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Ian Cartwright

By Andy Coghlan

A small finger bone found in the Saudi Arabian desert may rewrite a key part of the human story: how our species emerged from Africa and spread around the world

The bone is one of the oldest pieces of evidence of modern humans living outside Africa. Its discoverers argue that we must now reconsider our theories about when and how modern humans began spreading from our African birthplace.

Huw Groucutt of the University of Oxford in the UK and his colleagues found the finger bone at a site called Al Wusta in what is now the Nefud Desert. It is the second bone in from the fingertip, but it’s not clear which finger.

The team recognised the bone as human on sight, and later confirmed this by comparing it to finger bones of other humans, extinct hominins like Neanderthals, and other primates such as gorillas.

They were able to date the bone directly, using precise techniques that rely on the decay of radioactive uranium. The team estimates the bone is at least 85,000 years old.

The way out of Africa

For years, many archaeologists have believed that our species only left Africa around 70,000 years ago, and from there spread rapidly into Asia and Europe, and ultimately the Americas.

However, that story has looked increasingly shaky due to a series of finds in the Levant: the area east of the Mediterranean that includes countries like Israel and Syria. It seems humans lived in this area over 100,000 years ago.

In January this year, a team led by Israel Hershkovitz of Tel Aviv University in Israel and Mina Weinstein-Evron of the University of Haifa, Israel unveiled a human jawbone from Israel that was 177,000 years old. This showed that humans lived in the Levant for tens of thousands of years.

Nevertheless, many archaeologists believe the Levant was a bottleneck and that humans did not travel further until 70,000 years ago. There are ancient artefacts and fossils, some of which appear to be human, in parts of Asia including China and India – but in all cases, either the dating has been questioned or some do not believe the remains came from modern humans.

Because of this uncertainty, the Al Wusta finger bone is the oldest confirmed remnant of a modern human found outside Africa and the Levant. It shows that our ancestors progressed beyond the Levant “bottleneck” much earlier than thought, creating a new staging post in Saudi Arabia from which they could push on into the rest of Asia.

“It now seems likely that early modern humans were in southern China about 100,000 years ago, and they had reached Australia by about 65,000 years ago,” says Chris Stringer of the Natural History Museum in London, UK. The Al Wusta bone fits with that narrative.

Klint Janulis

People may have settled Arabia at this early date because it was an appealing place to live.

85,000 years ago, the Al Wusta site was rich grassland covered by permanent freshwater lakes, as Arabia’s climate was wetter than it is now. The researchers found at least 860 animal bones, and the most common were water-loving animals like hippos and buffalo.

The team also found 380 stone tools, suggesting that lots of our ancestors may have lived around the lakes.

“These were bands of hunter-gatherers, and they would have been living on the edge of lakes – but mobile, hunting for animals and gathering plants, perhaps existing off some aquatic resources,” says co-author Michael Petraglia of the Max Planck Institute for the Science of Human History in Jena, Germany.

North or south?

The study feeds into a long-running argument over the route humans took out of Africa.

“Did they leave Africa by travelling up the Nile, into the Sinai, and then across the Middle East to what is now Iran?” asks Stringer. “Or could they have travelled across or around the Red Sea into Arabia, and then directly accessed Iran via the Persian Gulf, when sea levels were periodically lower during the last Ice Age?”

Groucutt and Petraglia argue that the southern route through Arabia was the most important, whereas Hershkovitz and Weinstein-Evron favour the northern route through the Levant. A 2015 genetic study supported the northern route, but its methods were criticised.

“A more intriguing question,” says Weinstein-Evron, is whether the early humans of the Levant and of Arabia all belong to the same population, or whether they represent multiple migrations out of Africa.

“The Al Wusta research adds support to the notion that there were numerous, perhaps nearly continuous, pulses of Homo sapiens dispersals from Africa, and that regional moist episodes may have triggered the dispersals,” says Donald Henry of the University of Tulsa in Oklahoma.

Journal reference: Nature Ecology & Evolution, DOI: 10.1038/s41559-018-0518-2