Paleontologist Iyad Zalmout of the Saudi Geological Survey was walking through the Al-Wusta dig site in 2016 when he spotted a tiny bone eroding out of a layer of sediment. The 87,000-year-old fossil turned out to be a human intermediate phalanx—the middle section of your finger—from what was probably a middle finger. It's the earliest directly dated human fossil that has been found so far outside Africa or the Levant, and archaeologists say it's evidence that once humans ventured beyond Africa, they spread farther and faster than previously thought.

A green Arabia

According to uranium-series dating, the fossil is between 85.1 and 90.1 thousand years old. At that time, the Nefud Desert wasn't the 40,000-square-mile sea of sand that now stretches across the Northern Arabian Peninsula. Around 84,000 years ago, a shift in the climate brought stronger summer monsoons to Arabia. Based on evidence from layers of sediment at the site and hundreds of animal bones, Al-Wusta was the shore of a shallow lake, one of hundreds in an arid Pleistocene grassland. African antelope grazed here, and hippos wallowed in the muddy waters of the lake. And the site was home to a few dozen hunter-gatherers, according to Oxford University archaeologist Huw Groucutt, who directed the fieldwork at the site.

The people who dwelled here 87,000 years ago lived in a fairly densely populated landscape by the standards of the late Pleistocene. Groucutt and his colleagues have identified several other ancient lakes over the course of a decade of survey and excavation in the region, and many of them have their own stone tool assemblages, a sign that several hunter-gatherer bands roamed the lake-dotted landscape at around the same time. But Al-Wusta is the first site where archaeologists have found actual remains of those early settlers.

A team of biological anthropologists at Cambridge University took CT scans of the bone and compared its shape, dimensions, and proportions to the same bone in other hominins, nonhuman primates, and early and modern humans (if you want to compare the Al-Wusta bone to your own finger, it was 32.3mm long and 8.5mm wide at mid-shaft). Human fingers are much longer and more slender than those of Neanderthals and not even close to any of the nonhuman primates in the comparison group.

The team has pretty much ruled out DNA testing; since the bone has completely mineralized, it’s highly unlikely that there’s any DNA left. And the fossil doesn't reveal much about the individual's age beyond the fact that they were probably an adult. A finger bone isn't much to go on if you want a person's life story, after all, but the phalanx does offer one interesting detail: this ancient human did a lot of hard work with his or her hands. The finger bone sports a bony lump called an enthesophyte, which forms as a response to repeated physical stress where ligaments or tendons attach to the bone.

“We can speculate that it could be something from even, like, making stone tools from the site,” said archaeologist Michael Petraglia of the Max Planck Institute for the Science of Human History, the senior author of the study, "although there’s no way to be sure."

Humans spread faster, farther than thought

Conventional wisdom among paleoanthropologists is that Homo sapiens had ventured north along the eastern edge of the Mediterranean to Israel by 120,000 years ago, where they lived for a little while but eventually died out and were replaced by Neanderthals. Then, around 65,000 years ago, humans made a large, rapid migration out of Africa, through the Levant, and into Eurasia, where they met the Neanderthals who already lived there (spoiler alert: It didn't go as well for the Neanderthals this time around).

That model is based on a combination of fossil evidence from sites like Qafzeh and Skhul Cave in Israel and mitochondrial DNA studies that link most non-African populations to the group that left that continent 65,000 years ago. But genetic studies in the last few years, using whole genomes and ancient DNA, are starting to paint a more complex picture. Meanwhile, recent archaeological finds in India, Sumatra, and China have offered hints that modern humans may have made it all the way to Eastern Asia by as early as 80,000 years ago.

Those finds are still the subject of debate among paleoanthropologists. At some of the sites, the fossils haven't been directly dated; instead, their estimated ages come from dating the sediments around or above them. At others, it's not clear that the fossils are Homo sapiens and not another hominin species. In some cases, there are no fossils at all, only stone tools with no clear proof of whose hands shaped them.

These sites offer hints at an earlier, wider human dispersal into the world but nothing you could quite put a finger on, if you'll pardon the pun. At Al-Wusta, archaeologists have for the first time a fossil that they claim is definitely Homo sapiens, directly dated to 85-90 thousand years ago. That puts modern humans in the middle of the Arabian Peninsula after their earliest presence in the Levant and about 7,000 years before the first suggestion of their presence in Eastern Asia.

“So actually it does all fit together very neatly, with Al-Wusta being an early representative of a much broader process,” said Groucutt.

The picture that has emerged in the last few years at sites from Kenya to Sumatra is that humans dispersed sooner and wider than anthropologists previously suspected. Recent genetic studies suggest that Homo sapiens first emerged in Africa 260,000 to 350,000 years ago—not 220,000 years ago as previously thought. Fossils from Misliya Cave in Israel push back the date of human arrival in the Levant to 177,000 years ago, much earlier than 130,000 years suggested by the fossils found at Israel’s Skhul Cave and Qafzeh. And now there’s Al-Wusta.

Debate over timing and routes continues

But the find still leaves some questions unanswered. It's not clear, for instance, whether the sites at Misliya, Qafzeh, and Skhul Cave and the one at Al-Wusta are part of one long wave of human migration or a series of fits and starts that happened whenever shifting climate brought more rain to the arid belt of the Sahara and the Arabian Peninsula.

Petraglia leans toward the latter view. “It's possible there were multiple windows of opportunity for the movement of groups out of Africa. Don't forget these are small hunting and gathering groups of people, so there might have been a trickle during one period of time and a wave at another period of time,” he said.

On the other hand, University of Tulsa archaeologist Donald O. Henry, who commented on the study in a related paper, leans toward the other possible interpretation. He thinks that Al-Wusta is evidence of continuous occupation after humans expanded beyond Africa, especially because its dating places humans outside Africa and the Levant right between the dates of the two traditional waves of migration. He also says the fossil find in Saudi Arabia supports the idea that those early pioneers may have taken two routes into Eurasia: one northward along the Eastern Mediterranean Coast, and one to the south, across the narrow southern end of the Red Sea into Arabia.

During parts of the Late Pleistocene, when sea levels were much lower, that crossing would have been roughly the equivalent of a large river, so it’s certainly possible. But Al-Wusta doesn’t necessarily offer proof of one route or the other, Petraglia says. The site is just 650km from Skhul Cave and Qafzeh in Israel, after all. Ultimately, it's going to take much more evidence to answer those questions, which Groucutt says are too large for a single discipline, whether it's archaeology, anthropology, or genetics, to answer alone.

“I think the only way we can really address this is by researchers from different disciplines working together,” he said.

But there are probably more pieces of the answers waiting to be found in the deserts of the Arabian Peninsula. Groucutt and his colleagues are one of only two teams working in Saudi Arabia at the moment, and they say it's a large area that has been mostly unexplored by archaeologists so far.

Nature Ecology and Evolution, 2018. DOI: 10.1038/s41559-018-0518-2 and 10.1038/s41559-018-0539-x (About DOIs).