https://www.nature.com/articles/s41559-020-01319-6
Leece said it was notable that P. robustus appeared at roughly the same time as our direct ancestor Homo erectus, as documented by an infant H. erectus cranium that the team discovered at the same Drimolen site in 2015.
“These two vastly different species, H. erectus with their relatively large brains and small teeth, and P. robustus with their relatively large teeth and small brains, represent divergent evolutionary experiments,” Leece said. “While we were the lineage that won out in the end, the fossil record suggests that P. robustus was much more common than H. erectus on the landscape two million years ago.”
More broadly, the researchers think that this discovery serves as a cautionary tale for recognizing species in the fossil record.
A large number of fossil human species have been discovered over the past quarter century, and many of these new species designations are based on a small number of fossils from only one or a few sites in small geographic areas and narrow time ranges.
“We think that paleoanthropology needs to be a bit more critical about interpreting variation in anatomy as evidence for the presence of multiple species,” Strait said. “Depending on the ages of fossil samples, differences in bony anatomy might represent changes within lineages rather than evidence of multiple species.”