Our fossil finger discovery points to earlier human migration in Arabia

: Ian Cartwright/Michael Petraglia/Palaeodeserts Project

Our fossil finger discovery points to earlier human migration in Arabia

The Arabian Peninsula is a vast landmass at the crossroads of Africa and Eurasia. Yet until the last decade almost nothing was known about early humans in the area. In the last few years the team I work with have made many remarkable discoveries in Saudi Arabia, but one thing was always missing: fossils of ancient humans.

This changed when we discovered a small bone with big implications in Saudi Arabia’s Nefud Desert two years ago. As my colleagues and I explain in a new paper in Nature Ecology and Evolution, this 90,000-year-old Homo sapiens finger bone fossil shows human migration into Eurasia occurred earlier than previously thought. And it also highlights the role of climate change in our early expansions.

For the past several years I have been conducting research in Saudi Arabia, as a co-investigator and field director of the international Palaeodeserts Project. In 2014, we discovered the site of Al Wusta, close to another established archaeological site in the north-west of the country, and began serious research there in 2016.

We very soon found hundreds of animal fossils and human-made stone tools. Then we found a small fossil, one of the best preserved from the site. It had the characteristic shape of a part of a human finger bone, but could it really be that after so many years of looking, we had finally found an ancient human fossil?

Where the finger was found. Julien Louys/Michael Petraglia/Palaeodeserts Project

We used a technique called uranium series dating to determine that the finger bone was 90,000-years-old. This involved measuring how much of the tiny amount of uranium naturally found in the fossil had decayed into radioactive thorium and working out how long this must have taken.

The next challenge was identifying the species to which the fossil belonged. Was it a human or was it a Neanderthal, the only other hominin known in south-west Asia in this time period? It turns out that the finger bone belonged to our own species, Homo sapiens.

The part of the finger bone we had found, the middle section or “intermediate phalanx”, is very different in humans and Neanderthals. In basic terms, ours are longer and thinner while Neanderthals’ are shorter and squatter. We also CT-scanned the Al Wusta fossil to produce a 3D computer model. We then used a technique called geometric morphometrics to compare the fine details of the fossil’s shape with the same part from many humans, extinct hominins and non-human primates to confirm it really was from an ancient human.

New history

This finger wasn’t just an interesting find in its own right. It could also help change our understanding of when humanity first spread out from its earliest homes. According to the old textbook view, our species evolved in Africa about 200,000 years ago. Despite a brief, failed expansion to the edge of Eurasia about 100,000 years ago when humans first tried migrating to the lands at the eastern end of the Mediterranean (the Levant), we only successfully spread out of Africa around 60,000 to 50,000 years ago.

Recent evidence suggests that much of this narrative is wrong. Findings in Africa, such as from the site of Jebel Irhoud in Morocco, suggest that Homo sapiens appeared early, more than 300,000 years ago. Our origin does not seem to have occurred in only one small area, but across much of Africa.

Findings from the Levant, most recently the dating of a maxilla (upper jawbone) from Misliya Cave in Israel, suggest our species repeatedly expanded into the winter-rainfall fed, forested area just outside Africa. We don’t yet know if people survived long term in the Levant, which is a very small area. It seems more likely that there were repeated migrations from Africa.

But what about the areas beyond the Levant? Recent findings suggest that our species got to East Asia and Australia much earlier than had been thought. But determining the hominin species present and the age of these sites have proven challenging.

Hundreds of tools and animal bones were found but only one human fossil. Klint Janulis/Michael Petraglia/Palaeodeserts Project

Our finger fossil gives us a more specific time range to work with, which correlates with other evidence. Stone tools from Al Wusta are similar to those from the Middle Palaeolithic (Stone Age) period in the Levant and north-east Africa. They suggest that our early spread into Eurasia was not associated with some kind of technological breakthrough, such as the invention of projectile technology as some have suggested.

Together, these findings show that Homo sapiens had spread beyond the Levant much earlier than traditional accounts would have it. The Al Wusta phalanx is the oldest directly dated fossil of our species beyond Africa and the Levant and so represents a crucial reference point in understanding this topic.

The challenge for the future is working out what became of the population to which the Al Wusta human belonged. The Al Wusta human lived in a very different landscape from the current desert in which it was found. The kinds of animal fossils and features of the sediments show that the site was once a freshwater lake in a grassland environment.

How did these ancient humans respond to the dramatic environmental change which dried out lakes such as that at Al Wusta? How did they relate to other populations? A single discipline alone – be it archaeology, genetics or palaeontology – can’t robustly explain the evolution and spread of our species. But by working together, I am confident that we can make major inroads into understanding our origins over the coming years.