People occupied North America by roughly 11,000 BCE, but the exact timeline of how early humans first arrived on the continent is contested. Two new studies suggest that humans were living in North America as far back as 30,000 years ago—preceding some earlier estimates by more than 15,000 years.

According to the traditional narrative, the first North Americans were big game hunters who crossed a land bridge connecting Asia to North America around 13,000 years ago. They left behind distinct, fluted arrowheads and bone and ivory tools that were dubbed “Clovis” tools. “This narrative, known as ‘Clovis-first,’ was widely accepted for most of the 20th century until new archaeological evidence showed that humans were present in the continent before Clovis,” Lorena Becerra-Valdivia, an archaeological scientist with the Universities of Oxford and New South Wales and co-author of the new studies, tells Mental Floss. “Within academia, an earlier arrival of 16,000-15,000 years ago was generally accepted.”

Her new analysis pushes back that date by several millennia. The study, “The Timing and Effect of the Earliest Human Arrivals in North America,” published in the journal Nature, looks at radiocarbon and luminescence data from Beringia, a region that historically linked Russia and Alaska, and North America. A statistical model built with this data indicates that a significant human population was living on the continent long before the Clovis era. According to the study, these humans were likely present before, during, and after the Last Glacial Maximum—the period when ice sheets covered much of North America 26,000 to 19,000 years ago.

Stone tool found below the Last Glacial Maximum layer.Ciprian Ardelean

These findings also contradict the land bridge theory. Rather than making a straightforward journey from Asia to North America and populating the southern half of the continent as the Clovis people were thought to have done, the first humans may have entered the Americas by traveling down the Pacific Coast. “These are paradigm-shifting results that shape our understanding of the initial dispersal of modern humans into Americas,” Becerra-Valdivia says. “They suggest exciting and interesting possibilities for what likely was a complex and dynamic process.”

The second, related study in Nature, ”Evidence of Human Occupation in Mexico Around the Last Glacial Maximum,” supports this new narrative. In it, researchers from institutes in Mexico, the UK, and other countries share artifacts and environmental DNA uncovered from Chiquihuite Cave—a high-altitude cave in Zacatecas, central Mexico. The tools, plant remains, and environmental DNA collected there paint of picture of human life dating back 13,000 to 30,000 years ago. The evidence shows that the site was more than just a stopping point, and the people living there had adapted to the high altitudes and harsh mountain landscape.

The two studies not only offer insight on when the first North Americans arrived on the continent, but who they were and how they lived. The Americas would have looked a lot different to humans during the Last Glacial Maximum than they did to the Clovis people millennia later. The fact that the first North Americans left behind far fewer artifacts than the Clovis people shows that their populations stayed relatively small. “Humans at Chiquihuite Cave would have faced the harshness of the Last Glacial Maximum, the peak of the last Ice Age, which would have kept their population at a low density,” Becerra-Valdivia says. “Clovis peoples, in contrast, thrived well after the last Ice Age, expanding widely through the continent during a period of globally warmer temperatures. Their life ways and subsistence patterns, therefore, would have been very different.”