On Father’s Day in June 2018, Samantha Good was working on an excavation in the Drimolen cave in South Africa’s Cradle of Humankind. She uncovered what appeared to be a canine tooth jutting out from the sediment. She kept digging until she found two more teeth and a partial palate, and then alerted her instructors.

“I think I said ‘There’s something interesting happening,’ ” remembered Good, an undergraduate student studying anthropology at Vancouver Island University.

Angeline Leece, a paleoanthropologist at La Trobe University in Melbourne, came to see what Good had found. “I think my breath stopped for a second,” Leece said.

Good would learn that she had unearthed a 2-million-year-old skull that belonged to Paranthropus robustus, our large-toothed, small-brained ancient human cousin. The skull provides the best-known evidence of an ancestor of humanity evolving to adapt to a changing climate, which researchers detailed in the journal Nature Ecology & Evolution.

Around 2 million years ago, this area transformed from more lush conditions to arid ones. In order for a species like P. robustus to survive, it probably would have needed to be able to chew on tough plants.

Some scientists suggested that since they had found mostly large males in Swartkrans and mostly small females in Drimolen, that, more or less, only males lived in Swartkrans and only females in Drimolen.

“What it looked like to me instead is that … the Drimolen ones were just overall smaller,” Leece said.

Jesse Martin, a doctoral student, pieced the skull bones back together. DNH 155 was so well-preserved that one of his team members, David Strait, remarked that it had intact nasolacrimal ducts. He said, “This Paranthropus could have cried.”

DNH 155’s cranium indicated its chewing muscles were not as strong as the Swartkrans. Martin said the differences suggest DNH 155 and the other P. robustus found at Drimolen were smaller because they were earlier forms of the species belonging to a different population that hadn’t yet been subjected to the new environmental pressures.

Such a morphological change, the scientists said, was likely the result of P. robustus adapting to that changing climate, with members who were able to get enough nutrition from a change in their food supply surviving, and passing their traits to offspring.