For more than a decade, a team of archaeologists and anthropologists scoured the Arabian Desert for evidence that some of the earliest members of our species once traversed these formerly green lands. Now, they may have it. An ostensibly modern human finger bone uncovered in Saudi Arabia in 2016 has been dated to about 88,000 years old, making it the oldest directly dated fossil of our species found outside Africa or its immediate vicinity in the eastern Mediterranean. The discovery supports the idea that early modern humans spread into Eurasia earlier and more often than many previously believed.
Although some say it’s hard to identify our species, Homo sapiens, by a single bone, the findings appear unimpeachable, says John Shea, an anthropologist at the State University of New York in Stony Brook who studies human origins, but wasn’t involved in the study. “This isn’t one of those cases where someone dashed off into the field, found something after a day or two of fieldwork, and then ran to the media with it,” he says. “They earned this find the old-fashioned way: hard work.”
Several competing theories explain when and by what routes our earliest ancestors migrated out of Africa after they evolved there as early as 300,000 years ago. For decades, the fossil evidence favored the hypothesis that anatomically modern humans stayed on the continent, with an occasional jaunt into neighboring Israel, for hundreds of thousands of years until a wave of migrants swept into Eurasia—and then throughout the world—between 50,000 and 60,000 years ago.
But the tantalizing discoveries of 100,000-year-old stone tools found in the mountains of Oman and decidedly human fossils in the Israeli Levant dating to 177,000 to 194,000 years ago forced anthropologists to consider the possibility of earlier migrations. Teeth found in Chinese caves have been dated to between 80,000 and 120,000 years, although the dates are based on the caves’ stalagmites, not the teeth themselves.
Some have tried to reconcile these findings with the late-exodus narrative by claiming there may have been an early, but ultimately doomed, first wave migration out of Africa some 120,000 years ago, after which humans more or less stayed put on the continent for another 60,000 years. Others have argued there were several migrations in and out of Africa throughout this whole period.
Yet proponents of the multiple-migration hypothesis have so far lacked the archaeological equivalent of a smoking gun: a directly dated early modern human fossil found far outside Africa’s borders. That’s what Huw Groucutt, a paleoarchaeologist at the University of Oxford in the United Kingdom, and his team were looking for when they began excavating sites in the Arabian Desert more than 10 years ago.
In 2014, they discovered a site named Al Wusta in Saudi Arabia’s arid Nefud Desert that once sat on the banks of a large freshwater lake. Dozens of sharpened stone tools buried in the sediment hinted that it might be a special place. Their hunch paid off 2 years later, when study co-author and paleontologist Iyad Zalmout of the Saudi Geological Survey in Jeddah found a small bone stuck in the sediment. “He said, ‘This is a human finger,’” Groucutt recalls. “That night back at the hotel, we were Googling ‘human finger bone’ and, yeah, it looked like our species.”
It was an intermediate phalanx, the bone between a fingertip and finger knuckle. It’s 3.2 centimeters long and was probably was part of a middle finger. Professional anatomists analyzed 3D scans of the bone and concluded that it was a match for our own species, rather than another early hominins such as Neandertals or a member of Australopithecus.
Next, the team used a technique called uranium series dating to gauge the bone’s age. Researchers bored a microscopic hole into it with a laser and measured traces of radioactive elements within. By comparing the ratios of uranium and thorium present in the bone, scientists can tell its age. The Al Wusta finger bone clocked in at 88,000 years old, the researchers report today in Nature Ecology & Evolution. A hippopotamus tooth found at the site was dated to 90,000 years old, as were the sediment layers surrounding the stone tools. If humans were migrating out of Africa about 90,000 years ago—a time intermediate to the 120,000- and 60,000-years-ago hypotheses—“that shows there were probably multiple dispersals,” Groucutt says.
John Hawks, a paleoanthropologist at the University of Wisconsin in Madison, says the authors have convincingly shown the finger bone is likely a hominin of some sort. “Still, I doubt whether anyone can identify a single isolated finger bone as a modern human, as opposed to any other form of hominin,” such as Neandertal, he says.
Katerina Harvati, a paleoanthropologist at the University of Tübingen in Germany, agrees, noting there’s so much anatomical overlap between hominin species that she’d like to see additional fossils confirm it. If the findings hold up, she says, they fit nicely with other lines of evidence pointing to multiple out-of-Africa migrations. They also raise questions about how long these early migrants’ descendants lived on.