Small oval depressions—376 in all—are scattered across a patch of dry ground nestled between the dunes of northern Saudi Arabia. At a glance, these pockmarks don't seem particularly impressive, so much so that a team of scientists surveying the region almost passed them by in 2017. But upon further examination, the team realized that the depressions were left by an array of ancient animals, and among them were traces of our own species, Homo sapiens.
The discovery of human footprints, if confirmed, would mark the oldest trace of our species yet found in the Arabian Peninsula, which sits at the gateway to early humans’ spread around the world. As described in a new study published in Science Advances, the frozen footfalls preserve a striking snapshot of time some 115,000 years ago, as animals and humans congregated near a shallow lake, perhaps with a shared purpose of slaking thirst and sating hunger.
While the prints aren’t easy to see, the largest of the lot caught one team member’s eye. Stomped into an ancient lakeshore, the track appeared to be from an elephant larger than any that tromp across the landscape today.
“As soon as we saw one, we could see them all,” says Mathew Stewart, a zooarchaeologist at the Max Planck Institute for Chemical Ecology and first author of the study.
A closer look revealed the elongated prongs of camels, and perhaps faint traces from giant buffalo and ancient horse relatives. As the team was packing up for the day, they spotted what became the most exciting find: Seven imprints likely left by members of our own species.
While the prints aren’t the oldest evidence of humans outside of Africa, the vignette inscribed in mud provides a peek into the once lush landscape and creatures that may have greeted humans during their early forays.
“This is like a moment in time, if you will,” says Michael Petraglia, a human evolution researcher at the Max Planck Institute who led the team. “The imagination really runs wild: What did these people look like? What were they doing? ... Once all these lakes dried up, what happened to them? That really gets our curiosity going.”
The continental crux
The research in Saudi Arabia is part of a more than decade-long effort led by Petraglia to unearth hominin history in the Arabian Peninsula and better understand our species’ first steps out of Africa.
Most non-African people today can trace their genetic roots to a wave of H. sapiens that ventured from the continent roughly 60,000 years ago. But they weren’t the first to leave. Early H. sapiens were likely trickling out of Africa tens of thousands of years before. An upper jaw found in Israel suggests humans arrived in the region by 180,000 years ago. And a controversial but stunning find of a human skull in Greece dating to some 210,000 years old hints of even earlier waves.
As the usual story goes, these ancient explorers likely crossed out of northeastern Africa over the present-day Sinai Peninsula, spreading into the Levant—the region just north of Arabia that includes Israel, Syria, Lebanon, Jordan, and the Palestinian territories—before migrating into Europe and Asia. But some suggest early humans instead crossed near the Horn of Africa to the southern Arabian Peninsula, spreading around the Indian Ocean rim.
At this crucial continental juncture sits Arabia—a vast stretch of land that long went unstudied. “If we were thinking about stepping stones out of Africa, we needed to know more about Arabia,” Petraglia says.
Petraglia and his team have started to fill in that gap, unearthing clues to a time when the now arid peninsula was vastly different. Lush grasslands blanketed a landscape crisscrossed by rivers and dotted by some 10,000 lakes, making it an alluring place for hominin explorers. Stone tools have been found scattered around many ancient lakeshores, yet their makers remain unknown.
“That has been keeping us going for years,” Petraglia says.
The allure of lush landscape
The tracks were identified during a survey of ancient lake Alathar, and scientists analyzed the system in a battery of tests. Study of ancient algae suggests it was once filled with fresh water—a vital resource for humans and animals alike. Yet the waters were likely in the process of drying up, hinting that the track makers visited when the season was hot, perhaps chasing the dwindling resource around the region.
Four of the seven hominin tracks cluster in a southerly route near the lake’s edge, all likely left by two or three individuals. Both animals and humans seemed to move without purposeful direction, congregating around the lake’s shores, Petraglia notes. All the footprints were likely left within hours or days, based on modern experiments.
Fossil tracks provide a window into the past in a way other ancient remnants cannot. “To a geologist or a paleoanthropologist, footprints are traces of past behavior,” says Cynthia Liutkus-Pierce, an Appalachian State University geologist who recently led work on a trove of tracks in Tanzania as a National Geographic grantee. “And that’s something you often don’t get with bones and stones.”
Comparison of the hominin footprints’ shape and size to ancient tracks and modern footprints helped untangle the explorers’ identities. Stewart, who conducted the footprint analysis, also used the measurements to estimate the track makers’ height and build. Overall, it seems the prints were left by tall, lanky hominins like H. sapiens rather than short, stocky Neanderthals, though the tracks are not preserved well enough to say with certainty, according to Stewart.
However, identification of the tracks as H. sapiens fits with the current picture of where different kinds of humans were living some 115,000 years ago, when Neanderthals, Denisovans, and even perhaps Homo Erectus all inhabited various corners of the world. In this swath of the Middle East, only H. sapiens remains have been found from around that time. The footprints lie near another lake where the team recently uncovered a finger bone dating to some 90,000 years old that may also be H. sapiens.
While ascribing the tracks to H. sapiens is a reasonable conclusion, the fossil record is rife with gaps, and fresh evidence could still flip the muddy script of the ancient hominins in this region, says Rutgers’ Craig Feibel, who specializes in ancient environmental reconstructions. "It’s not impossible to have those sort of wild cards suddenly appear and change our thinking,” says Feibel, who was not part of the study team.
If the identification of H. sapiens holds, the tracks and fossil finger point to groups of early humans who didn’t just bolt through the Levant, but ventured into the Arabian interior. “It’s probably very much habitat dependent,” Feibel says. Rather than “packing up and marching north,” these early humans were likely seeking ecosystems that could support expanding populations.
The Arabian story remains incomplete, but the latest discovery is an exciting clue to how much more may be hiding in plain sight, says Kevin Hatala, an evolutionary biologist at Chatham University who specializes in ancient hominin footprints and was not involved in the new study.
“I think this kind of opens everyone's eyes to a new form of data that might be present,” he says. Such ancient muddy scenes are easy to miss if you aren’t looking for them. “It wouldn’t surprise me ... if they did find more.”