Archaeologists have hard evidence that humans lived in North America much earlier than previously thought, and a Chicago researcher played a key role in nailing down the dates.

The earliest North Americans were long thought to be the Clovis people, who lived about 12,000-13,000 years ago. Now archaeologists have dug up stone tools and debris from underneath a Clovis site in central Texas.

Steven Forman brought samples back to his lab at the University of Illinois at Chicago. He used a technique called optical dating to determine when the sediment around the objects was last exposed to sunlight. The artifacts turn out to be about 15,000  years old, from millennia before the Clovis people. And they appear to provide a missing link in understanding how some Clovis technology developed.

It’s not the first evidence of cultures older than Clovis, but Forman said it may be the strongest.

“It appears to be that this might be kind of watershed piece of science in which people say, yes, there is really compelling evidence for pre-Clovis occupation in North America,” said Forman, a professor of earth and environmental sciences at UIC. “It’s no longer a red herring.”

The new find will likely overturn the history of ancient humans in North America. The results are out today in the journal, Science.