Further analysis of the earliest group of tools—and the techniques used to make them—showed a strong similarity to known tools from that time period in East Africa. The presence of two techniques, Levallois flaking and bifacial reduction—which in the outer chunks of a rock are flaked off to separate a useable core—underscore the makers' African origins. "In East Africa and northeast[ern] Africa bifacial reduction is a constant part of the technological repertoire of people," Marks said. "In Africa they produce hand axes, but also what are called foliates—leaf-shaped bifacial pieces," he added, noting that "either you have a connection to East Africa or you have an independent invention of this technique from a group that has no history in the area."
And the later layers of tools, groups A and B, did not bear evidence of the use of these techniques. "These marked differences led us to conclude that assemblages A and B developed locally and in basic isolation," Marks noted.
Even though the tools were not accompanied by any direct H. sapiens fossil evidence, the research team is confident that they were not made by another more primitive human relative, such as Neandertals. "There's no evidence for any Neandertals south of that [Russian-Asian] temperate zone to the east," Marks said. Certainly another group cannot be ruled out but, he noted, that would have meant that an errant group of Neandertals "took a turn south, went several thousand kilometers into what at the time was desert," he said. "It seems to me a very difficult explanation and one that doesn't follow any reasonable logic."
Not everyone in the archeology field, however, is convinced that the stone tools in the C grouping have an undeniable connection to Africa. "I think the verdict here is ambiguous," says John Shea of Stony Brook University's Department of Anthropology, who was not involved in the research but is familiar with the paper. Given that all H. sapiens originated in Africa, he adds, "it is likely that either [the tools'] makers or their makers' ancestors came from Africa"—but not necessarily directly. "When the Earth offers up what you have long been seeking," he adds, "a good archaeologist needs to be on guard."
Marking the time of sands
Because the evidence for human occupation at the Jebel Faya is not biologically based—from bone or other animal or plant material—the samples could not be radiocarbon dated (aside from some shells that turned up in an overlaying layer, which were dated at some 9,700 to 10,400 years ago). The research team thus had to find a different means to date the tools. "Archaeology without ages is like a jigsaw with the interlocking edges removed," Simon Armitage, of Royal Holloway, University of London, and co-author of the paper, said on Wednesday. "You have lots of individual interesting pieces of archaeological information, but you can't fit them together to produce the big picture."
He used single-grain optically stimulated luminescence (OSL) dating to determine how long the surrounding sand grains had been buried. "When a sand grain is buried it's exposed to very low levels of naturally occurring radiation from the surrounding sediments," Armitage explained. The radiation separates electrons from their host atoms and entraps them in a different part of the crystal. Once exposed to sunlight again, the electrons are freed, "and in doing so, they emit a tiny amount of light called luminescence," Armitage said, likening the process to discharging a rechargeable battery. If collected without being exposed to sunlight, the sand grains' luminescence can be measured with OSL dating, revealing the time that has passed since it was last buried.
Many of the recent brushstrokes of human migration have been painted not with sand grains, but rather with the help of DNA analysis. Sampling of genetic material from people around the world has put the first major human exodus out of Africa at some 60,000 years ago. But as defenders of rock-hard stone evidence, researchers behind the new paper stand behind the accuracy of their findings. "Genetics can only tell us which event happened before another or after another, and the estimates of time are very rough," Uerpmann said. He said he was not surprised that their results contradicted DNA estimates and suggested that it will "stimulate thinking" about the genetic approach.
Nevertheless, the researchers acknowledge that fragmentary artifact-based evidence is not perfect for following early human dispersal around the globe. "To track them archaeologically would be extremely difficult," Marks said. "What we have been able to do here is a first step."
The findings in the U.A.E. do not preclude the Nile route out of Africa from being taken as well. "Our findings open a second way," Uerpmann said, adding that in his opinion the Arabia route "is more plausible for massive movements than the northern route."