Sept. 18 (UPI) -- Paleontologists have discovered a diverse assemblage of 120,000-year-old human and animal footprints in an ancient lake deposit in Saudi Arabia's Nefud Desert, offering new insights into the trajectories of human migrations out of Africa, according to a study published Friday in the journal Science Advances.
A mounting body of evidence, compiled and published over the last two decades, has upended early theories that humans migrated out of Africa in one or two giant waves.
"As more and more fossils are discovered, it seems that humans repeatedly dispersed out of Africa and did so much earlier than previously thought," study co-author Mathew John Stewart told UPI in an email.
"Precisely when, how often and under what conditions remain open questions," said Stewart, a researcher at the Max Planck Institute for Chemical Ecology in Germany.
For answers to these questions, researchers have mostly looked to Africa and Eurasia, ignoring the Arabian Peninsula. Though it neighbors both Africa and Asia, evidence of human occupation in the region is scant.
"The area today is a hyper-arid desert, characterized by very little rainfall and large, expansive sand dunes," Stewart said. "The conditions are not very amenable to the preservation of material and sediments. Significant erosion of sediments and the subsequent destruction of material, such as fossil remains, is unfortunately common."
Paleoclimate data suggests that Arabia wasn't always as dry as it was today, and a scattering of fossil discoveries has confirmed that humans were able to make forays into the Arabian interior when shifts in climate turned the peninsula's deserts into grassland.
The ancient footprints found in the Nefud Desert, fossilized in an ancient lake deposit known as 'Alathar' -- Arabic for "the trace" -- suggests humans made one of those forays roughly 120,000 years ago.
"The age of the footprints are consistent with Homo sapiens fossils in the Levant, and suggests that there were multiple routes that humans took upon expanding beyond Africa," study co-author Richard Clark-Wilson told UPI in an email.
"There is earlier evidence for our species moving into the Mediterranean environment of the Levant and southern Greece, but this is the earliest evidence of our species moving into a semi-arid grassland as Arabia would have been," said Clark-Wilson, a postgraduate research student at Royal Holloway in Britain.
In addition to human footprints, researchers uncovered footprints left by elephants, horses and hippos, suggesting Homo sapiens weren't the only species drawn to the open grasslands and water resources of northern Arabia. Research suggests it's possible humans were following animals when they first moved into the region.
"Whats exciting about the animal footprints is that it closely ties human and animal movements around lakes in northern Arabia," Stewart said. "Unlike most other records, footprints provide very high-resolution information, on the order of hours or days. Also, the animal footprints provide information on what the environment and ecology was like when these people were moving through the landscape."
While the discovery of ancient footprints in Arabia suggests human movements out of Africa extended eastward into northern Arabia, Stewart said plenty of questions remain unanswered.
"Precisely what happened to these people during the more arid periods? How long did they occupy the Arabian interior? Where did they go?"