Claim over 'human ancestor' sparks furore

Researchers dispute that hominin fossil is a new species.

Michael Cherry

Witwatersrand in Johannesburg, South Africa, has revealed two remarkably well preserved hominin fossils aged just under two million years old. The fossils were discovered at Malapa cave, part of a site known as the Cradle of Humankind, some 40 kilometres west of Johannesburg. But the researchers' suggestion that the fossils represent a transitional species in human evolution, sitting between Australopithecus  and Homo  species, has been criticized by other researchers as overstated.

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Controversially, the researchers have named the fossils as a new species, Australopithecus sediba. 'Sediba' means fountain or wellspring in Sotho, which is one of the 11 official languages of South Africa. Berger deems this an appropriate name, as he says that A. sediba  is a good candidate for being the transitional species between the southern African hominin, Australopithecus africanus, and early Homo  species — either the earlier Homo habilis  or even a direct ancestor of the more recent Homo erectus. The research is published in Science^1,2.

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But palaeoanthropologist Tim White of the University of California, Berkeley, says that A. sediba  and A. africanus are merely chronospecies: names given to describe slightly different anatomy in fossils from a single evolving species. White says that the suggestion by Berger and his team that this lineage split before the emergence of Homo  is "fossil-free speculation", adding that "the obsession with Homo  in their title and text is difficult to understand outside of a media context".

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Anthropologist Fred Grine of Stony Brook University in New York says that the authors have "not undertaken any competent analysis of variation within A. africanus  — something I do not understand in the context that three further skeletons have been found by the same team at Malapa".

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But in a press conference on 7 April, Berger defended the classification of A. sediba as a new species. The fossils "morphologically fundamentally differ from other fossil species that have been found", he said. The arms of the new fossils are long, like those of A. africanus, but the pelvises are strikingly like those of H. erectus.

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Alan Morris of the University of Cape Town in South Africa is more upbeat about the findings. "It is the presence of modern behaviours and the concomitant expansion of the brain and restructuring of the pelvis for running that defines Homo, but the evolving line that led to these developments has not been clearly visible. The Malapa specimens will rekindle the debate about the validity of the taxon Homo habilis, and will make us look more carefully at the variability of Australopithecus africanus  and her sister species," he says.

Francis Thackeray, director of the Institute for Human Evolution at the University of the Witwatersrand, agrees that the latest finds raise important questions about the ancestry of humans. "The new fossil has a suite of characters which confirm that there is no clear boundary between Australopithecus africanus  and Homo," he says.

References

1. Berger, L. R. et al. Science 328, 195-204 (2010). | Article | ChemPort |


2. Dirks, P. H. G. M. et al. Science 328, 205-208 (2010). | Article | ChemPort |


3. Smith, H. F.&Grine, F. E. J. Hum. Evol. 54, 684-704 (2008). | Article