Саудын Арабын цөлөөс олдсон 115,000 жилийн настай хүний мөр нь эртний нүүдлийн шинэ баримт боллоо

Along the southwestern edge of a dried-up lake in the Nefud Desert of northern Saudi Arabia, four footprints sit side by side. They point the same way and lie close together. Not all four are the same size.

Researchers read the cluster as two people, or possibly three, walking together.

They were not alone at the water. The same surface holds 107 camel tracks and 43 elephant tracks. Prints of horses and of bovids are there too, possibly including a giant buffalo. The survey recorded 376 footprints in all.

The lake bed is called Alathar, and the footprints in it are between about 112,000 and 121,000 years old. Seven are human, and the team that studied them, writing in Science Advances in September 2020, argues that they are the oldest securely dated evidence of Homo sapiens in Arabia.

Sand Grains That Keep Time

Erosion exposed the prints in 2017 after stripping away the sediment that had covered them. “We immediately realized the potential of these findings,” said Mathew Stewart of the Max Planck Institute for Chemical Ecology. He led the study.

The team photographed the tracks and built three-dimensional models of them, matching 177 of the 376 prints to a kind of animal. To date the surface, they used optically stimulated luminescence. The method measures how long grains of sand have been hidden from daylight.

“Sand grains act as natural clocks as they record the time elapsed since they were last exposed to sunlight,” said Simon Armitage of Royal Holloway, University of London. “Once the sands are buried in the lake sediments, they are protected from sunlight and the natural clock begins ticking.”

The layer beneath the footprints gave an age of 121,000 years, give or take 11,000. Above them, the layer dated to 112,000, give or take 10,000. Both land in the last interglacial, a humid stretch in the region’s past.

A Lake in a Grassland

The Nefud is sand today. It was not then.

“At certain times in the past, the deserts that dominate the interior of the Arabian Peninsula transformed into expansive grasslands with permanent freshwater lakes and rivers,” said Richard Clark-Wilson, also of Royal Holloway.

Today the monsoon rains reach only the southern and southwestern edges of the peninsula. Climate records indicate that, several times over the past 130,000 years, they pushed into the desert interior.

Alathar was a shallow freshwater lake in semi-arid grassland. Some tracks record animals walking to and from its shoreline, southwest of the surface exposed today. Around it, the mud is heavily trampled, a pattern the study attributes to a dry season, when herbivores crowd around shrinking water.

Elephants had disappeared from the Levant, to the northwest, about 400,000 years ago. At Alathar, their tracks run mostly north to south, while the other animals’ prints head in every direction.

The team also collected 233 fossils from the site, among them the remains of oryx and elephants, and some of the bones carry the tooth marks of carnivores.

Seven Prints And No Tools

The study gives two reasons for assigning the human prints to Homo sapiens. Our species had spread into the Levant and Arabia between 130,000 and 80,000 years ago, during a period when Neanderthals were absent from the Levant. The size of the footprints also fits early Homo sapiens better than it fits Neanderthals.

Nothing else the people left has been found. The site has yielded no stone tools. Stewart called that unusual for the area. None of the fossil bones shows butchery marks.

“We know people visited the lake, but the lack of stone tools or evidence of the use of animal carcasses suggests that their visit to the lake was only brief,” Stewart said. The authors conclude that the lake was a place to drink and to forage.

The prints themselves put a hard limit on the time involved. In an experiment with modern footprints on mud flats, fine details were gone within two days. By four days, the prints were unrecognizable.

“Footprints are a unique form of fossil evidence in that they provide snapshots in time, typically representing a few hours or days, a resolution we tend not to get from other records,” Stewart said.

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