Ancient human footprints discovered in Arabia

We’re 'anatomically modern' humans - Homo sapiens - and our species arose about 250,000 years ago in Africa. At some point, our ancestors left the continent to spread across the rest of the world. Previous fossil and other evidence suggested that this began to happen from about 55,000 years ago, although the science was shaky. That’s why the recent discovery of a cornucopia of fossilised footprints in Arabia, which is of course outside of Africa, and dating from a hundred thousand years ago, is a huge step forward in our understanding. The tracks include ancient elephants, horses, and what appear to be modern humans. Mathew Stewart is one of the discoverers, and spoke to Chris Smith...

Mathew- What makes these findings in particular special is the age of the deposits, I would say. We dated the footprints to around about 125,000 years ago. This was a particularly humid period, and it's quite an important period for human dispersals out of Africa.

Chris - Can you tell us a bit about the area you were working in when you made the discovery? What does it actually look like today?

Mathew - This is one of the largest deserts in Arabia. So you can picture - very, very arid, little in terms of vegetation and animals, and just rolling sand dunes.

Chris - And when you went there, obviously you didn't go there anticipating you were going to find an amazing collection of footprints. So what did you expect to find?

Mathew - The footprint findings were sort of quite a lucky finding. We were surveying a palaeo lake - so an ancient lake deposit - as we've done hundreds of times in this area. And in this instance, we had happened to notice that the surface of the palaeo lake was covered in these footprints and trackways.

Chris - It must've been pretty exciting for you when you realised what you'd stumbled on.

Mathew - Yeah, it was, it was very exciting. In fact, we were at the sight for quite some time before one of our senior colleagues noticed the footprints. And as soon as we saw a couple of them, we realised that the entire ancient lake surface was covered in them.

Chris - And how do you know how old they are?

Mathew - The footprints were being exposed by the erosion of these overlying lake sediments, but there were still some of those overlying sediments left. So we were lucky enough to be able to date the sediments below and above. And so we used this method known as optically stimulated luminescence dating, targeting these layers, which led us to conclude that the footprints were between 121-112 thousand years old.

Chris - And who made the footprints?

Mathew - We argue that these are footprints made by our species, Homo sapiens, on two grounds. There is no evidence for Neanderthals in that region at that time. What we do know is that we have Homo sapiens dispersing out of Africa at around about that time. The second line of evidence is the size of the footprint. So we did some stature and mass estimates based on the footprints themselves and compared those to estimates of Homo sapiens and Neanderthals based on fossilised bone. They come out more closely to Homo sapiens.

Chris - Critically though, when for years we've been talking about how humans left Africa - because we agree that anatomically modern humans like us almost certainly had their origins on the African continent, and that's based on a whole range of different lines of evidence isn't it, including genetic evidence - we've been telling people as a community scientifically for years that the exodus out of Africa to populate the rest of the world seems to be around about 50-60 thousand years ago. But you're pointing to a different geography from Africa, and you're pointing to a timeline 60 thousand years earlier. So does this then suggest that actually we had it wrong?

Mathew - Yeah. This sort of joins a growing corpus of evidence suggesting that this traditional view of this exodus out of Africa at around 50 thousand years ago isn't entirely correct. There is growing both archaeological and fossil evidence to suggest that we dispersed out of Africa earlier, all the way to Northern Australia by around about 65-70 thousand years ago. The picture is just becoming much more complex. It appears that we left multiple times; that there were dispersals back into Africa; it's adding to this much more complex picture.

Chris - And presumably if these footprints were in what is now a dried up lake, it was once not a dried up lake. And it presumably then was not in a desert. So what was the environment there like, and does that point towards why these early humans could have been making a beeline for that geography?

Mathew - Absolutely. The Arabian peninsula, much like the Sahara, wasn't always the hyper arid desert that they are today. And in fact numerous times over the past million years the conditions have changed drastically, during what are known as interglacials - quite humid periods. The last interglacial, which is what the footprints date to, being one of these very humid periods; changing the Sahara and Arabian deserts into big open grasslands, large permanent rivers and lakes, and vastly different flora and fauna as well.