Tools have been found in Arabia marking the presence of humans 400,000 years ago, the oldest evidence for human habitation found there yet. The tools' presence coincides with changes in the climate, indicating the timing for humanity's movements out of Africa.
Dr Huw Groucutt of the Max Plank Institute has been studying the Khall Amayshan 4 site in northern Arabia. Although the location is now an inhospitable desert, Groucutt and co-authors report in Nature a lake has been sustained there six times over the last half a million years, and on five of those occasions, stone tools were dropped around its edges, indicating humans were present there almost every chance they got. Each time the tools appear they are different from those that came before, marking a change in culture in the intervening time.
Whether the toolmakers were Homo Sapiens or some extinct branch of the human family, is unclear, although the most recent toolmakers were probably our own species. Evidence from Arabia's relatively wet south-western coastal strip suggests some sites were occupied by modern humans and Neanderthals at different times. It's also likely some of the Khall Amayshan occupations coincided with migrations that allowed new branches of humanity to conquer the world beyond Africa.
Luminescence dating reveals the ages of the tools as approximately 400, 300, 200, and 55 thousand years old, with a longer human presence 130-75,000 years ago. It is the first time a site in Arabia has been associated with multiple bouts of occupation.
The site also preserves fossils of animals who inhabited the area at the same times as humans. Most of these represent species living in Africa at the time, confirming the theory that Arabia's brief wet periods coincided with northern African conditions that allowed animals and human inhabitants of Africa to expand their range. However, some species from nearby parts of Asia were also found at Khall Amayshan during these periods.
It's, therefore, possible some tool-makers were early humans who had left Africa during previous migration windows, and reclaimed parts of Arabia during bursts where this was possible. However, in most eras, the style of tool-making more closely resembles those in Africa at the relevant times than in areas to the north of the site that remained more continuously habitable by humans.
"Arabia has long been seen as empty place throughout the past," Groucutt said in a statement. "Our work shows that we still know so little about human evolution in vast areas of the world and highlights the fact that many surprises are still out there."
In an accompanying editorial the University of Exeter's Professor Robin Dennell, who was not involved with the research, notes; “Ten years ago no dated archaeological sites more than 10,000 years old had been recorded in the three million square kilometers [almost 1.2 million square miles] of the Arabian Peninsula.” Since then we have found 85,000 year-old footprints, 190,000 year-old tools that resemble those of earlier eras, and a solitary finger bone.
Dennell also notes the most recent tools could have come from a time when modern humans and Neanderthals overlapped in the area, possibly representing one of the times when interbreeding took place.
We're lucky to be able to find Khall Amayshan 4's treasures. Migrating sand dunes covered the site during dry periods, protecting it from erosion and providing clear markers distinguishing the different periods of habitation.