When researchers first arrived at a cave high in the desert mountains of north-central Mexico, they hoped to learn what the environment was like there thousands of years ago. But the unexpected discovery of what they believe is an ancient projectile point led to a decade-long excavation that could rewrite the history of the Americas.

According to a paper published today in the journal Nature, the site, known as Chiquihuite Cave, may contain evidence of human occupation that places people in North America around 30,000 years ago—roughly twice as early as most current estimates for when the first humans arrived on the continent.

The question of when people first arrived in the Americas has been debated for more than a century.