100,000-year-old human settlement in U.A.E. overturns what we know of our evolution

Human artifacts recently discovered in the United Arab Emirates date back at least 100,000 years, which means our ancestors might have left Africa up to 125,000 years ago...twice as long ago as previously thought. What's going on here?

The tools discovered during an excavation in the U.A.E., located in the southeastern part of the Arabian peninsula, have been reliably dated to 100,000 years ago. Genetic evidence has suggested modern humans did not leave Africa until about 60,000 years ago, but these tools appear to be the work of our ancestors and not other hominids like Neanderthals.

If they are the work of our ancestors, then they've been found outside Africa at least 40,000 years ahead of schedule. But, as the paleontologists behind this discovery are quick to point out, the 60,000 year figure is one based on only one strand of evidence, and that's genetic data. It's a useful tool, to be sure, but using genetics to reconstruct a species's history can be tricky - genetic data once said domestic dogs were 120,000 years old, but more recent evidence has shown they're actually much closer to 20,000 instead.

This find is one of the first major archaeological discoveries that seems to place anatomically modern humans out of Africa - but, helpfully, still close to Africa, so it's a bit easier to reconstruct their path and timing of migration. That automatically makes this an intriguing find, although we can't instantly dismiss the old 60,000 years figure. This is an extraordinary claim and, as one of the best scientific maxims points out, it requires extraordinary evidence.

Well, I can't guarantee their evidence is sufficiently extraordinary, but at a press conference yesterday the researchers involved did lay out some compelling reasons to believe the basics of the find - that modern humans lived in Arabia 100,000 years ago - even if they were reluctant to discuss the wider implications.

They answered a number of questions one might have about this discovery, so let's dive in:

How do we know anatomically modern humans made these tools?

Paleontologist Tony Marks explains how they identified the likely makers of these tools, which were classified assemblage C:

"There were two possibilities for assemblage C. First, that it was made by local people who'd been there for a long time and who would have left similar artifacts around the landscape. Or second, it was made by people moving into the area. Since assemblage C was 120,000 years old, we looked at what was in southeastern Arabia at that time, there was literally nothing. Long before 120,000 in western Arabia there was what we call the Acheulean, but it had disappeared about a half million years ago, leaving a 400,000 year gap between it and assemblage C. Thus it seemed that assemblage C was made by people coming from somewhere outside southern Arabia, either from the north or from the west.

"A comparison of contemporaneous Paleolithic assemblages from the north showed they totally lacked the bifacial tool production found at assemblage C. Their technique was quite different. Thus, they were unrelated. In east Africa, however, there were contemporaneous Paleolithic assemblages that not only used bifacial techniques to make some of their tools, but also used the other two techniques, blade production and radial (levaloir). An origin in east Africa for assemblage C people therefore was most plausible based on the stone tools and how they were made."

But couldn't it have been another hominid species that had already left Africa, such as the Neanderthals?

Marks offers some logical reason why Neanderthals are very unlikely candidates to be behind these tools, even leaving aside the fact that the tools fit the more human style:

We can look at it from a broad point of view. If these tools were not made by modern man, who might have made them? Well, could Neanderthals have made them? Well, at 120,000 years ago, beginning of the inter glacial, Neanderthals had pretty well developed their facial characteristics and body characteristics to be recognizable as Neanderthals and not the yet classic Neanderthals. But they're mainly in Europe at about the beginning of the last interglacial there's a movement, a spread of Neanderthals along the temperate zone to the east. That is the Crimea, southern Russian plain out to central Asia. There is no evidence for any Neanderthals south of that temperate zone to the east. It is only in OSI4, that is when it starts getting cold that you have movements of Neanderthals out of the highlands of the temperate zones down into the (levant). Into lower elevations where the environment is better. Here is a group of Neanderthals who instead of going into this temperate zone, which was getting better, they took a turn south, went several thousand kilometers into what at the time was desert, really dry areas, until they reached southern Arabia, which happened to be very good because of monsoons that were coming up from the south. It seems to me a very difficult explanation and one that is – doesn't follow any reasonable logic.

