Stewart et al. 2020
About 120,000 years ago, two or three people walked along the shores of a shallow lake in what is now northern Saudi Arabia. They left at least seven footprints in the mud, and today those marks are the oldest known evidence of our species' presence in Arabia.
A Pleistocene walk by the lake
Imagine you were a hunter-gatherer about 120,000 years ago and you went to Eurasia from East Africa. Paleoanthropologists are still debating exactly why you decided to do something like this, and you almost certainly don't have a goal in mind, but for now, we're assuming it just takes a really, really long time to go. You almost inevitably come to the Levant at the eastern end of the Mediterranean. From this important geographic crossroads, you have a few options: you can travel north through Syria and Turkey, then turn east to Asia or west to Europe. They could also strike east across the northern end of the Arabian Peninsula.
That was a better option then than it sounds now. During the Pleistocene, the Arabian Peninsula repeatedly had a more humid climate than it does today. Evidence of ancient sediments, pollen, and animal fossils suggest that today's deserts were once grasslands and forests crossed by rivers and dotted with lakes, such as at Alathar in the western Nefud Desert.
As a result, the Arabian Peninsula was an important avenue for hominin expansion beyond Africa, beginning with Homo erectus and eventually ending with Homo sapiens. 300,000 year old stone tools at one point in Nefud likely indicate an early wave of hominins, likely Homo erectus. And last year, archaeologists found an 85,000-year-old finger bone fossil in Nefud – the oldest directly dated Homo sapiens fossil outside of Africa or the Levant. The footprints in Alathar suggest that our species had reached Arabia earlier.
How can we be sure? Neanderthals had wider, heavier feet with flatter arches compared to most Homo Sapiens, and the proportions of the prints in Alathar are more similar to Homo Sapiens' feet. There's no trace of anyone else in the area at this point (so far) either, so our species looks more likely. And based on the varying sizes of the tracks, the group likely included at least two or three people.
Using a method called optically stimulated luminescence, paleoecologist Mathew Stewart from the Max Planck Institute for Chemical Ecology and his colleagues dated the sediment layer directly above the layer with the footprints and the layer directly below it. For tens of thousands of years or more, quartz grains trap electrons in their crystal structure (the electrons are trapped because natural radiation makes them jump around until they get stuck). When scientists come by and zap a sample of sediment with light, these electrons are released and emit photons in the process. By measuring the photons, scientists can determine how long it has been since a rock or sediment layer last saw the light of day. This provided a handy set of brackets for the possible age of the footprints: anywhere between 112,000 and 121,000 years old.
At that time Alathar was a shallow lake at a low point between the dunes. The silicic acid fossils of unicellular algae in the footprint layer suggest that the lake was drying up during a visit by the small group of people in the wake of climate change. But there was still enough fresh water to make an attractive stop in the arid landscape for humans and a menagerie of other Pleistocene wildlife.
The ground around the lake deposit is "heavily trampled" with at least 376 tracks from different animals. Much of the trail had been mixed up by erosion and trampling, but the 177 that Stewart and his colleagues were able to identify provided a snapshot of the ecosystems that early humans belonged to as they expanded into Southwest Asia.
In Alathar people would cross paths with entire herds of elephants and camels, along with at least one giant buffalo and one wild donkey. It is likely that all of these animals visited Alathar within days or even hours, as footprints in the mud quickly fade unless they are somehow preserved. (If you're an Ichnology enthusiast, it's worth noting that experiments on a watt show that human footprints lose their fine details in two days and become completely unrecognizable in four days.)
"Human and mammalian movement and the use of the landscape in Arabia were inseparable," wrote Stewart and colleagues.
Although both humans and animals in Alathar went in all directions – along the coast, to and from the lake, and on other paths – the trails eventually all go north to south. Stewart and colleagues suggest that the large herbivores such as elephants, camels, and buffalo may have moved south to follow a seasonal shift in rainfall. Modern elephants in East Africa are doing the same thing today, following sea chains like the chain that Alathar was part of 120,000 years ago.
The small group of people who passed by Lake Alathar may have followed the water, the herds, or both, but there was no way they stayed long. There is no trace of stone tools, animal butchery, fire pits or anything else to suggest that people actually lived and worked here. At other ancient lakes in Nefud, archaeologists have found stone tools and other evidence that people stayed here for a while, but not in Alathar.
"It appears that Alathar Lake has only been briefly visited by people," wrote Stewart and colleagues. "It could have served as a stopping point and place to drink and forage during long-distance travel, possibly triggered by the arrival of dry conditions and dwindling water resources."
Advances in Science, 2020 DOI: 10.1126 / sciadv.aba8940 (About DOIs).