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Kate Wong, Scientific American
Kate Wong, Scientific American
Of all the human species that have ever lived, only one, Homo sapiens, conquered the entire planet. The apelike australopithecines (Lucy and her ilk) never ventured out of Africa. The Neandertals never ventured in. Homo erectus got around, colonizing much of the Old World. But, so far as is known, it never reached Australia or the New World. Our own species, though, went everywhere. When, researchers have long wondered, did our ancestors begin to spread across the globe? And how did they do it?
In a paper published in the January 26 Science, researchers report on a fossil from a cave in Israel they say pushes the earliest known occurrence of our species outside of Africa back by tens of thousands of years. The discovery could add to a growing body of evidence that is rewriting the origin and evolution of H. sapiens. But critics caution the identity of the fossil hinges on scant evidence, and that the implications of the find for understanding the rise of our species are limited.
A decade ago the leading theory of modern human origins held our species arose around 200,000 years ago in eastern Africa and (apart from short forays into Israel around 90,000 to 120,000 years ago) did not begin spreading out of Africa in a major way until around 60,000 years ago. When they did finally begin to disperse across the globe, according to this model, they replaced the archaic human species they encountered along the way, including the Neandertals, without interbreeding with them.
But a flurry of discoveries in recent years has forced scientists to rethink that scenario. Just last year researchers reported that H. sapiens fossils from a site in Morocco called Jebel Irhoud date to more than 300,000 years ago, pushing the origin of our kind back by more than 100,000 years and moving the spotlight from eastern Africa to north Africa. Other finds indicate our species began colonizing far-flung lands well before 60,000 years ago, reaching China by perhaps 120,000 years ago, Indonesia as early as 73,000 years ago and Australia by 65,000 years ago. Moreover, researchers studying DNA recovered from fossils have shown that as H. sapiens entered new lands it did in fact interbreed with archaic human species it encountered, including Neandertals and the mysterious Denisovans.
The new fossil—a piece of upper jaw with several teeth, found in Misliya Cave on the western slopes of Mount Carmel in Israel—adds another wrinkle to the story. Israel Hershkovitz of Tel Aviv University and his colleagues dated the fossil to between 177,000 and 194,000 years ago using three independent methods. When they compared the anatomical details of the Misliya specimen with those of fossils belonging to modern humans and their relatives, the jawbone grouped most closely with H. sapiens. Taken together these findings push back the earliest known occurrence of our species outside of Africa by more than 50,000 years, the authors contend.
“It’s exciting to find Homo sapiens outside of Africa this early,” says paleoanthropologist Shara Bailey of New York University, an expert on early human teeth, who was not involved in the new Misliya cave study. But she warns that the case for the fossil belonging to H. sapiens rests on meager evidence. Although it aligns more closely with H. sapiens than with the other species in the researchers’ comparative analysis, it is only one fragmentary bone. And the teeth look surprisingly modern for their age, Bailey observes. What the rest of this individual looked like is anyone’s guess. Bailey notes recent discoveries of far more complete fossil humans from South Africa, representing previously unknown members of the human family—Australopithecus sediba and Homo naledi—show evolution mixed and matched modern and archaic traits in unexpected ways in the past. Had researchers recovered only one bone from either of these species, they might have arrived at a very different conclusion about the identity of the owner.
Still, Bailey says, the available evidence (such as it is) does suggest the Misliya jaw came from a H. sapiens individual, which meshes well with the recent discoveries from Morocco that push back the origin of our species. Previously scientists surmised that the apparent long delay between when H. sapiens originated and when it began to spread around the world might have had to do with climate and environmental conditions. If the species originated in sub-Saharan Africa around 200,000 years ago as was thought, the bone-dry Sahara Desert could have been a formidable barrier to migration out of the motherland. But “if sapiens is in north Africa 300,000 years ago, there is nothing to keep it from leaving,” Bailey says.
The Misliya find may also suggest a certain technological breakthrough helped fuel our march to world domination. Archaeologists have long considered the advent of the Levallois method of making stone tools—a strategy for obtaining broad, thin, sharp flakes from a chunk of stone called a core—to be a significant development in human prehistory. Which human species invented this strategy has been unclear; Levallois tools have turned up alongside fossils of Neandertals and other extinct species, in addition to early modern humans. The discovery of Levallois implements with both the earliest H. sapiens on record, at Jebel Irhoud, and the oldest known H. sapiens outside of Africa, at Misliya, could be taken to indicate Levallois tools were invented by modern humans, and that they may have facilitated the spread of H. sapiens out of Africa.
But not everyone buys that argument. Archaeologist John Shea of Stony Brook University, who was not involved in the new work, notes Levallois-like stone tools have been found at sites in Africa dating to 500,000 years ago, and sites in Armenia dating to more than 300,000 years ago—long before H. sapiens is known to have appeared on the scene. If the Levallois technique was a game changer for H. sapiens, he says, then it should have also been a game changer for other human species. “Why didn’t Levallois help Neandertals get into Africa?” he says. “What’s good for the goose is good for the gander.”
It remains to be seen whether the presence of H. sapiens at Misliya represents a first tentative step out of Africa that fizzled, or part of a larger wave of migration that carried our ancestors out into parts unknown. Or maybe it was just business as usual. Shea notes that from the standpoint of climate and environment, Israel and its neighbors were, in essence, a part of Africa, harboring mostly the same kinds of animals. Perhaps H. sapiens evolved out of ancestral human populations that inhabited this larger region encompassing Africa and the Arabian Peninsula. “You can walk from Africa to Misliya,” Shea says. “It would take about two weeks.”
This article is reproduced with permission from Scientific American. It was first published on Jan. 26, 2018. Find the original story here.
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