Close examination of the lower jawbone, teeth and skeleton of the hominid species Australopithecus sediba proves conclusively that it is uniquely different from a closely related species, Australopithecus africanus, according to a series of papers authored by a scientific team that includes several Texas A&M University researchers.
Darryl de Ruiter, associate professor in the Department of Anthropology, is the lead author or co-author in a series of six papers detailing the findings in the current issue of Science magazine. Also included in the authorship are associate professor Thom Dewitt of the Department of Wildlife and Fisheries Sciences, as well as Keely Carlson, a current doctoral student, and Juliet Brophy, a recent doctoral graduate in the Department of Anthropology at Texas A&M.
In 2010 the team, comprising researchers from the United States, Africa, Europe and Australia, discovered skeletal remains in a South African cave located about 30 miles from Johannesburg and dated to about 1.98 million years ago. The team named the new species Australopithecus sediba and showed that it displayed a mosaic of both human-like and ape-like characteristics shared with other forms of Australopithecus and modern-day humans.
However, some researchers contended the new skeletons belonged to a closely related species, Australopithecus africanus, and therefore did not represent a new species.
"We looked at the jawbone area and found definitive proof that the two are not the same species," de Ruiter says of the findings.
"Australopithecus sediba is unique in size, shape and pattern of growth, though it does share features with older australopiths, as well as later specimens referred to Homo erectus. It represents a transitional form between australopiths and Homo, the genus to which we humans belong, though it looks more like Homo than any other australopith ever found."
Australopiths belong to Australopithecus, a genus of hominins now extinct. Ape-like in structure and lifeways, yet walking bipedally similar to modern humans, they are believed to have played a significant role in human evolution, and it is generally held among anthropologists that some form of Australopithecus eventually evolved into Homo. They are just not sure which form of Australopithecus, which is why Australopithecus sediba, with its unique arrangement of Homo-like features, is so intriguing.
"We examined the remains and found several distinct individuals -- possibly representing a family group. They all seemed to have died suddenly in the same event about 1.98 million years ago, but the remains are in surprisingly good shape."
de Ruiter adds that the findings "show very strong support of Darwin's theory of evolution by natural selection."
The team used a method called morphometrics that uses math and 3-D models to form a precise replica of the jawbones of species of Australopithecus and early Homo, but showing distinct differences in size and shape between them. In addition, they were able to demonstrate the growth trajectory from a juvenile to an adult form was unlike that of any other hominin species known, further supporting the unique appearance of Australopithecus sediba.
In 2012 several of these same researchers, including de Ruiter, proved that Australopithecus sediba had a forest-based diet of leaves, fruits, nuts and bark, one similar to that of a present-day chimp. The diet of early Australopithecus is a key component central to the study of human origins.
The team's work was funded by the South African National Research Foundation, the Palaeontological Scientific Trust, the L.S.B. Leakey Foundation Baldwin Fellowship, the National Geographic Society, the Institute for Human Evolution at the University of Witwatersrand in Johannesburg, the Program to Enhance Scholarly and Creative Activities and the International Research Travel Assistance Grant of Texas A&M, and the Ray A. Rothrock '77 Fellows Program in the College of Liberal Arts at Texas A&M.
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