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A new study claims certain features of Little Foot’s skull mark it as a member of the contested species Australopithecus prometheus.

Ron Clarke

Identity of Little Foot fossil stirs controversy

More than 20 years ago, paleoanthropologists began to painstakingly excavate the rock-encased skeleton of an ancient hominin from deep inside a South African cave. Last week, they offered the first in-depth measurements of the skeleton dubbed “Little Foot,” the most complete ancient hominin in the fossil record. Now, researchers say the skeleton is about 3.67 million years old, a member of the genus Australopithecus, and from an eldery female. But how she fits into the broader picture of hominin evolution—and which species she belongs to—has sparked fierce debate among competing teams.

The specimen, known formally as StW 573, was discovered in South Africa’s Sterkfontein cave system in 1998. Ronald Clarke, a paleoanthropologist at the University of the Witwatersrand in Johannesburg, South Africa, and his team have spent the past 20 years carefully freeing it from rocky material that had entombed it over millions of years.

Now, in four papers under review at the Journal of Human Evolution and published to the bioRxiv preprint server last week, Clarke and colleagues offer a long-awaited analysis of the skeleton. They say—based on the age of the sediments around the fossil—Little Foot lived some 3.67 million years ago, about a million years earlier than previous claims. The skeleton’s relatively small stature and certain skull features suggest it was probably a female of advanced age, with a brain size of about 408 cubic centimeters, about one-third the size of modern human brains. Little Foot apparently suffered a forearm injury early in life, and her relatively long legs, in proportion to her arms, suggest she probably walked upright more than she swung through trees.

Based on earlier data, many paleoanthropologists thought Little Foot was a member of Australopithecus africanus, a well-established lineage of upright walkers that lived between 3.3 million and 2.1 million years ago. Many other South African specimens, including some from the very same cave, have been assigned to the species.

But Clarke argues that a number of features differentiate Little Foot—and at least a dozen other nearby fossils—from A. africanus. These include larger, flatter faces with a wider distance between eye sockets; larger canines and forward-tilting incisors; larger mandibles; and slightly concave foreheads. Differences in teeth wear indicate A. africanus was omnivorous, whereas Little Foot and her kin were mostly vegetarian, Clarke argues. Together, he says, that suggests two species of hominins were living near the caves some 3 million years ago.

Clarke says Little Foot’s features most closely match A. prometheus, a species proposed in 1948 by anthropologist Raymond Dart. Clarke makes a “convincing,” argument, says Dean Falk, an evolutionary anthropologist at Florida State University in Tallahassee who wasn’t involved in the work. If Clarke is right, “Everybody has been lumping two species into one species.” Knowing that they are actually separate, she says, could shed light on which species gave rise to later ones in the region, filling in several evolutionary gaps.

Yet the designation drew swift condemnation from paleoanthropologists Lee Berger, also at the University of the Witwatersrand, and John Hawks at the University of Wisconsin in Madison. In a paper slated to be published this week in the American Journal of Physical Anthropology, the pair argues that the name A. prometheus was originally poorly defined and shouldn’t be used to classify the remains. Berger further says the fossil is unlikely to be as old as Clarke’s team claims. And he thinks the skull has been too distorted over millions of years to be accurately measured without major reconstructive work. “These papers have a dearth of data,” he says. “They’re making a lot of ‘just believe me’ claims.”

William Kimbel, an anthropologist at Arizona State University in Tempe and an expert on another ancient female hominin—the famous A. afarensis skeleton Lucy—disagrees. “The work is certainly not shoddy … [it’s] incomplete.” He says the next step should be to examine the hundreds of other Australopithecus fossils found throughout South Africa and east Africa and calculate just how much variation would be expected within a species. Then, researchers could determine whether Little Foot falls outside that range. If so, it may indeed warrant being labeled a distinct species, Kimbel says. Whatever the species, this remarkably complete skeleton includes so many body parts that future study is sure to reveal a lot about australopithecines, others say.

Carol Ward, an evolutionary anatomist at the University of Missouri School of Medicine in Columbia, largely agrees with that assessment. She trusts Clarke’s measurements, but, “There’s nothing that neatly separates these fossils into species piles. … I think the jury is still out.”