Archaeologist Tom Dillehay didn’t want to return to Monte Verde. Decades ago, his discoveries at the famous site in southern Chile showed that humans occupied South America by 14,500 years ago, thousands of years earlier than thought, stirring a long and exhausting controversy. Now, Dillehay, of Vanderbilt University in Nashville, has been lured back—and he is preparing for renewed debate. He reports in PLOS ONE today that people at Monte Verde built fires, cooked plants and meat, and used tools 18,500 years ago, which would push back the peopling of the Americas by another 4000 years.

If his team is correct, the discovery will “shake up both the archaeology and genomics of the peopling of the Americas,” says archaeologist Jon Erlandson of the University of Oregon in Eugene. Genetic studies suggest that the ancestors of Paleoindians first left Siberia no earlier than 23,000 years ago (Science, 21 August, p. 841), so Dillehay’s new dates suggest they wasted little time in reaching the southern tip of the Americas. And the find raises questions about the North American record, where no one has found widely accepted evidence of occupation before 14,300 years ago. “Where the hell were the people in North America at that hour?” wonders archaeologist David Meltzer of Southern Methodist University in Dallas, Texas.

When Dillehay began his work at Monte Verde in the 1970s, most researchers thought the Clovis people, who hunted big game in North America starting about 13,000 years ago (using calibrated radiocarbon dates), were the first Americans. When Dillehay reported traces of huts, hearths, human footprints, and artifacts that were thousands of years older, he was forced to defend every detail of his dig to skeptical colleagues. By now, though, most archaeologists accept the older occupation at Monte Verde and a few other sites.

When the Chilean government invited Dillehay to survey the full extent of Monte Verde, he at first refused. “I was tired of it,” he says. But in 2013, fearing another team’s survey might damage the site, he returned, hoping to spend a few weeks collecting new evidence of ancient plants and climate by digging 50 small test trenches across a 20,000-square-meter area. But the dig turned up 39 stone artifacts, including flakes, a “chopper,” and cores, embedded near plants or animal bones that had been burned in small fires at 12 areas. This suggests a “spotty, ephemeral presence,” he says.

His team radiocarbon dated the plants and animal bone to between 14,500 and 18,500 years ago, and perhaps as early as 19,000 years ago. The last ice age was only just starting to wane at that time, leaving a cool temperate rain forest at Monte Verde, about 60 kilometers from the Pacific Ocean. Dillehay speculates that early Paleoindians moved along deglaciated corridors between the coast and the Andes, hunting paleo llamas and elephantlike gomphotheres.

Not everyone is convinced. Archaeologist Michael Waters of Texas A&M University in College Station questions whether the stone artifacts were actually humanmade, and says that the team hasn’t eliminated the possibility that the fires were natural.

Dillehay concedes that his team found few unequivocal stone tools, which are the strongest evidence of a human presence. But he notes that about one-third of the tools were made from exotic materials such as limestone and white quartz from outside the area, suggesting that people transported the stone. Meltzer finds this compelling. “The specimens don’t scream out ‘made by human hands,’” he agrees, “but Dillehay’s group has made a careful assessment of their form and raw material … It’s evidence we cannot ignore.”

Much is at stake, which suggests that the onus is on Dillehay once again to prove his case. “I guess that part of my destiny is that this damn site simply will not let go of us,” he says.