A dental study of fossilized remains found in South Africa in 2008 provides new support that this species is one of the closest relatives to early humans.
The teeth of this species – called Australopithecus sediba – indicate that it is also a close relative to the previously identified Australopithecus africanus. Both of these species are clearly more closely related to humans than other australopiths from east Africa, according to the new research.
The study, published in the journal Science, revealed that both africanus and sediba shared about the same number of dental traits with the first undeniably human species.
"Our study provides further evidence that sediba is indeed a very close relative of early humans, but we can't definitively determine its position relative to africanus, said Debbie Guatelli-Steinberg, co-author of the study and professor of anthropology at The Ohio State University.
The research was led by Joel D. Irish, professor of natural sciences at Liverpool John Moores University.
The sediba fossils were found in South Africa in 2008 and first described in a series of articles published in Science in 2010. That study was led by Lee Berger of the University of Witwatersrand in South Africa, who is also a co-author of this new study.
In this study, Irish, Guatelli-Steinberg and their colleagues extended that work by examining the teeth from sediba and comparing them to eight other African hominin species, which include modern humans from Africa, and extinct species of Homo, Australopithecus, and Paranthropus. In all, the researchers examined more than 340 fossils and 4,571 recent specimens. They also examined teeth from 44 gorillas for comparison.
The focus was on 22 separate traits of tooth crowns and roots that can give clues as to the relationship between the different species studied.
For example, they measured how much one of the incisors was shovel-shaped. Depending on the species in this study, the incisor may have no depression in the back of the tooth, a faint shovel shape, or a trace of that shape.
Researchers use standardized measurements from the Arizona State University Dental Anthropology System to compare the teeth on these 22 traits.
The researchers found that on 15 of these traits, sediba and africanus scored the same. Sediba shared 13 traits with Homo erectus, an early human species, which was comparable to how africanus scored.
Sediba and africanus shared five dental traits that weren't found in earlier australopiths, further showing their close relationship. Both also share five traits with early humans – Homo habilis/rudolfenis and Homo erectus—which weren't shared with earlier ancestors, demonstrating the close relationship between these two australopiths and the first humans.
Teeth are an excellent way to study relationships between different species, Guatelli-Steinberg said. They are well preserved in the fossil record, and researchers can compare large samples, at least for many ancient species.
In addition, most of the dental traits the researchers used in this analysis don't have a selective advantage that could help one species survive over another. That means if researchers see a similar trait in two species, they can be more confident that they shared a common ancestor and that the trait didn't evolve independently.
In many ways, these new dental data support the earlier research on sediba, which included analysis of the inside of the skull, hand, spine, pelvis, foot and ankle, Guatelli-Steinberg said.
"All of the research so far shows that sediba had a mosaic of primitive traits and newer traits that suggest it was a bridge between earlier australopiths and the first humans," she said.
Guatelli-Steinberg said their dental analysis showed that both africanus and sediba are more closely related to humans than the famous "Lucy" skeleton fossil found in East Africa in 1974. This fossil represented a species, Australopithecus afarensis, that was at one time was thought to be the closest relative of humans.
Lucy is estimated to have lived 3.2 million years ago. Sediba lived 1.977 million years ago, while africanus lived between 3.03 and 2.04 million years ago.
"Our research on teeth can't definitively settle if either sediba or africanus is more closely related to humans than the other species," Guatelli-Steinberg said. "But our findings do suggest that both are closely related to each other and are more closely related to humans than afarensis.
"We need to find more sediba remains to help fill in the missing pieces of this evolutionary puzzle."
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