To the untrained eye, the palm-sized quartz stones appear unremarkable. But archeologists believe that 96 roughly sharpened rocks dug from a cliffside deposit in China represent the oldest known evidence of ancient human tools outside of Africa.

How and when our human ancestors first migrated from Africa is one of the greatest remaining scientific and historical mysteries. The first bipedal (standing on two legs) primates emerged in Africa around 7 million years ago, branching off from apes to form hominins, an evolutionary tribe that includes all previous species of humans—neanderthals, Homo erectus, etc.—plus our own, Homo sapien.

Until now, the earliest evidence of hominins outside of the African continent were tool and bone fragments from Dmanisi, Georgia dating back an estimated 1.85 million years. But if the China find proves true, early humans were hunting and gathering in northeastern China—8,000 miles from Africa—as far back as 2.1 million years ago.

“Our discovery means it is necessary now to reconsider the timing of when early humans left Africa,” said Robin Dennell of the University of Exeter, a paleoanthropologist with the China excavation, in a press release.

The cache of tools unearthed between 2004 and 2017 by lead archeologist Zhaoyu Zhu of the Chinese Academy of Sciences and his team include hammer stones, scrapers, rounded cobbles and numerous pointed pieces. Although the exact use of the tools is unknown, animal bone fragments from ancient deer, pigs and antelopes were found nearby.

No human remains, however, were identified, making dating a bit more difficult—but not impossible.

The oldest artifacts discovered at the Loess Plateau in China. (Credit: Zhaoyu Zhu/Nature International Journal of Science)

To pinpoint the rough time frame that the stone tools were hewn, archeologists used a technique called paleomagnetic dating. Conveniently, the Earth’s magnetic field randomly reverses every couple of million years—the North Pole becomes the South Pole and vice versa—leaving its lasting imprint on magnetic minerals locked in the geological record.

In this case, the early stone tools found in China were scattered throughout 17 layers of soil deposits bookended by two known geomagnetic reversals, the Reunion Subchron dating back 2.14 million years and the later Olduvai Subchron that occured around 1.85 million years ago. By analyzing how the tools were clustered in the soil layers, the archeologists dated the oldest tools at 2.1 million years.

While the China find is exciting—pushing back the exit of early humans from Africa at least 270,000 years—plenty of questions remain about who these humans were and what evolutionary advances enabled them to wander thousands of miles across continents.

Anthropologists previously believed that Homo erectus, the first fully upright, large-brained species of human, was the first to leave Africa. But the fossil record in Africa and elsewhere in Asia dates the earliest appearance of Homo erectus to around 1.89 million years ago.

If the Chinese tools are indeed more than 2 million years old, they were likely chiseled by smaller, more ape-like hominins that predated erectus. Smaller-brained species of hominins began sharpening and using stone tools as far back as 3.3 million years ago.

Dennell of Exeter told The New York Timesthat the tools themselves may have been the key that unlocked the first long-distance migrations.

“Suddenly you had a primate that could obtain meat from a carcass, and it opened up a new world for them,” said Dennell. “That simple technology was enough to get them out of Africa and right across Asia.”