A characteristic of reports about human ancestors has been the hailing of a particular fossil species as a critical link in the evolutionary chain. Invariably, this gets plenty of media publicity. With the passing of time, the discoveries do not seem so dramatic and the puzzles and challenges appear unresolved. We are currently witnessing this pattern of discourse in relation to Australopithecus sediba, known from two specimens recovered in 2008 from the Malapa Cave site in South Africa and published in 2010. The most recent analyses appeared in Science in April 2013, and some have suggested "that A. sediba may just be the most important hominin (modern humans and their extinct relatives) discovery yet." The New Scientist report commenced thus:
"Our closest non-human ancestor lived in South Africa. That's the conclusion of a battery of studies carried out on two strange skeletons discovered near Johannesburg in 2008. They represent a likely stepping stone between the ape-like australopiths and the first members of our own genus."
Fossil elements of the two partial skeletons of Australopithecus sediba. The grey shadows represent skeletons of A. africanus, adjusted for different body proportions. (Credit: Science/AAAS. Source here)
It is customary for the discoverer and co-workers to present the strongest arguments they can find to attract interest in their research. In this case, the thesis is that A. sediba is a "mosaic of ancient australopith and modern Homo features" and that this makes it convincing as a transitional form. An independent assessment of the findings has been made by William Kimbel in Nature. He is not convinced.
"[The two skeletons] have been the focus of scrutiny because of both their excellent preservation and claims that this hominin - a species more closely related to humans than to chimpanzees - lies at the base of the Homo lineage. A series of reports published in Science sheds light on the morphology of A. sediba but, in my view, does little to elucidate its role in later human evolution." (p.573)
First, Kimbel looks at dental morphology. He finds methodology problems that raise serious questions about the conclusions drawn.
"The researchers take] the unconventional step of using only the Arizona State University Dental Anthropology System - a graded series of minor crown variants originally devised to distinguish recent human populations from one another - to decipher relationships between hominin species that are millions of years old. I have serious doubts about the phylogenetic meaning of morphological similarity in this case. These concerns are compounded by the authors' reliance on the gorilla as the sole outgroup in their cladistic analysis." (p.573)
Second, analysis of the mandible suggests that insufficient attention has been given to the samples comprising a sub-adult (with only its second molar erupted) and a presumed adult female.
Third, the upper and lower limb dimensions show many ape-like affinities and suggest arboreal climbing behavior. The New Scientist report refers to this as an "unusual feature of A. sediba: its arms and legs show it was far more comfortable swinging in the trees than most australopiths."
Fourth, the rib cage and vertebral column was claimed to be Homo-like. Kimbel raises questions about what is primitive and what derived and suggests that there is nothing in the fossil material that can be regarded as unexpected.
Finally, the issue of gait is considered. Ann Gibbons drew attention to sediba's unusual way of walking in an article for Science:
"If you happened to be in South Africa about 2 million years ago, you might have seen an odd sight: an older female hominin sashaying down a wooded slope, perhaps in search of water. She walked upright, but she wasn't human, and she moved with what to our eyes would have looked like a distinctly strange gait. She was a member of Australopithecus sediba, and [. . .] she may have twisted from side to side, rolling her feet inward with each step. "Sediba's got swag," says paleoanthropologist Lee Berger of the University of the Witwatersrand in Johannesburg, South Africa. (p.132)
Kimbel is not impressed. He describes the research conclusions as controversial.
"The proposed 'hyperpronation' of the foot and extreme inward rotation of the leg and thigh suggest an ungainly bipedal stride that might have made it into Monty Python's 'Ministry of Silly Walks' sketch. The presumed inversion of the foot at the heel-strike of the surprisingly ape-like calcaneus, combined with a vertical shank (tibia), outwardly angled thigh (femur) and a long, lordotic lower back - all hallmarks of terrestrial bipedality in Australopithecus and Homo species - constrains the reconstruction. Prominent osteophytic growths on the pelvis and fibula at the attachment sites of the thigh musculature raise the possibility of a gait that was pathologically impaired, but DeSilva and colleagues argue that this locomotor pattern was adaptive. However, if A. sediba was a descendant of A. africanus, which, similarly to the even older A. afarensis (dating to between 3.7 million and 3.0 million years ago), shows no trace of this pattern, then it is hard to imagine the selective advantage that would accrue from such a kinematically peculiar gait." (p.574)
The general conclusion from Kimbel is that the grounds for claiming A. sediba to be a transitional fossil in the human lineage are weak. The research tells us more about the Australopithecenes, but little more.
"Given the mix of features seen in A. sediba, it is difficult to understand why these researchers insist that it lies at the base of the Homo lineage. Similar intellectual gymnastics are required to comprehend the authors' argument that no African Homo fossils exist from before the time of A. sediba. Although the recent papers constitute a fascinating further analysis of the A. sediba fossils, I do not think that they provide compelling evidence that this species is anything other than an unusual australopith from a Pliocene-Pleistocene time period that is already populated by a fair number of them." (p.574)
Hesitation on hominin history
William H. Kimbel
Nature, 497, 573-574 (30 May 2013) | doi:10.1038/497573a
Extensive studies of fossil skeletons of Australopithecus sediba provide fascinating details of the anatomy of this hominin species, but do not convincingly indicate its position on the evolutionary route to modern humans.
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