100,000-year-old human settlement in U.A.E. overturns what we know of our evolution

If these tools date back to 100,000 years, why then do they think humans left Africa 125,000 years ago?

Adrian Parker explains how ancient climate limited the times when humans could leave Africa, and that about 125,000 years ago was an ideal time to move into Arabia:

We need to go back to where modern humans emerged in east Africa. This occurred approximately 200,000 years ago. The period between 200,000 years ago until 130,000 years ago corresponds to time when there was a global ice age. During ice ages global sea levels fall as water becomes locked up in the vast ice sheets in the northern and southern hemispheres. When ice ages occur, the world's major desert belts also expand and thus modern humans would have been restricted to east Africa as the deserts of the Sahara and Arabia posed major geographical barriers that prevented movement out of the region.

By 130,000 years ago global climatic conditions changed and we moved into an interglacial, a period of warmer, global temperatures. At this time, the Indian Ocean monsoon system was forced northwards, bringing rainfall into Arabia. The previously arid interior of Arabia would have been transformed into a landscape covered largely in savannah grasses with extensive lakes and river systems. At the onset of the inter glacial, sea levels in the southern Red Sea were over 100 meters lower than today. this led to a brief window of time when sea levels were still low and Arabia experienced a wetter climate, thus humans would have been able to cross a much narrower Red Sea, perhaps as little as four kilometers wide before sea levels rose sufficiently to make the crossing more difficult."

Did this particular population of anatomically modern humans then spread out further, or could they have been an isolated population that just died out, with the successful population only leaving at the accepted 60,000 year date?

Lead researcher Hans Pedro Ortun says there's no real way to know that from just one site, but one can still speculate a little:

"We only can speak about the site that we are dealing with and not with where these people would have gone. But as you can easily imagine the route to the north was easier than the route back to the south because the desserts there would have become worse in that period of time. So the natural answer to your question would be that they moved towards the Persian Gulf which was smaller then, and along the gulf, either towards the north or west into Mesopotamia or towards the east along the Iranian coast and the Indian subcontinent."

What does this discovery tell us about human migrations?

Again, Dr. Ortun says this pushes the first human migrations back at least 20,000 years, but it also illuminates a lot about their mechanics:

Well, the mechanisms of getting out of Africa should be understood in a different way. Up till now we thought of cultural developments leading to the opportunity of people to move out of Africa. Now we see, I think, that it was the environment that was the key to this and the change from a glacial period into an interglacial opened the other possibility to leave Africa though the southern corridor and this certainly not only happened once, this happened many times during the (quarternerly) and this leaves a lot of possibilities for human migrations and keeping this in mind, might change our view completely. There are not many exits from Africa. You can only exit by the Sinai or by the south. That's the only – that's the only points where you can leave it. so either it's the route that we propose, or it could be the route from Egypt to Sinai and both are possible, both have their problems and in any case, our findings open a second way, which in my opinion is more plausible for massive movements than the northern route.

But how does this fit with the 60,000 year figure produced by genetic data?

As I outlined above, there are some reasons to be cautious about the genetic data, and Nicholas Wade pushes it further. He points out that there may be no genetic evidence to back up their archaeological discovery, but it's not as though there's any archaeological data to bad up the genetic claims:

If we take this 60,000 year expansion and we say OK, instead of looking at it from a genetic point of view, let's look at it from an archeological point of view. what were these people carrying in the way of culture at 60,000 years ago in Africa and can you find it anyplace outside of Africa? And the answer is no, you can't. There is no archeological evidence for movements at 60,000 out of Africa. Now, it may turn up next year, but at the moment it's simply not there.

For more on how we got here, check out our earlier piece on the even more extraordinary claim that a 400,000 year old human tooth had been discovered in a cave in Israel. Whether either of these claims ultimately stand up - and this new finding seems to have a better chance of that than the cave teeth - it looks like the story of human evolution is about to get a lot more interesting.

[Science; images via Science/AAAS]