Scientists believe they have found our oldest immediate ancestors, a finding that sheds fresh light on Homo sapiens
' rise out of Africa and his conquest of the globe.
The skulls of two adults and a child, found in 1997 in the Middle Awash area of central Ethiopia, have been carbon-dated to between 154,000 and 160,000 years old, around 50,000 years earlier than the previous oldest finds of Homo sapiens.
That provides solid proof for the "Out of Africa" theory and confirms that the enigmatic hominids known as Neanderthals were not our distant parents, the authors say.
The fossils "provide crucial evidence on the location, timing and contextual circumstances of the emergence of Homo sapiens," they report on Thursday in Nature, the British weekly science journal.
"They... represent the probably immediate ancestors of anatomically modern humans. Their anatomy and antiquity constitute strong evidence of modern-human emergence in Africa."
Pieced together from fossilised fragments in an agonisingly long game of 3-D jigsaw puzzles, the skulls have deep faces and long, rugged cases that enclosed large brains.
Homo sapiens idaltu
They resemble modern crania in the face, top of the skull and brain capacity, which at around 1,450 cubic centimetres compares with anatomically modern man's average today of between 1,350 and 1,400 cc.
Those similarities, but also subtle differences in skull characteristics, mean that the fossils are a member of Homo sapiens yet, like us, they are also a subspecies of it.
The finds have been dubbed Homo sapiens idaltu ("idaltu" meaning "elder" in the Afar language of Ethiopia).
Anatomically modern man -- a subspecies called Homo sapiens sapiens -- gradually emerged from Homo sapiens idaltu, his body shaped by diet and other evolutionary forces, the authors suggest.
The earliest remains of Homo sapiens sapiens, found in South Africa, Ethiopia and the Middle East, have been dated to around 100,000 years ago.
The study's leaders were Tim White and Clark Howell, who are professors of anthropology at the University of California at Berkeley, and Giday WoldeGabriel, a geologist at Los Alamos National Laboratory, New Mexico.
Much needed evidence
They argue the discovery is a much-wanted chunk of evidence in a heated 25-year debate.
Molecular studies, based on the evolutionary changes in DNA over a long period, have suggested that modern humans appeared on the scene around 150,000 years ago in eastern Africa.
They then migrated to other regions, starting with the Middle East and then heading northwards and westwards into Europe and eastwards into Asia, eventually crossing into the Americas via Alaska.
Critics laughed at the idea that a tiny population of humans living on a subsistence diet could migrate and survive such distances, and questioned assumptions about the gene flow and mutation rate.
They especially targeted the theory's biggest weakness -- the lack of good fossil evidence to support it.
Their alternative was a "multiregional" model, in which modern humans arose in different parts of Africa and Europe at roughly the same time, emerging from the local population of hominids.
In the case of Europe, that would have meant that Homo sapiens sprung out of the Neanderthals, who showed up around 400,000 years ago but then puzzlingly disappeared around 30,000 years ago.
Did the Neanderthals fail to leave a mark?
Did the Neanderthals disappear because they had evolved into or interbred with modern humans, as some have suggested?
No, not if the Idaltu fossils are any indication.
"They show that near humans had evolved in Africa long before the European Neanderthals disappeared," says Howell. "They thereby demonstrate conclusively that there was never a 'neanderthal' stage in human evolution."
If Idaltu has now condemned the Neanderthals to an unsuccessful branch of human evolution, there remain many questions.
One of them: fine, precise cuts on the skulls inflicted by a stone tool, which suggests that, after their death, the two adults and the child were stripped of their faces, muscles and other tissues, and their skulls were repeatedly handled, leaving a deep polish on their sides.
Could this have been some funeral ritual, an early form of ancestral worship?
Or could it have been more macabre -- was our earliest known ancestor a cannibal?
Middle Awash has been a treasure trove of human fossils, including the apelike hominid Ardipithecus, dated up to 5.8 million years old, and Australopithecus garhi, around 2.5 million years old.
The Idaltu fossils came from the village of Herto, on the floor of the Afar rift valley, near the shore of what was a shallow, freshwater lake.