The story of how and when modern humans first left Africa may be more ancient and more complex than anyone knew.
This week, anthropologists excavating a collapsed cave in Israel described a Homo sapiens fossil fragment that has been dated to between 194,000 and 175,000 years ago.
It is the earliest known modern human fossil to be found outside Africa.
The discovery, detailed Thursday in the journal Science, provides the first physical evidence that Homo sapiens migrated out of the African continent tens of thousands of years earlier than previously thought.
"For more than 50 years most anthropologists thought modern humans left Africa around 100,000 years ago," said Israel Hershkovitz, the professor of anthropology at Tel Aviv University in Israel who led the work. "This changes the whole concept of modern human evolution."
Rick Potts, a paleoanthropologist and head of the human origins program at the Smithsonian National Museum of Natural History, said the new study supports a growing body of fossil and genetic evidence that suggests our species made several short-lived forays out of Africa before ultimately dispersing around the globe starting roughly 70,000 years ago.
"The find leaves open the possibility that Homo sapiens ventured long distances but were not successful in taking up permanent residence in western or eastern Asia," said Potts, who was not involved in the work. "They apparently became extinct."
The recently excavated fossil was found embedded in sediments at an archaeological site known as the Misliya Cave in northern Israel, about 7 miles south of Haifa. The cave, which probably served as a shelter for these hominids, is located on the western slope of Mt. Carmel and is part of a network of caves in the area.
Archaeologists working at the site found evidence that the cave's inhabitants hunted large game like wild cattle and gazelle and were able to control fire. Several small stone tools similar to those associated with Homo sapiens remains in Africa from the same time period were found at Misliya as well.
The newly discovered fossil contains an upper left jawbone with eight teeth still attached. It also includes part of the cheekbone, the roof of the mouth and the bottom of the nasal cavity.
Ideally, anthropologists would examine a full skull before determining how to categorize an ancient specimen. For example, a small face tucked underneath a forehead and a rounded brain case clearly indicates Homo sapiens rather than another species, Potts said.
In this case, however, the research team did not have that option. Instead, researchers compared precise measurements of the size and shape of the teeth and the jawbone with other ancient hominid fossils from Europe and Africa. In this way, they were able to show that the fossil's features had the most in common with our species.
"Nothing we found was inconsistent with it being Homo sapiens, and some features clearly indicated it was Homo sapiens," said Rolf Quam, a paleoanthropologist at Binghamton University in New York who worked on the study.
Potts said the multiple lines of evidence that the researchers used to make the ID were persuasive for him.
"Barring definitive evidence of a complete brain case plus face, the authors have done their best to convince us that Homo sapiens is the best classification. Or at least I'm convinced," he said.
The research team used three different methods of dating the fossil and the sediments around it. The scientists dated the age of tooth samples using uranium-thorium, turned to combined electron spin resonance techniques to date enamel samples, and relied on thermoluminescence to determine the age of burnt tools associated with the fossil. Together, they suggest that the jaw fragment is between 175,000 and 194,000 years old.
"Before the Misliya fossil, the earliest known human fossils outside of Africa were 100,000 years old," Quam said. "This pushes that back by at least 75,000 years."
He added that along with the recent discovery of the oldest human fossil in Africa dating to about 300,000 years ago, and genetic analysis that shows Neanderthals were mating with humans at least 220,000 years ago, the new find suggests the story of human evolution and migration goes back much further than anyone realized.
"You have several different lines of evidence coming together that the origin of Homo sapiens is earlier than we thought, and now we have fossil evidence of an earlier migration of modern humans out of Africa," Quam said.
Potts, who described the findings as important, said they have already gotten him thinking about more lines of inquiry.
"The big question for me is, why weren't those pioneering groups successful in a longer-term way?" he said. "And what was it about African Homo sapiens after 75,000 years ago that made the difference, leading to the rapid and more permanent colonization of the planet?"
In other words, the mystery of our origins, and our dispersal, continues.
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