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The remnants of a remarkably petite skull belonging to one of the first human ancestors to walk on two legs have revealed the great physical diversity among these prehistoric populations.
But whether the species Homo erectus, meaning "upright man", should be reclassified into several distinct species remains controversial.
Richard Potts, from the National Museum of Natural History at the Smithsonian Institute in Washington, DC, and colleagues discovered numerous pieces of a single skull in the Olorgesailie valley, in southern Kenya, between June and August 2003.
The bones found suggest the skull is that of a young adult Homo erectus who inhabited the lush mountainside some 930,000 years ago. The prominent brow and temporal bone resemble other Homo erectus specimens found elsewhere in Africa, and in Europe, Indonesia and China.
But the skull itself is around 30% smaller, which is likely to have corresponded to a similar difference in body size. The specimen helps fill a gap in the fossil record as very few Homo erectus specimens of this age have been found in Africa so far.
Many stone tools of similar age to the skull fragments have been found at the same site, and Potts' team suspect these may have required considerably more strength to manufacture than the small Homo erectus probably possessed. If so, this would imply a considerable physical variation within the local population.
Some experts even go so far as to suggest that a complete rethink of the human genealogical tree may be in order. "Recognising that Homo erectus may be more a historical accident than a biological reality might lead to a better understanding of those fossils whose morphology clearly exceeds the bounds of individual variation," says Jeffrey Schwartz of Pittsburgh University.
But Fred Spoor, at University College London, UK, disputes this interpretation, saying there is probably similar variation among modern human populations and ape species. "It's completely justified to call it Homo erectus," he told New Scientist. "This just gives some insight into the great variation of later specimens."
Spoor notes that the paucity of the fossil record means that many conjectures about Homo erectus remain unproven.
He hypothesises that a Homo erectus of this size may in fact have been muscular enough to make the stone tools found in the Olorgesailie valley. "They may have been small individuals, but incredibly powerful," he says.
Journal reference Science (vol 305, p 75)
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