A team from the University of the 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 Homospecies, has been criticized by other researchers as overstated.
The fossils, a juvenile male and an adult female, were found together in the Malapa cave, part of an eroded cave system, leading to speculation that the two fell to their deaths while searching for water. The fragile parts of the skeleton that are often missing from fossils this old, such as the hands and feet, have been preserved (see videos showing fossilized cranium here and here). The collar bone of the first specimen was discovered by team leader Lee Berger's nine-year-old son during a visit to the site in 2008.
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 earlyHomo species — either the earlier Homo habilis or even a direct ancestor of the more recent Homo erectus. The research is published in Science1,2.
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 withHomo in their title and text is difficult to understand outside of a media context".
Anthropologist Fred Grine of Stony Brook University in New York says that the authors have "not undertaken any competent analysis of variation withinA. africanus — something I do not understand in the context that three further skeletons have been found by the same team at Malapa". He says that although Berger and his colleagues compare the A. sediba specimens to other hominins in terms of morphological characters that he and co-author Heather Smith defined in a paper two years ago3, the team's analysis raises questions about the character states used and how they have been coded. White agrees, pointing out that of the characters that the authors say are shared with Homo, the key ones required to draw this kind of link — such as facial morphology and brain size — are at odds with their conclusions.
Both Grine and White are critical of the fact that most of the diagnosis of A. sediba is on the basis of a single immature individual, and point out that because anatomy changes during growth, the jury will be out until more complete adult remains are described.
Grine adds that a further issue relates to the suggestion that A. sediba was ancestral to Homo. The earliest fossil evidence for Homo— a maxilla or upper jaw bone from the Hadar formation in Ethiopia that is attributable to the species H. habilis — is 2.3 million years old. This is perhaps 500,000 years older than the A. sediba fossils. "If A. sediba is supposed to represent a better candidate for ancestry thanA. africanus, it is a little late in time," Grine says.
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.
And although there's no doubt that anatomy changes during growth, he adds, the juvenile is nearly an adult in terms of the development of key characteristics. "In human developmental terms, it's in its early teens. Its brain has clearly reached about 95–98% of its adult capacity," he said.
As for the age of the specimens, Berger says that as this is the first time that fossils of A. sediba have been discovered, he and his colleagues have not established over what period of time the species existed. "The site we have found is simply a point in time and does not in any way represent the first appearance of this species nor the last."
He also suggests that earlier Homo fossils from a similar period may have been misclassified because they are incomplete. "Fossils from that period are very fragmentary," he added. "We now know that you need a lot more than just one part of the morphology to define a genus or a species."
One mystery solved
Meanwhile, White says that the real significance of the find is that it finally shows what happened to A. africanus in southern Africa — the species survived longer than previously thought, and in a slightly changed form. He explains that before this, A. africanus was generally sitting as a point in southern Africa, at 2.5 million years ago to 2.8 million years ago: "Its parentage was thought by most people to be Australopithecus afarensis and its downstream relationships debated: some thought it was ancestral to Homo, some a dead end, others an ancestor of Australopithecus robustus. It is most commonly depicted in trees as a question mark, even though it was the first described species. Now it is clear that A. africanus not only persisted but evolved slightly in South Africa between 2.5 and 2.0 million years ago," 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.http://www.nature.com/news/images/end-of-item.gif); background-attachment: initial; background-origin: initial; background-clip: initial; background-color: initial; background-position: 100% 50%; background-repeat: no-repeat no-repeat;">