Giving human origins the finger

The story of how - and when - the first anatomically modern humans migrated out of Africa and spread around world has been challenged by a new fossil discovery from Saudi Arabia. A bone from a middle finger, together with hundreds of stone tools, have been uncovered and dated to a time much earlier than scientists had believed that our ancestors first left the African continent. Georgia Mills spoke with study author Huw Groucutt from the University of Oxford.

Huw - The textbook view of human origins at the moment suggest that our species - homosapiens - evolved in Africa and that all non-Africans are descended from a migration out of Africa about 50 or 60 thousand years ago, probably following the coastline out of Africa. We know that about 100 thousand years ago there was a very short-lived localised expansion to the Levant, and this is mostly known from a few sites in, what is today, Northern Israel. But the idea is this is just sort of a flash in the pan, it’s very localised and really that’s irrelevant to the major story of how Eurasians came to be.

Georgia - What have your found then that challenges this notion?

Huw - We’ve been working in Saudi Arabia that’s part of a big project for almost 10 years now and, until very recently we knew almost nothing about the area. We found, over the last few years, lots of archaeological sites and lots of animal fossils, but we were always missing human fossils. We found a site called Alwosta, and there we found a human finger bone; it’s the middle bone of the middle finger and it’s dated to 90 thousand years ago. So, even though it’s only one fossil, it’s enough to identify the species. It’s our species, homosapiens, so it’s showing that we were in that area 90 thousand years ago.

Georgia - Right. So you’ve placed at least one human at a place where we didn’t previously believed there would have been humans at this time?

Huw - Yes, that’s right. It was a very different sort of environment to where we knew people were living. We knew they were in the forests on the sort of doorstep of Africa in the Levant, and now we know they were deep in the interior of Arabia. Although it’s only one fossil, we found hundreds of stone tools so we know there were quite a few people living at this site, and there were many other sites like it in the area.

Georgia - How did you date it, and how are you sure it’s human? I’m guessing if it’s just a finger bone it’s not a particularly obvious fossil, if that makes sense?

Huw - We got very lucky really. In this region there are two sorts of possibilities: us or neanderthals. It fortunately turns out that this bone is quite different in us and neanderthals. Our middle finger bone is more narrow and elongated, whereas a neanderthal bone is shorter and squatter. But, to really be sure, we did CT scanning to get a 3D model, and then we compared this using a technique called geometric morphometrics to the same bone from various humans, various extinct hominins, and even things like monkeys and chimpanzees, and this very clearly showed that the fossil aligned with homosapiens; it’s a very similar shape. If I hold it to my finger it’s exactly the same shape and size.

With the dating, we used a method called uranium series dating. When a bone is buried in the ground it absorbs uranium and, over time, this uranium decays into thorium, and we can measure the precise ratio of these two isotopes to determine the age; it decays in a predictable rate. This gave us a date of 88 thousand years ago and to confirm that date we used other techniques to date the sediments of the site and animal bones at the site as well. And all of these techniques agreed with each other that the fossil was about 90 thousand years, which was great because often different techniques are applied and the don’t agree with each other, so it gets complicated. So this is a very well dated fossil in a very well dated site.

Georgia - What does this tell us then; what can we now infer about human history?

Huw - I think the major change is that the sort of textbook views suggesting we left Africa about 50 or 60 thousand years ago was based on the idea that we couldn’t leave Africa until there’d been some kind hi-tech sort of revolution - a human revolution and there are different forms of this. But, for example, some people argue that we needed bows and arrows or complex symbolic material culture to survive in Eurasia, which seems sort of odd but that was a common view. What we found is that we were spreading much earlier, using very simple kinds of material culture.

So our migration wasn’t based on some kind technological breakthrough, it was based much more on patterns of climate change. We know, periodically, North East Africa and Arabia witnessed very extreme climate change. Basically, monsoonal rains moved far inland and this transformed the area so there were lakes, and rivers, and grasslands, and abundant animals, and it was this transformed landscape that early humans could follow. So it changes the narrative from a kind of human revolution to one based much more on the environment and climate.