PARIS (AFP) — The 1.77-million-year-old remains of three adults and a teenager unearthed in the Caucasus point to a far greater variation in early humans than once suspected, according to a study released Wednesday.
But the bones also reveal a species with a startling mix of primitive and advanced features that does not fit neatly into any evolutionary continuum, an ambiguity sure to spark debate among scientists.
The research team that made the discovery, led by David Lordkipanidze of the Georgian National Museum, reported their findings in the British journal Nature.
In many respects, the well-preserved fossils found near the Georgian town of Dmanisi resemble Homo erectus -- a species from the genus Homo that first appeared in Africa some two million years ago and quickly spread throughout Europe and much of Asia.
The Dmanisi specimens are the oldest hominins, a taxonomic term comprising humans and chimpanzees, to be found outside Africa.
They have remarkably human-like spines and lower limbs that would have been well suited for long distance travel. Their feet had well-developed arches.
An apparently small gap in the size of males and females also put them in the same company as H. erectus and H. sapiens.
At the same time, however, they share many traits with the earlier Homo habilis -- "skilful man", named for the tools he used -- and even with the more primitive species in the genus Australopithecus, which first appeared in Africa some four million years ago.
Their brain-to-body mass ratio is on the low end of the scale, as is their body size, not much larger than a chimpanzee.
The largest adult in the group would have weighed 48-to-50 kilogrammes (96-110 pounds), and stood 147 to 161 centimeters (56.4-64.4 inches) tall.
And another key measure of separation from modern humans was their inability to pivot the forearm forward in relation to the body.
Discovered in the early 1990s, the Dmanisi site has proved a rich source of remains and artifacts from the dawn of the Pleistocene period.
The partial adolescent skeleton presented in the study is especially valuable, as fossil remains of "post-cranial" material -- meaning everything except the skull -- are very rare.
These sharp differences raise vexing questions for paleoanthropologists trying to piece together the puzzle of hominin evolution.
Taken together with recent find of about the same age near Lake Turkana in Kenya, the Dmanisi fossils suggest that the Homo genus that spread across Africa and Eurasia "was less modern and more variable than sometimes supposed," suggests Harvard scientist Daniel Lieberman in a commentary, also published in Nature.
If the two fossils come from the same species -- which Lieberman said is likely -- then "early H. erectus was not only quite widespread but also unusually variable in both body and brain size."
The unusual mix of primitive and advanced characteristics of the Georgia specimens may also point to an ability to adapt to local environments, the authors suggest.
As to where H. erectus fits into the larger evolutionary scheme, these new fossils are likely to raise as many questions as they answer.
It seems less, not more, likely, for example, that H. erectus and H. habilis co-existed and sprang from earlier antecedents, rather than the first evolving from the latter, as has long been thought.
The recent discovery of the two other fossils in Africa -- a H. erectus skull dated to 1.55 million years ago and a H. habilis jaw that is 1.44 million years old -- extended the period by which the two species overlapped.
"Their co-existence makes it unlikely that Homo erectus evolved from Homo habilis," argues Meave Leakey, who reported the African findings in Nature last month.
"The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition."
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