Friday, January 12, 2007

Skull provides signs of when humans left Africa

John Noble Wilford:

From a new analysis of a human skull discovered in South Africa more than 50 years ago, scientists say they have obtained the first fossil evidence establishing the relatively recent time for the dispersal of modern Homo sapiens out of Africa.

The migrants appeared to have arrived at their new homes in Asia and Europe with the distinct and unmodified heads of Africans.

An international team of researchers reported today that the age of the South African skull, which they dated at about 36,000 years old, coincided with the age of and closely resembled the skulls of humans who were then living in Europe and the far eastern parts of Asia, even Australia.

The timing, the scientists and other experts said, introduced independent evidence supporting archaeological finds and recent genetic studies showing that modern humans left sub-Saharan Africa for Eurasia between 65,000 and 25,000 years ago, probably closer to 45,000 to 35,000 years ago in Europe.

Until now, however, paleontologists had been frustrated by the absence of fossils to test the hypothesis of most geneticists that the people of sub-Saharan Africa and in Eurasia at this time were one and the same — modern humans. The human fossil record in Africa from 70,000 to 15,000 years ago had been virtually blank.

Some scientists, on the other hand, have contended that the migration could have begun as early as 100,000 years ago and that in the intervening time, contact with more archaic populations like the Neanderthals could have produced recognizable changes in what became the modern humans of Eurasia. But no scientists in the migration controversy have disputed that ancestors of the human species originated in Africa.

In a report in Friday’s issue of the journal Science, a research team led by Frederick E. Grine of Stony Brook University in New York concluded that the South African skull provided critical corroboration of the archaeological and genetic evidence indicating that humans in fully modern form originated in sub-Saharan Africa and migrated, almost unchanged, to populate Europe and Asia.

Dr. Grine and his colleagues said in an announcement by Stony Brook that the skull was the first fossil evidence “in agreement with the out-of-Africa theory, which predicts that humans like those that inhabited Eurasia should be found in sub-Saharan Africa around 36,000 years ago.”

Ted Goebel, an anthropologist at Texas A & M University who was not connected to the research, said the skull opened the way to important insights about “the missing years of modern humans.”

Writing in an accompanying commentary in the journal, Dr. Goebel said, “Here is the first skull of an adult modern human from sub-Saharan Africa that dates to the critical period, and one that can speak to the relationship of early moderns from Africa and Europe.”

The new findings pivoted on fixing the skull’s age. When it was uncovered in 1952 near the town of Hofmeyr, South Africa, the cranium was almost complete but the bone degraded. Not enough carbon remained for scientists at the time to extract a radiocarbon date.

Using new technology, Richard Bailey and other researchers at the University of Oxford, England, measured the amount of radiation that had been absorbed by sand grains that filled the braincase since its burial. They calculated the yearly rate at which radiation had collected in the sand and checked this with data from a CT scan of the bone. In this way, they determined that the Hofmeyr skull belonged to a human who lived 36,000 years ago, plus or minus 3,000 years.

Another member of the team, Katerina Harvati of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, made a detailed examination of the shapes, sizes and contours of all parts of the skull. She compared these three-dimensional measurements with those of early human skulls from Europe and with skulls of living humans in Eurasia and southern Africa, including the Khoe-San, commonly known as the Bushmen.

Because the Bushmen are well represented in the more recent archaeological record, Dr. Harvati said, they were expected to bear a close resemblance to the Hofmeyr skull. Instead, the skull was found to be quite distinct from all recent Africans, including the Bushmen, she said, and it has “a very close affinity” with fossil specimens of Europeans living in the Upper Paleolithic, the period best known for advanced stone tools and cave art.

“Much to my amazement,” Dr. Grine said in an interview, “the skull linked very closely with those from Europe at the time and not with South African remains 15,000 years on.”

Dr. Grine said these modern humans probably originated in East Africa, which is rich in fossils of ancestors of the species, and had then moved into Eurasia and also south to the tip of Africa.

New Kostenki and South African finds in Science magazine (earliest occupation of Eurasia by modern humans)

Fossil skull, artifact, help date human migration

Clues found for early Europeans

Excavation Finds Earliest Europeans in Russia 45,000 Years Ago

Ivory carving pushes back date of modern man's venture into Europe

New signposts on the path of early human migration

New Clues From an Old Skull


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