The species had a brain that was slightly larger than a chimp's and about 60 percent smaller than a modern human's.
P. robustus is a descendent of Australopithecus afarensis, a group first chronicled by the discovery of the famous "Lucy" fossil in Ethiopia (get Lucy fast facts).
Lucy's kin also gave rise to the highly adaptable, tool-using genus Homo, which includes Homo sapiens, or modern humans.
Matt Sponheimer, an anthropologist at the University of Colorado at Boulder who led the study, says the very different fates of "Lucy's children" have always puzzled scientists.
(Read a related feature on human migration in National Geographic magazine.)
"There's this question about this tale of two children, as it were, one of which flourishes [and] the other which doesn't," Sponheimer said. "And the question is, Why?
"What I think we've done in this study is show that traditional explanations as to why [P. robustus went extinct] are probably, at the very least, dramatically oversimplified and possibly just outright incorrect."
Sponheimer and his colleagues suggest that other biological, social, or cultural factors may explain the ultimate disappearance of P. robustus.
As for the success of the Homo species who lived at the same time as P. robustus, the paeloanthropologists says that any number of factors may explain why our ancestor survived and P. robustus did not.
"It would be very interesting if it turned out that the main difference was that we just were able to procreate more quickly—we were the rabbits to [P. robustus's] apes," Sponheimer said.
The new findings are also significant because they showcase the powerful new dental investigative technique known as laser ablation.
The method was advanced by study co-authors Benjamin Passey and Thure Cerling at the University of Utah in Salt Lake City.
Sponheimer says the innovation enables scientists to uncover a trove of previously untapped data that lie preserved in the fossil teeth of early humans and other ancient and modern animals.
By studying the growth rings and chemical composition of teeth, for example, researchers can reveal dietary habits, detect changes in climate, and possibly track migrations.
Stanley H. Ambrose, an anthropologist at the University of Illinois, Urbana-Champaign, said the study's findings are "very exciting."
He says the new dental technique opens new possibilities to learn not only what early humans and ancient animals ate but also how they interacted within their communities and how their environments changed over time.
"It will be a great tool because it will allow a fairly high-resolution monitoring" of dietary and seasonal climate changes within ancient animal communities, he said.
In a related perspective article in Science, Ambrose says the dental detective technique could also explore whether environmental changes helped drive the evolution of modern humans.
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