The world鈥檚 largest sand sea, the Empty Quarter in the Arabian peninsula, was once dotted with freshwater lakes and lush oases 鈥� but prehistoric humans appear to have avoided settling around them.
Today, scientists have found out why: the early inhabitants chose to live near useful deposits of rock instead. After gathering data during a 1500-kilometre expedition across the vast desert and using computer modelling to understand the evidence, researchers have concluded that even plentiful fresh water wasn鈥檛 enough to attract early humans if they couldn鈥檛 also find exposed formations of suitable tool-making stone like flint and quartz.
鈥淧eople who live by stone tools never stray far from good stone,鈥� says at the University of Algarve in Portugal, who was involved in the analysis. 鈥淵ou just really don鈥檛 see Stone Age sites unless you鈥檙e within a few kilometres of an outcrop.鈥�
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The Sahara is the world鈥檚 largest hot desert, with landscapes ranging from rocky highlands to vast gravel plains. But the Rub鈥� al-Khali 鈥� the Arabic name for the Empty Quarter 鈥� is the world鈥檚 largest continuous expanse of wind-swept dunes, known as a sand sea. It covers 650,000 square kilometres of the southern Arabian peninsula.
Today, the region is overwhelmingly dry, other than for a few permanent lakes in its interior that are fed by groundwater. But over the past century, scientists have uncovered evidence of numerous ancient lakes that formed during wetter periods, when stronger Indian Ocean monsoons helped .
Researchers have long suspected that these greener periods opened routes for early humans spreading out of Africa and across the Arabian peninsula 鈥� an idea supported by the . However, all these sites line the desert鈥檚 margins, whereas its vast interior is still largely unexplored by researchers.
In 2023, Rose鈥檚 colleague at the University of Vienna and his team spent 35 days crossing the Empty Quarter鈥檚 dunes, examining 125 locations for traces of ancient lakes. They focused on lakes that were present between about 122,000 and 67,000 years ago, when early humans were spreading into Arabia.
The team found evidence of ancient lakes at 55 of the locations. This evidence included freshwater snail and clam shells, ostrich eggshell fragments and traces of animals that once burrowed into the muddy lake beds. But despite these signs of a once-thriving environment, they found no stone tools or other evidence that prehistoric humans had settled by any of the 55 lakes.
鈥淚f we could spot a snail shell the size of a fingernail, we would have spotted a stone tool,鈥� says Dellmour. 鈥淏ut there were none.鈥�
To test whether the shortage of suitable stone could explain the lack of human settlements, Rose and Dellmour and their colleagues turned to computer modelling. They mapped ancient water sources and the geology of the Arabian peninsula, then compared the results with the distribution of hundreds of archaeological sites found across the region in earlier research.
The analyses revealed that 57 per cent of the peninsula鈥檚 terrain within reach of water lacked suitable stone for making tools 鈥� and that prehistoric human settlements were concentrated in places offering both resources.
That could be partly due to ancient tool-making being extremely 鈥渨asteful鈥�, requiring abundant access to raw material, says Rose. 鈥淭hey weren鈥檛 thinking about efficiency,鈥� he says. 鈥淪o, you鈥檇 have a pile of garbage produced for one spear point.鈥�
Groundwater-fed lakes in the Empty Quarter wouldn鈥檛 have brought flint, chert, obsidian or quartz into the sand sea like rivers could have, says Rose. And while people could have carried tools into resource-poor areas, this would probably have supported only small groups for short periods.
The findings show that geology was potentially just as important as climate in shaping prehistoric human migrations, says Rose.
The study sheds important light on a long-standing debate among archaeologists about the importance of raw stone in ancient human sites, especially in harsh environments like the desert, says at the University of T眉bingen in Germany. 鈥淧robably what鈥檚 most surprising to me is the lack of archaeological stone tools in otherwise perfectly fine circumstances, and close access to fresh water,鈥� he says. 鈥淚t really shows that early humans were picky with where they went and that they did not settle for just any spot.鈥�
Nature
