Palaeontologists say a new fossil find from Olduvai Gorge in Tanzania could simplify our understanding of the origin of humans.
By Dr David Whitehouse
BBC News Online science editor
The remains of the 1.8-million-year-old hominid are said to rank among the best specimens yet discovered of the earliest members of our genus, Homo.
The upper teeth are complete
Writing in the journal Science, Rutgers University anthropology Professor Robert Blumenschine says the new fossil shows that at least some of the examples of two early groups of Homo should now be reunited into a single species.
The new find, designated OH 65, consists of a portion of the lower face and upper jaw with all the teeth present.
"This is an important ancestor that comes from a crucial time in prehistory - when we first began to see stone tools, when hominids had just begun to exploit larger animals as a food source, and when brain size was just beginning to expand significantly," says Professor Blumenschine.
Professor Blumenschine and colleagues describe in their Science paper a hominid specimen they found at Olduvai Gorge in 1995, a region that gained prominence through the discoveries of Louis and Mary Leakey in the early 1960s.
Forty years ago, Louis Leakey and colleagues unearthed a series of fossils at Olduvai that were identified as the then oldest member of our genus.
It is not entirely clear how the Homo genus, which includes modern humans, relates to older hominid groups
Because of its large brain size and stone tools found nearby, the fossil they dug up was called Homo habilis, "handy man".
Ten years later Louis Leakey's son Richard found a more complete H. habilis skull in northern Kenya, showing the species ranged across much of the Rift Valley of eastern Africa.
And in the mid-1980s, new findings split H. habilis into two groups. The fossils found at Olduvai Gorge kept their original designation while others, now recognised as displaying slightly different features - a flattened sub-nasal area and more robust dentition took on a new name, Homo rudolfensis.
However, the latest work by Blumenschine's team could now lead to some members of these two hominid species being rejoined.
"Any time you make a find like this, complete enough to show so many important diagnostic features, we get very excited," says Professor Blumenschine.
Increasingly large brain
OH 65 provides a key anatomical link between H. rudolfensis and the original H. habilis.
The researchers maintain that another Kenyan fossil, KNM-ER-1470, currently designated as H. rudolfensis, should be put back with fossils from Olduvai Gorge into H. habilis.
This is because the new fossil and 1470 share several features, including the flat face below the nose and dental structure, which once led to the species' separation.
"OH 65 allows us to reshuffle the specimens that belong in the ancestral genus and tie together rudolfensis and habilis," says Professor Blumenschine.
"It shows that all three specimens are likely to be members of the same species - Homo habilis."
Crucially, OH 65 was found with stone tools and with bones from larger animals that clearly show the marks made by those tools.
Professor Blumenschine says this corroborates the work of other scientists in demonstrating the hominid's capacity both to make tools and to use them in butchering meat for food, even at this early time.
"As we learn more about the paleoecology, we may begin to understand what environmental conditions were selecting for adaptive traits in early Homo, traits like an increasingly large brain, that eventually gave rise to what we are today."