‘Father’s Day Fossil’ overturns rule of attributing hominin skull differences to species differences

They called it the “Father’s Day Fossil.”

It definitely is a family thing – the family of man. A 2 million-year-old skull discovered in South Africa on June 17, 2018, has provided researchers with evidence of evolutionary adaption to climate change.

The skull of a male Paranthropus robustus was found in the Drimolen Main Quarry, a paleocave in South Africa. It is smaller than skulls unearthed at the nearby Swartkrans site.

The discovery was reported in an article published in the journal Nature Ecology & Evolution on Nov. 9. Stephanie Baker of the University of Johannesburg, who is a co-director of the project, said they quickly realized the scope of what they had found.

“The fossil was discovered on Father’s Day 2018 by a student, Samantha Good,” Baker said. “The whole skull was inverted and the dental row was exposed first – which was awesome because we immediately knew what it was! It could only be Paranthropus with big teeth like that.

“Thereafter, myself and Angeline Leece meticulously extracted the specimen over the course of two weeks. We prepared it out in chunks, which we then scanned and passed on to our colleague Jesse Martin. He is a reconstruction expert. He then worked on cleaning each individual piece and gluing it to the adjacent fragments with a fossil consolidant glue called paraloid.”

This discovery has a very important implication for the field of palaeoarchaeology: skull differences don't always equate to species differences. After their analysis and conclusions with DNH 7, the research team is now cautioning palaeoarchaeology researchers about attributing skull differences to species differences. It’s a complex issue, Baker said.

“This is best answered with a bit of background," she noted. "The first Drimolen P. robustus skull ever discovered is DNH 7, the former most complete cranium of the species ever found. Then there are a couple well-preserved specimens from places such as Swartkrans. Originally, it was believed that the differences we saw between DNH 7 and the others was because she was female, and they were all male. 

“The DNH 155 Drimolen find acts as one such instance where overall its shared morphology with other well-known examples of the species, and DNH 7 are characteristically P. robustus. The flat face, sagital crest, giant teeth. But there are some interesting nuanced features that it doesn't share with the Swartkrans individuals. These are the same difference that we saw in DNH 7 but put it down to sexual dimorphism. Because DNH 155 is male, it turns out that these differences are not led by the sex but rather by time. Drimolen is 200,000 years older than Swartkrans.”

Baker and her team report that they have observed evolution within a single lineage.

"It is exceedingly rare that we — as palaeoanthropologists — can tease out episodes of evolution on the lineage for any one ancient species," Baker said.

She said the researchers concluded the Drimolen population represents the beginnings of the species in southern Africa, and that they quickly adapted to chew very hard foods in response to a shifting climate. 

“As a long-winded way around to this point, the skulls are still the same species, just at different points on their evolutionary trajectory,” Baker said. “Naming them a new species would imply that they were not as closely related as they are.”

There are important implications for the field of paeleoarchaeology, she said. One important implication is that some hominin fossils may be incorrectly classified as different species, making researchers think they are not closely related.

“An argument could be made for continued dating at the various cave sites in the Cradle,” Baker said. “Constantly refining the dates of these sites allows us scientists tighter temporal control of the fossils that come from them. Dates are so important to understanding the flow [or lack thereof] of human evolution.”

Part of the thrill with this find was the unexpected nature of it, she admitted.

“We are always surprised when something new comes along. We don't know what to expect, which is what makes this field so exciting,” Baker said. “I doubt people saw this coming. It is so rare to have such fine-scale resolution on the dates of a cave site, that commentary on micro-evolution just don’t exist.”

In addition the experience of seeing this “small-brained extinct hominin” as he appeared in life is thrilling, she said.