"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," Martin said.

Hominin, in plain terms, refers to the group that includes modern humans and our extinct close relatives after we split from the ancestors of chimpanzees. Homo erectus is widely considered a pivotal member of that group: the first species that most experts agree belongs to our own genus, Homo, and a likely direct ancestor of ours.

A cousin evolving in a different direction

The second major finding concerns P. robustus. The researchers confirmed that this species underwent what scientists call microevolution — small, incremental changes within a species over generations — between 2 million and 1.8 million years ago. Specifically, its teeth and chewing muscles grew larger over that 200,000-year window.

The contrast with Homo erectus is striking. Martin notes that while Homo erectus was evolving larger brains and smaller molar teeth, possibly linked to eating meat, P. robustus kept a brain only slightly larger than a chimpanzee's and developed massive molars to grind tough, hard foods — functioning, as Martin put it, like a mortar and pestle.

The two species shared the same landscape, and both are preserved in the Drimolen cave deposits. But their fortunes at the site were wildly uneven.

"These two human species shared the same landscape and are both preserved in Drimolen cave. 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.

A rare window into fine-grained change

The paper also confirms that the P. robustus fossils from the 2-million-year-old Drimolen cave differ slightly but meaningfully from P. robustus fossils found at the nearby 1.8-million-year-old Swartkrans cave. During that 200,000-year span, P. robustus teeth grew even larger, and so did the muscles that powered chewing.

Martin points out why this comparison is unusual. Hominin fossils are so scarce that differences between known specimens tend to be large, forcing researchers to make educated guesses about what filled the gaps in the fossil record. The sheer number of Drimolen fossils, some described in this paper, offers what he calls an unprecedented look at the small-scale evolutionary changes that shaped our ancestors.

The scientific community had already recognized the distinctiveness of the Drimolen P. robustus population by assigning it a new subspecies name, P. robustus ukusa. The newly described fossils support that formal recognition.

The study was a collaboration between the Palaeoscience Labs in the Department of Archaeology and History at La Trobe University; the Palaeo-Research Institute at the University of Johannesburg; the Evolutionary Studies Institute at the University of the Witwatersrand; and the Department of Anthropology at Washington University in the United States.

The findings, while adding weight to the case for an early southern African origin for Homo erectus at Drimolen, rest on a still-limited fossil record — a caveat that researchers in this field routinely acknowledge. Each new specimen, however, narrows the gaps in a story that remains stubbornly incomplete.