This Science News Wire page contains a press release issued by an organization and is provided to you "as is" with little or no review from Phys.org staff.

An enigmatic phase in human evolution: Australopithecus sediba

The fossil site of Malapa in the Cradle of Humankind, South Africa, discovered by Lee Berger of the University of the Witwatersrand, Johannesburg, in August 2008, has been one of the most productive sites of the 21st century for fossils of early human ancestors or hominins. A new hominin species, Australopithecus sediba (Au. sediba), was named by Berger and his colleagues, following the discovery of two partial skeletons just under two million years old, a juvenile male individual—Malapa Hominin 1 (MH1) - and an adult female, Malapa Hominin 2 (MH2). The skeletons are under the custodianship of the University of the Witwatersrand, where they are presently housed. Each partial skeleton is more complete than the famous "Lucy," an Australopithecus afarensis or early hominin species found in 1974 in Ethiopia. Now, 10 years later after the discovery of Malapa, full descriptions of the hominin fossil material, as well as raw measurement data and surface scans of the fossils, available at Morphosource.org, are published in a special issue of the open access journal, PaleoAnthropology. Chaffey College Anthropology professor Dr. Marc R. Meyer co-authored one of the research articles on the vertebral column and ribcage of this enigmatic fossil species.

The special issue is comprised of nine separate papers analyzing: the skull; vertebral column and thorax; pelvis; upper limb: shoulder, arm and forearm; hand; and lower limb fossils of Au. sediba; along with descriptions of body size and proportions; and walking mechanics, including a 3-D computer animation of Au. sediba walking. The papers are co-authored by leading anthropologists, who are members of the main group of researchers that Berger had assembled for the study of the Malapa material. The research draws on approximately 135 specimens from MH1, MH2 and what may be a third individual, all of which were uncovered between 2008 and 2016.

The researchers find that Au. sediba is in fact a unique species, refuting earlier critics who questioned its validity as a species. Au. sediba is distinct from both Australopithecus africanus, with which it shares a close geographic proximity, and from early members of the genus Homo (e.g., Homo habilis) in both East and South Africa; yet, it also shares features with both groups, suggesting a close evolutionary relationship.

These new research papers address critiques of Au. sediba from other colleagues while correcting some initial observations and testing new ideas regarding this extraordinary collection. For example, other researchers hypothesized that this was more than one species due to the differences in the size and shape of the vertebrae. "The differences in these vertebrae can simply be attributed to their developmental age differences: the juvenile individual's vertebrae have not yet completed growth, whereas the adult's vertebra growth is complete," explained co-editor Scott A. Williams, an associate professor of anthropology in the Center for the Study of Human Origins at New York University, and co-author of two of the papers, including the one on the vertebral column.

The special issue also finds that Au. sediba was well adapted to terrestrial bipedalism or walking on just two feet but also spent significant time climbing in trees, perhaps for foraging and protection from predators. This larger picture sheds light on the lifeways of Au. sediba and also (whether directly or indirectly) on a major transition in hominin evolution, that of the largely ape-like species included broadly in the genus Australopithecus to the earliest members of our own genus, Homo.

More information:
The vertebrae, ribs, and sternum of Australopithecus sediba (Williams, S.A., Meyer, M.R., Nalla, S., García-Martínez, D., Nalley, T.K., Eyre, J., Prang, T.C., Bastir, M., Schmid, P., Churchill, S.E., and Berger, L.R.)

Provided by Chaffey College