Arabia was a legendary crossroads between East and West for centuries. Now scientists find it might have been pivotal at the dawn of history as the launching point for modern humans leaving Africa to expand across the rest of the world.
Artifacts dating back at least 100,000 years unearthed in the Arabian desert might be evidence of the first step our lineage took in our march across the globe. These new findings suggest modern humans first left Africa by at least 40,000 years earlier than researchers had expected, which could rewrite our understanding of ancient sites elsewhere on the planet.
Anatomically modern humans first arose about 200,000 years ago in Africa. When and how our lineage then dispersed out of Africa has long proven controversial, but past evidence had suggested an exodus along the Mediterranean Sea or Arabian coast some 60,000 years ago.
Now, an ancient toolkit of stone hand axes, scrapers and perforators discovered by an international team of researchers at a site in the United Arab Emirates suggests modern humans arrived in eastern Arabia as early as 125,000 years ago.
"Our findings should stimulate a re-evaluation of the means by which we modern humans became a global species," said researcher Simon Armitage at the University of London.
Rocky picnic spot
The site in question, an ancient rock shelter named Jebel Faya about 34 miles (55 kilometers) inland from the coast of the Persian Gulf, was originally known "as a nice shady picnic place for a weekend," said researcher Hans-Peter Uerpmann from Eberhard Karls University in Tübingen, Germany.
The fact that spots around Jebel Faya had stone tools suggested that artifacts might lie buried at the site.
"They were covered by layers of sand and gravel, which had accumulated since the Stone Age," Uerpmann explained.
They started digging trenches to excavate the site in 2003. "Once a camel fell into a trench and we had trouble with the Bedouins, but otherwise, the area is very safe and no problems at all with the locals, who come and are very interested," Uerpmann told LiveScience.
In 2006, the researchers discovered a stone hand ax that suggested the site might be far older than they suspected. Using a technique known as optically stimulated luminescence dating, which measures the minute amount of light long-buried objects can emit to see how long they have been interred, Armitage determined the artifacts were about 100,000 to 125,000 years old.
This hand ax and other artifacts the scientists discovered resembled technology used by early humans in east Africa, but not the craftsmanship that emerged from elsewhere in the Middle East, explained researcher Anthony Marks at Southern Methodist University in Dallas. The tools were probably not the creations of archaic humans such as Neanderthals, he noted, as the closest known Neanderthal band was thousands of miles away.
Early humans rafted out of Africa
By analyzing historical sea-level records for the region as well as details of past climate preserved in ancient lakes and rivers, sand dunes and cave stalagmites, the scientists reconstructed what the environment of the site was once like. Their findings suggest, that rather than technological innovations, a change in the environment was the key behind the expansion of modern humans out of Africa.
The investigators determined that sea levels in the southern Red Sea 130,000 years ago were more than 330 feet (100 meters) lower than today. This meant that the Bab al-Mandab Strait, which separates Arabia from the Horn of Africa, would have dried up and been much narrower, perhaps just 2.5 miles (4 km) wide, enough to allow safe passage with rafts or boats, Uerpmann said.
Although this site is now arid to hyper-arid, it was far wetter and greener in the past, "covered largely in savannah grasslands with extensive lakes and river systems," said researcher Adrian Parker at the University of London. There would have been great numbers of prey there for people to hunt, Uerpmann added — the oryx antelope, the wild ass, mountain ibex goats and several species of gazelle.
Instead of exiting Africa by traveling farther north over the Sinai Peninsula, "our findings open a second way, which in my opinion is more plausible for massive movements than the northern route," Uerpmann said. Ultimately, early humans could then have wandered into the Fertile Crescent and India and into the rest of Europe and Asia.
"These artifacts are a good sample pointing in an interesting direction," said paleoanthropologist John Hawks at the University of Wisconsin at Madison. "It'd always be nice to have skeletal materials to analyze, but it's so rare that we do."
"The longer timeline they propose matches the genetics quite well in some ways," added Hawks, who was not involved in this study. If you look at DNA from inside the human cell nucleus, "it points to a divergence time for populations inside Africa and present-day non-Africans of about 140,000 years."
However, when it comes to DNA in human mitochondria — the powerhouses of the cell, which come from each person's mother — "that has for a long time pointed at a date of 60,000 to 70,000 years ago for dispersal out of Africa," Hawks said. "So at present, we can't explain why that is." [Age Confirmed for 'Eve', Mother of All Humans ]
"It's possible that the people who initially left Africa were a small isolated population who became quite limited in mitochondrial diversity, or there were selective pressures acting on people with those mitochondria," he added. "Or it could just be we've gotten the timescales with mitochondrial DNA wrong. We don't know as of yet."
The fact that modern humans may have circulated around the world much earlier than thought raises the question of how they might have interacted or even interbred with extinct lineages such as the Neanderthals or Denisovans (a recently discovered new branch of humanity).
"The simple model of modern humans dispersing out of Africa just got more complicated," Hawks said.
"The full scope of potential conclusions, which may derive from our findings, cannot be foreseen at the time being," Uerpmann said.
The scientists detailed their findings in the Jan. 28 issue of the journal Science.
You can follow LiveScience on Twitter @livescience.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.