The standard story of how humans arrived in the Americas is that they marched 1,500 km across the Bering Land Bridge, a now-vanished landmass between Siberia and Northern Canada that emerged roughly 15,000 years ago in the wake of the last ice age. But for the past decade, evidence has been piling up that humans arrived in the Americas by traveling in boats along the Pacific coast. Some 14,000-year-old campsites like Oregon’s Paisley Caves have been found near rivers that meet the Pacific, suggesting that early humans came inland from the coast along these waterways. Now, a new study published in Nature provides more solid evidence the first humans to reach the Americas could not have come via the Bering Land Bridge.

A group of geoscientists, anthropologists, and biologists led a massive effort to study the environment on the Bering Land Bridge when humans were supposedly crossing it 15,000 years ago. They used a common method for sampling ancient environments called coring. Using hollow tubes, they drilled deep into the sediment at the bottom of two frozen lakes in British Columbia, looking for fossils of plant and animal life from the era when humans could have crossed the Land Bridge. They picked these two specific lakes—Charlie Lake and Spring Lake, to be exact—because they were in a region where the last remaining ice sheets melted. The very first humans to pass into the Americas would have had to cross through this area.

Carefully analyzing the layers of sediment, the researchers were able to determine what kind of life inhabited the region. Radiocarbon dating allowed them to recreate a timeline for the ancient ecosystem there, too. In their paper, the researchers write: