The world’s largest sand sea, the Empty Quarter in the Arabian peninsula, was once dotted with freshwater lakes and lush oases – 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’t enough to attract early humans if they couldn’t also find exposed formations of suitable tool-making stone like flint and quartz.
“People who live by stone tools never stray far from good stone,” says Jeffrey Rose at the University of Algarve in Portugal, who was involved in the analysis. “You just really don’t see Stone Age sites unless you’re within a few kilometres of an outcrop.”
The Sahara is the world’s largest hot desert, with landscapes ranging from rocky highlands to vast gravel plains. But the Rub’ al-Khali – the Arabic name for the Empty Quarter – is the world’s 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 transform parts of the peninsula into what scientists call Green Arabia.
Researchers have long suspected that these greener periods opened routes for early humans spreading out of Africa and across the Arabian peninsula – an idea supported by the presence of stone tools near ancient lakes. However, all these sites line the desert’s margins, whereas its vast interior is still largely unexplored by researchers.
In 2023, Rose’s colleague Rudolf Dellmour at the University of Vienna and his team spent 35 days crossing the Empty Quarter’s 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.
“If we could spot a snail shell the size of a fingernail, we would have spotted a stone tool,” says Dellmour. “But there were none.”
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’s terrain within reach of water lacked suitable stone for making tools – and that prehistoric human settlements were concentrated in places offering both resources.
That could be partly due to ancient tool-making being extremely “wasteful”, requiring abundant access to raw material, says Rose. “They weren’t thinking about efficiency,” he says. “So, you’d have a pile of garbage produced for one spear point.”
Groundwater-fed lakes in the Empty Quarter wouldn’t 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 Manuel Will at the University of Tübingen in Germany. “Probably what’s most surprising to me is the lack of archaeological stone tools in otherwise perfectly fine circumstances, and close access to fresh water,” he says. “It really shows that early humans were picky with where they went and that they did not settle for just any spot.”