Ancient Desert Lakes in Arabia Left No Human Trace. Missing Stone May Be Why

Researchers who walked across Arabia's Empty Quarter looking for the camps of early humans found ancient lake deposits but no sign that people ever lived beside them. A new study in Nature suggests water alone was not enough to hold prehistoric groups in place: they also needed the right stone for making tools, according to ScienceAlert.

The result challenges a long-standing idea that temporary lakes and grasslands in the desert would have drawn people moving out of Africa and into Asia.

What the team did in the Empty Quarter

The Rub' al Khali, or Empty Quarter, is the world's largest continuous sand desert and stretches for 1,000 kilometers (620 miles) across the Arabian Peninsula. During wetter phases of the Pleistocene, ScienceAlert reported, temporary green corridors appeared there, fed by lakes that have since vanished. Researchers have long proposed that early humans could have used those corridors to disperse out of Africa.

In the new study, a team traveled roughly 1,500 kilometers across the desert in search of ancient lake deposits and signs of hominin settlement. They investigated 125 locations and found evidence of ancient lakes at 55 of them. Earlier research had dated lake deposits in the region to roughly 122,000 to 67,000 years ago.

They found no traces of ancient humans. Geoarchaeologist Rudolf Dellmour of the University of Vienna said the team stood on the shore of one ancient lake after another without finding anything made by human hands.

Why stone appears to matter

The researchers point to geology. Of the 125 locations, only 10 had deposits of knappable stone, the kind that can be shaped into stone tools, and all 10 lay on the outer margins of the desert. The surface geology of the central basin is made up of carbonate and evaporite formations, which the team says are unsuitable for making stone tools. Stone-tool production, the study argues, would have been a prerequisite for sustained settlement.

The authors note that access to knappable stone has been linked to human occupation in several other settings, including Libya, Tibet, Egypt, South Africa and North America. They describe a "tethering" effect, in which people who depend on stone tools become tied to sources of raw material. In many of those places water was plentiful, so the location of stone played a more visible role in where people settled.

A model that combines water and rock

Alongside the fieldwork, the team built a model to refine predictions of where ancient settlements might have been, factoring in both water availability and geology. By the researchers' calculations, 57 percent of the terrain that would have had access to water in prehistoric Arabia lacked suitable stone for toolmaking.

The authors describe a patchwork of separate zones where water and stone occurred together. They call "Green Arabia" a "lithic archipelago," with islands defined as much by geology as by climate.

Why it matters for the story of human migration

The study speaks to how scientists reconstruct the routes early humans took out of Africa. The authors warn that dispersal models that leave out the distribution of stone raw materials may systematically overestimate the area that people dependent on stone tools could have lived in.

In practical terms, a map of ancient lakes and grasslands may show where people could have found water, but not necessarily where they could have stayed. As ScienceAlert put it, a lake appearing in the desert does not necessarily make a town.

Prehistoric archaeologist Emanuele Cancellieri of the University of Algarve in Portugal, who was not involved in the study but wrote a commentary on it in Nature, said the idea that humans need more than a water supply is hardly new. He said the value of the work lies in testing that idea in an ambitious study.

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Around Prague, based on reporting by ScienceAlert