Five hominid fossils found in Dmanisi, Georgia, could erase a number of African hominid species from the tree of human evolution and rule out Australopithecus sediba as a possible human ancestor.
A. sediba, found in the Cradle of Humankind in Gauteng in 2008, was heralded to much fanfare as the most likely candidate for a human ancestor, with its mixture of modern human and ape characteristics. However, the Dmanisi skulls – hailed by international and local palaeoscientists as "spectacular" – cast doubt on the prevailing story of human evolution.
"Skull 5", described by the international team of researchers in the journal Science on October 18, is the only complete adult skull of a human ancestor discovered for the Pleistocene period. This period, which ranges from about 1.8-million years ago until about 11 500 years ago, was when modern humans, Homo sapiens, evolved.
With its thick-toothed grin and small brain, Skull 5 offers researchers a complete reference point to measure variation in the other skulls found at the site, which they believe to have been from the same time and place, and its species, Homo erectus. H. erectus are the first known hominids to have the same upright body proportions as modern humans.
And what they found has the palaeoscience community in an uproar: hominid species had as much variation between individuals as modern humans do, and many of the unique species discovered in Africa – such as Homo habilis and Homo rudolfensis – are in fact part of the same species lineage.
Some of these species were present in Africa more than two million years ago and scientists had been trigger-happy in naming new species, according to the researchers.
The University of Zurich's Christoph Zollikofer, who worked on the Dmanisi fossils, told the Mail & Guardian: "There has been one key find in Ethiopia dated to about 2.3-million years ago. It is described as H. habilis … at the time, there was nothing to compare it with."
Skull 5 has led lead author David Lortkipanidze, from the Georgian National Museum, to argue that the "close morphological" similarities between Skull 5 and the Ethiopian hominid point to them being of the same species.
"This evidence speaks to a much earlier origin of the Homo genus and this is much before 1.9-million years," Zollikofer says, meaning that A. sediba, whose remains have been dated to 1.98-million years ago, was around at the same time as the Ethiopian H. erectus candidate and that A. sediba could not have been the precursor to a species that already existed. However, South African experts say that A. sediba was not analysed in the Dmanisi study, even though it is mentioned in the paper.
The idea that the number of hominid species is improbably large and that it is more likely that the specimens represent one species, is not new.
"Most of the fossils represent single fragmentary finds from multiple points in space and geological time of at least 500 000 years," Zollikofer says. "This ultimately makes it difficult to recognise variation among species in the African fossils as opposed to variation within species."
Director of the Institute of Human Evolution at Wits University Francis Thackeray has long argued that there were fewer species than palaeoscientists thought. "We're thrilled and excited by the [Dmanisi] discovery and the discussion of the possibility the H. Habilis and H. erectus [are] the same species," he says.
His work has focused on statistics, and in a paper published in the journal Antiquity earlier this year, he and co-author Eddie Odes write: "There is clearly a need for an approach whereby the degree of similarity between specimens can be reassessed in the context of a species definition which is applicable to hominid fossils."
Thackeray says: "My statistics show there is a high probability that [H. erectus and H. Habilis] both belong to the same species."
The institute's Professor Lee Berger, who discovered A. sediba, points out that there was no comparison between the Dmanisi fossils and A. sediba, although it is discussed in the analysis.
"They didn't include sediba, even though they had access to all the data … [and] the specimens have similar brain size and facial features.
"I'm disappointed that they didn't use all the available evidence … What it means is that we will have to replicate the study and understand the results with the entirety of the evidence, rather than just part of it … I think the inclusion of sediba would dramatically alter the conclusion."
Zollikofer declined to comment on whether his group would be comparing A. sediba with H. erectus georgicus, which is what the Dmanisi fossils have been called, saying that the issue had become fractious.
"Given the evidence that is available now, we have come to this specific conclusion [that A. sediba is not a human ancestor]," he says.
"Most of the fossils represent single fragmentary finds from multiple points in space and geological time of at least 500 000 years"Rock of ages: This well-preserved hominid skull from 1.8-million years ago was found in Dmanisi, Georgia, and offers new evidence that early humans were a single species.