After 5 years of scouring the Andes mountains, two graduate students have found the oldest solid evidence that humans were living at extreme elevations by 12,800 years ago. These early settlers of the Americas, known as Paleoindians, camped in a rock shelter and manufactured stone tools in an open-air workshop almost 4500 meters above sea level, indicating that humans lived at least for part of the year at high elevations 1000 years earlier than previously thought.

The high-altitude basins and ranges of the Andes of South America, the Himalayas of Asia, and the Ethiopian plateau of Africa were among the last frontiers for human settlement. At elevations more than 2500 meters, humans risk dying of hypoxia (lack of oxygen), exposure to frigid temperatures, high solar radiation, and other hazards. Above 4000 meters in the Andes, Paleoindians—the first humans to cross Beringia to the Americas from Asia—would have lived well above the tree line, making it difficult to find wood for campfires or plants to eat. As a result, researchers have thought those challenges prevented people from living even seasonally at high altitude until 10,000 to 11,500 years ago in South America.

An earlier archaeological site has been dated to about 15,000 years ago in Tibet, but the elevation is lower at 3300 to 3500 meters, and it’s not clear if humans lived there or camped there briefly, says archaeologist Mark Aldenderfer of the University of California, Merced, who was not involved in the work. He says the “question of real significance is not when people entered the highlands or how high they got but when they started living there permanently,” because that indicates when humans had to adapt physiologically to life in thin air.

Strong evidence that humans were hunting in the Andean highlands at least seasonally for 1000 years came first with the 2009 discovery of “fishtail” points, distinctive arrowheads with a fluted stem in the shape of a fish that may have been set atop wooden spears for hunting, that researchers first reported at a meeting this year. When archaeologist Kurt Rademaker and geologist Gordon Bromley, both then graduate students at the University of Maine, Orono, found these points at 4355 meters, they knew they had solid evidence that early Paleoindians were capable of living at high altitude soon after they arrived in South America. That’s because fluted fishtail points are always older than 11,500 years in South America, and thus are the handiwork of some of the earliest Paleoindians. “I knew we had found a high-altitude occupation that was earlier that expected,” says Rademaker, now a postdoctoral researcher at the University of Tübingen in Germany.

Over the course of several years, Rademaker and his colleagues found hundreds of stone tools and projectile points at three archaeological sites in the Pucuncho Basin in the southern Peruvian Andes, they report online today in Science. In the Cuncaicha rock shelter, the team also discovered rock art and ceilings covered in soot from campfires, suggesting it was a base camp. At all three sites, they found starchy plant remains, which had been brought from lower elevation, as well as the bones of taruka deer and vicuña, and guanaco camelids, which are close relatives of alpaca and llamas. About 7 kilometers west, they found 260 stone tools, including projectile points and scrapers made of local obsidian and andesite, which suggest that the Pucuncho Basin was a “high-altitude oasis ideal” for specialized hunting, the authors write.

Rademaker sent samples of the same bones to three different radiocarbon labs to make sure that the dates were solid. The dates from all three labs matched putting the human presence at the rock shelter at 12,000 to 12,400 years ago and the open-air tool workshop at 11,500 to 12,800 years ago. “The dates seem very solid,” says archaeologist Michael Waters of Texas A&M University, College Station, who was not involved with the work.

This was just 2000 years after the oldest widely accepted archaeological site for humans in South America—at Monte Verde in Chile—and at a time when the climate was warming. An analysis of the obsidian tools found atop an outcrop of the obsidian also shows that the Pucuncho Basin was the source for the stone used to make tools found at the Quebrada Jaguay, a marine campsite used by early Paleoindians on the coast of Peru as early as 13,000 years ago. This suggests that the people who made the tools moved between the coast and the mountains where they hunted vicuña and guanaco for meat and pelts, perhaps after the rainy season when calves were born in the spring. “These people were more capable than we thought they were,” Rademaker says.

It also suggests that Paleoindians risked life at high altitude even before genetic mutations that helped them survive better in low oxygen swept through the highland people, Rademaker says. Biological anthropologist Cynthia Beall of Case Western Reserve University in Cleveland, Ohio, who studies the evolution of high-altitude adaptations in highland people in Tibet, agrees. “It is fascinating to think of people using the highlands so soon after the occupation of Monte Verde,” says Beall, who says that the work presents “a natural experiment” to test hypotheses about how long it takes a population to adapt genetically to high altitude. It also shows that as the climate warmed, humans spread into new frontiers. “It is a neat story of how people were adapting at the end of the last ice age,” Waters says.