La Trobe archaeologists have described 18 new hominin fossils, including a second Homo erectus skull fragment that confirms the species first appeared in southern Africa 2.0 million years ago—200,000 years earlier than traditionally thought.

The fossils also include representatives of another species, P. robustus, and show that it was undergoing microevolutionary change in response to climatic pressures.

The Drimolen Main Quarry in South Africa’s “Cradle of Humankind” is one of the richest deposits of human ancestors in the world. Yet many of the fossils excavated between its discovery in 1992 and 2019 were unpublished and therefore unknown to science.

La Trobe archaeologists, together with colleagues from South Africa and the United States, have now described 18 more fossils from the site and attributed them to Homo erectus and P. robustus.

Adult fossil confirms earlier Homo erectus appearance

In a paper published in Annals of Human Biology, the scientists report identifying the first adult Homo erectus fossil at Drimolen, dated to 2 million years ago. It is 200,000 years older than other adult Homo erectus fossils found elsewhere, including those from Dmanisi in Georgia, which are dated to 1.8 million years ago.

“Homo erectus is the first species of hominin that researchers broadly agree fits within our own genus Homo and is likely directly ancestral to us. Drimolen preserves evidence for the first appearance of this species 2 million years ago. However, the original fossil, DNH 134, belonged to a child between ages 2 and 4, and there was uncertainty about what an adult may have looked like,” lead researcher Dr. Jesse Martin said.

“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.”

P. robustus evolved larger teeth and chewing muscles

The researchers have also confirmed that P. robustus underwent microevolution, developing larger teeth and chewing muscles between 2 million and 1.8 million years ago.

Martin said that while Homo erectus was evolving larger brains and smaller molar teeth, possibly associated with eating meat, P. robustus maintained a brain size slightly larger than that of chimpanzees and evolved massive molar teeth to process tough, hard foods—like a mortar and pestle.

“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.

“This paper confirms that the P. robustus fossils from the 2-million-year-old Drimolen cave are slightly but importantly different from the P. robustus fossils at the 1.8-million-year-old Swartkrans cave.

“During this 200,000-year period, P. robustus teeth became even larger, and so did the muscles associated with chewing. 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.

“However, the large number of fossils from Drimolen, some of which are published in this paper, allows an unprecedented insight into the microevolutionary changes that shaped human ancestors.”

Martin said the uniqueness of the Drimolen P. robustus population had previously been recognized with a new subspecies name, P. robustus ukusa, and the fossils described in the research paper support this formal recognition.

The study was conducted by scientists from the Palaeoscience Labs, Department of Archaeology and History, La Trobe University; Palaeo-Research Institute, University of Johannesburg; Evolutionary Studies Institute, University of the Witwatersrand; and Department of Anthropology, Washington University.