Lush wetlands lured waves of early humans out of Africa, stone tools suggest | Science

If you know what to look for in dappled satellite images of desert—slight depressions, subtle color shifts—the dried-up ghosts of prehistoric lakes pop out against the sand fields of the Arabian Peninsula. Eight years ago, one ancient multihued lake in the Nefud Desert caught the eye of researchers. When scientists excavated its ancient shorelines, a new study reports, they found thousands of stone tools—and evidence that multiple waves of Homo sapiens and their relatives have been migrating across the Arabian interior for at least the past 400,000 years.

The results bolster the idea that the periodic greening of this typically harsh desert played a pivotal role in humans’ dispersals out of Africa—and provide the best evidence yet that different groups of humans pulsed out of the continent through the Sinai Peninsula, says Jessica Thompson, a paleoanthropologist at Yale University who wasn’t involved with the study. “Where there are lakes, there will be people,” she says. “They find their way there.”

Today, the sparsely populated Nefud is filled with wind-whipped sand dunes and spindly, drought-tolerant shrubs. But past excavations and paleoclimate models have revealed that over the past half-million years, brief periods of wetter, warmer conditions dumped seasonal rainfall over the region, turning its low basins into lakes and its ditches into rivers. In short order, the harsh desert became a lush grassland—a “green Arabia”—only to wither back to sand when arid weather inevitably returned.

In 2013, remote imaging specialists keyed in on several ancient lakebeds in the western Nefud in northern Saudi Arabia that appeared unusually colorful in satellite images. Archaeologist Michael Petraglia at the Max Planck Institute for the Science of Human History (SHH) suspected its marbled bands of sediment reflected several periods of draining and refilling.

He and colleagues piled into four-wheel-drive vehicles and headed to a site known as Khall Amayshan 4, huddled between large dunes. “Once we got there, there were just stone tools everywhere, so we knew straight away this was an amazing site,” says archaeologist Huw Groucutt, Petraglia’s colleague at SHH.

Using an excavator, the researchers dug trenches in the lakebeds. They dated the layers with optically stimulated luminescence, a technique that determines when sand grains were last exposed to sunlight, and they noted the stone tools associated with each layer. Their digging revealed that the paleolakes at Khall Amayshan 4 had formed and dried up six different times; stone tools were associated with five of those long-lost lakes, dating to 400,000, 300,000, 200,000, 100,000, and 55,000 years ago, the researchers report today in Nature. At another paleolake about 150 kilometers to the east, the Jubbah oasis, they found stone tools in layers dating to 200,000 and 75,000 years ago.

A stone tool
Scientists found thousands of stone tools on the shorelines of these ancient lakes, including stone hand axes—pictured here—that date to some 400,000 years ago.Ian Cartwright/Palaeodeserts Project

In addition to tools, the researchers found fossilized animal bones at many of the dry lakes, suggesting large African animals like hippopotamuses, elephants, and ostriches also followed this green route out of Africa, at least in the wet season. No hominin fossils were found in the new digs.

It’s unclear which hominin species was responsible for the tools in the two oldest layers, Groucutt says, but the relatively crude pointed hand axes are normally attributed to close ancestral relatives of our own species, such as H. erectus. The tools found in layers dating to 200,000, 100,000, and 75,000 years ago were smaller, more precisely worked stone flakes that resemble tools thought to have been made by our own species, the authors note. What happened to these early migrants remains a great unknown, Groucutt says.

Nevertheless, archaeologist and study co-author Abdullah Alsharekh with King Saud University says even with those unknowns, the work is starting to reveal “much of what has been hidden under the sand” for millennia.

The most recent tools—those from 55,000 years ago—closely resemble those linked to Neanderthals. Scientists have found Neanderthal remains in caves in the woodland-covered Middle East, toward the northwestern edge of the Arabian Peninsula. If the new findings hold up, they would suggest that Neanderthals, too, moved into the peninsula’s periodic grassland paradise, and may have even encountered our own species there.

Although scientists have known for years that Arabia likely played an important role in early human migration, the new study provides the most orderly mapping out yet of movement through this critical region, says Graeme Barker, an archaeologist at the University of Cambridge who was not involved in the work. It for the first time points to successive waves of specific species at specific times.

And the location of most of the paleolakes—in the northern reaches of the Arabian Peninsula—points to the Sinai Peninsula as the most likely route for human migration out of Africa, Groucutt says, displacing the Arabian Peninsula’s southern tip, which some researchers have proposed. Thompson agrees with that interpretation. “Through the Sinai seems to be the best supported model based on present evidence,” she says.

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