated to 2 million years ago. It is 200,000 years older than adult specimens found elsewhere and supports a first-appearance date that had rested on a child's skull from the same cave.
The Scientist · Science desk
What happened
- La Trobe archaeologists and colleagues from South Africa and the U.S. described 18 hominin fossils from Drimolen Main Quarry, attributing them to Homo erectus and P. robustus.
- Lead researcher Jesse Martin said H. erectus was evolving larger brains and smaller molars, possibly tied to eating meat, while P. robustus evolved massive molars for tough, hard foods.
- Drimolen's 2-million-year-old P. robustus fossils differ slightly from those at 1.8-million-year-old Swartkrans, where the teeth and chewing muscles are larger.
- The researchers say the new fossils support P. robustus ukusa, a subspecies name already given to the Drimolen population.
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Why it matters
- capability A P. robustus sample this large lets researchers track small changes inside one species. Sparse hominin records usually leave only wide gaps between specimens to guess across.
- constraint The tooth trend rests on two caves at two dates, so it cannot by itself separate change over time from difference between sites. The climate explanation needs environmental evidence of its own.
- constraint Only two H. erectus specimens are known from Drimolen. If either one were re-dated or re-attributed, the southern African first-appearance date would move with it.
- precedent Added support for P. robustus ukusa makes it more likely that later work treats the Drimolen and Swartkrans robust hominins as separate subspecies.
Drimolen's Homo erectus record now stands at two specimens, the child DNH 134 and the adult DNH 127. That is a small base for a species' first appearance [2]. Lead researcher Jesse Martin of La Trobe University presented the new fossil as support for a date already on record. "This paper publishes a new adult fossil attributed to Homo erectus, DNH 127, providing further confirmation that this species first appeared in southern Africa 2 million years ago," he said [14]. Among the adult fossils it outdates are those from Dmanisi in Georgia, dated to 1.8 million years ago [3].
The other hominin in the cave was far more common [8]. "While we know that Homo erectus and the lineage leading to us were ultimately successful, 2 million years ago P. robustus outnumbered Homo erectus at Drimolen by more than 10 to 1. During this period, our ancestors looked like long shots to survive," Martin said [8]. At that ratio, H. erectus accounts for fewer than one in eleven of the hominins counted, under about 9 percent [1].
Because P. robustus is so abundant at Drimolen, the team can look for change within a single species. "Hominin fossils are so rare that generally the differences between them are huge, and researchers are left to make educated guesses about what existed in the missing parts of the fossil record," Martin said [12]. The tooth trend comes from comparing two caves, Drimolen and the younger Swartkrans [7]. A two-site design ties time to place. Larger molars at Swartkrans fit 200,000 years of change in one lineage [7]. They also fit two populations living in different settings, and two data points cannot separate those readings on their own. The phys.org summary of the work says the change was a response to "climatic pressures" [13]. The press account does not describe the climate record behind that link, or how the fossils were dated.
I think the P. robustus series is the more useful of the two results, provided the dental trend holds as more material is published. Drimolen, in South Africa's Cradle of Humankind, is one of the world's richest deposits of human ancestors [15]. Many fossils excavated there between its discovery in 1992 and 2019 went unpublished [6]. The new paper appears in Annals of Human Biology, with authors from La Trobe, the University of Johannesburg, the University of the Witwatersrand and Washington University [5][11].
What to watch
- Publication of more of the Drimolen fossils excavated between 1992 and 2019, especially any further adult H. erectus remains or additional P. robustus teeth.
- Environmental records for the Cradle of Humankind between 2.0 and 1.8 million years ago that could test the climate explanation for larger P. robustus teeth.
- Independent assessments of whether the features of the DNH 127 skull fragment place it in Homo erectus.