Males weren’t always bigger than females in this hominid species

An ancient, distant human cousin from southern Africa called Paranthropus robustus has for the first time revealed molecular clues to its evolutionary status.

Protein sequences preserved in four partial P. robustus teeth from different individuals that lived roughly 2 million years ago indicate that larger and smaller fossils of this hominid species cannot always be classed as male or female, as previously thought, researchers report in the May 29 Science.

Sequences of a protein derived from a gene located only on the Y, or male, sex chromosome in present-day humans enabled the scientists to identify two teeth as having belonged to males, molecular biologist Palesa Madupe of the University of Copenhagen and colleagues say. One of those teeth was previously thought to have come from a female, based on its small size. Closer analyses of the two teeth lacking that male-specific protein indicated that those fossils, which are around the same size as the smaller male tooth, came from females.

“Paleoanthropologists have long known that our use of tooth size to estimate sex was fraught with uncertainty, but it was the best we had,” says paleoanthropologist Paul Constantino of Saint Michael’s College in Colchester, Vt. “Being able to accurately identify the sex of fossils using proteins will be hugely impactful.”

Proteins survive far longer in fossils than DNA does, especially in hot climates where genetic material degrades rapidly. The oldest DNA recovered in Africa dates to around 18,000 years ago.

Analysis of another protein indicated that one of the four fossil teeth came from a P. robustus individual more distantly related to the other three species members, the researchers say.

Two teeth carried a specific protein variant that the researchers initially thought characterized all members of P. robustus. But another tooth yielded a different version of that protein. And a fourth tooth displayed both protein variants.

Those signs of genetic diversity suggest that distinct groups of P. robustus inhabited southern Africa, the scientists conclude. Mating among different groups may have resulted in the range of protein variants observed in the four fossilized teeth, or perhaps the tooth identified as a protein outlier came from another Paranthropus species. Protein analyses of a larger number of P. robustus specimens are needed to explore those possibilities, the researchers say.

Other studies have identified skeletal differences in P. robustus finds from different South African sites, suggesting to some investigators that these fossils encompass more than one species.

“A combination of [fossil] and protein analysis on more specimens from P. robustus sites will further illuminate that theory,” says study coauthor Claire Koenig, also a University of Copenhagen molecular biologist.

The new study represents a first step toward using ancient protein sequences to check whether larger- and smaller-bodied fossil hominids belonged to different sexes or to different species, says paleoanthropologist Bernard Wood of the George Washington University in Washington, D.C., who was not part of Madupe’s group. Wood calls this advance “a big deal.”

Paranthropus fossils generally thought to represent three species — P. aethiopicus and P. boisei in East Africa and P. robustus at several sites in South Africa — date to between about 2.7 million and 1 million years ago. Characteristic features of these species include large jaws and teeth, wide faces and bony crests atop the skull that anchored chewing muscles.

Proposed evolutionary relationships between Paranthropus species and comparably ancient but more humanlike species, such as Australopithecus africanus in South Africa, are controversial. Madupe and colleagues suspect that protein comparisons of these species will take place. In another investigation, the researchers extracted protein sequences from an A. africanus tooth fragment that identified it as having belonged to a male.

If protein sequences can be removed from East African P. boisei fossils, comparisons to P. robustus will help to clarify whether these geographically distant populations were closely related, Wood says.