Henry said, "I see no reason to suspect that there were one, two, and even a few population dispersals from Africa with the same cultural signatures, detailed physical confirmations and genetic compositions. More likely, in my mind, there was a nearly continuous flow of Homo sapiens groups out of Africa and perhaps even back again beginning as early as suggested by the Homo sapiens evidence from Morocco."
That evidence, reported last year in the journal Nature, consists of fossils from the Moroccan cave Jebel Irhoud. The remains date to about 300,000 years ago and represent the oldest known fossils for our species. A skull found at a site called Florisbad in South Africa dates to approximately 260,000 years ago and is also believed to be early Homo sapiens.
"I think the combined archaeological, fossil, and genetic evidence indicates that Homo sapiens originated across Africa," Groucutt said. "It is a model that Chris Stringer has called 'African multiregionalism.'"
Stringer is an anthropologist at the Natural History Museum in London.
Multiregionalism appears to extend beyond the African continent. "For now, at least,” Henry said, “it would appear that the bones, stones, and genes point to North Africa and the Levant as the most likely settings for the emergence of Homo sapiens."
He added that an intriguing issue involves understanding the relationship of anatomically modern humans with Neanderthals. The recent fossil finds, including the newly discovered finger bone, suggest that these two hominid populations coexisted in certain areas for at least 20,000 years. Henry said that the evidence of Neanderthals tends to be found in the "wooded, rugged Mediterranean hill zone," whereas evidence of modern humans mostly is situated in lower land regions.
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He theorizes that "because of their short, lower limbs and robust stature, Neanderthals had less efficient locomotion and a higher, base-metabolic rate that acted to constrain their site exploitation territories."
Their anatomy might therefore have restricted their ability to expand into other locations. It did not, however, stop them from mating with Homo sapiens. Because of interbreeding, people of European, Asian, and North African heritage today retain a small percentage of Neanderthal DNA.
A greater puzzle is: What happened to the population represented by the Al Wusta finger fossil?
"Many geneticists would say that they were a 'failed dispersal' who died out, but I think that is quite unclear," Groucutt said. "Genetic interpretations are evolving very fast. To really resolve these issues, several disciplines — such as archaeology, paleontology, genetics, and paleoenvironmental science — need to work together."
Groucutt and his team are continuing their research in Saudi Arabia, which involves close collaboration between not only the University of Oxford and the Max Planck Institute for the Science of Human History, but also the Saudi Commission for Tourism and National Heritage, the Saudi Geological Survey, and numerous colleagues from across the globe.
"We are also looking at many different time periods," Groucutt said. "We have evidence that humans repeatedly spread into Arabia during periods of improved climate, going back hundreds of thousands of years, so we are currently conducting various studies on this."