The world’s largest continuous sand desert was not always so unforgiving.
The Rub’ al Khali, or “Empty Quarter”, stretches 1,000 kilometers (620 miles) of endless sand across the Arabian Peninsula.
But in the Pleistocene, it was a different story.
In wetter phases of deep history, the world’s largest sand sea sometimes became a more hospitable place.
During these windows, temporary green corridors emerged in the Empty Quarter, through which early humans could have dispersed out of Africa, supported by the life-giving waters of ancient ephemeral lakes that have long since disappeared.
But there is something mysterious about these ancient lakes.
In a new study in Naturea team of researchers traveled around 1,500 kilometers of the Empty Quarter, searching for clues about these ancient lake deposits and signs of their hominid colony.
In total, they found evidence of ancient lakes at 55 of the 125 sites surveyed, with previous research dating lake deposits in the area to around 122,000 to 67,000 years ago.
But no traces of ancient humans could be seen.
“We found ourselves at one ancient lake after another, but we found nothing man-made,” says geoarchaeologist Rudolf Dellmour of the University of Vienna in Austria.
For decades, researchers have hypothesized that the existence of ancient temporary lakes and nearby grasslands in the Rub’ al Khali would have attracted human settlements, as groups migrated from Africa, through Arabia and into Asia.
But while water is certainly vital to life, the new findings suggest that ancient humans needed more than water alone to set up camp.
Of the 125 sites surveyed during their journey through the Empty Quarter, the researchers found only 10 sites with geological deposits of cutting stone that would have allowed ancient humans to develop stone tools, and all of them were located on the outer margins of the desert.
Thus, while many ancient freshwater lakes could have supported life, the surface geology of the central desert basin – composed of carbonate and evaporite formations – was not suitable for the manufacture of stone tools, which would have been a prerequisite for lasting human settlement.
“The fact that humans need more than just a water supply is not a new archaeological discovery,” says prehistoric archaeologist Emanuele Cancellieri of the University of the Algarve in Portugal, who was not involved in the study but wrote a commentary on it in Nature.
“Rather, the importance of this research lies in putting this idea to ambitious study.”
This ambitious study goes beyond fieldwork in the Rub’ al Khali.
Alongside their desert expedition, the researchers developed a model to refine predictions about ancient settlement sites in the region, incorporating both water availability and geological considerations.
According to the researchers’ calculations, 57 percent of the land that would have been accessible to water in prehistoric Arabia lacked suitable stone for making tools.
In their study, they note that the availability of knapping stone has previously been linked to human occupation in several historical and geographic scenarios, including Libya, Tibet, Egypt, South Africa and North America.
“These convergent patterns, across different geological substrates, hydrological regimes, and chronological contexts, provide analogous support for a tethering effect by which populations dependent on stone tools become linked to raw material sources,” the researchers write.
In many of these contexts, water was widely available, so the distribution of whittling stones played a more visible role in determining where humans settled.
In Arabia the same rules applied, but the presence of the world’s largest continuous desert meant that human migration faced a more formidable and fragmented challenge.
“The Arabian Peninsula appears to have constituted a mosaic of distinct areas where water and lithic raw materials coexisted,” the researchers explain.
“These results indicate that Green Arabia was a lithic archipelago whose islands were defined as much by geology as by climate; dispersal models that omit the distribution of lithic raw materials may systematically overestimate the area habitable by populations dependent on stone tools.”
In other words, if a lake appears in the desert, it does not necessarily make a city.
The results are reported in Nature.
This article was fact-checked by Rachel Garner and edited by Rebecca Dyer. Although we are proud of our process, we are only human. If you spot an error, please let us know.
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