From 30,000 years ago until around 11,000 BCE, the earth was subjected to a cold snap that sucked up the sea into glaciers and ice sheets extending from the poles. This period is known as the Last Glacial Maximum, when the reach of the most recent Ice Age was at its fullest. By drilling mud cores out of the seabed, we can reconstruct a history of the land and the seas, notably by measuring concentrations of oxygen, and looking for pollen, which would have been deposited on dry ground from the flora growing there. We think therefore that sea level was somewhere between 60 and 120 meters lower than today. So it was terra firma all the way from Alaska to Russia, and all the way down south to the Aleutians—a crescent chain of volcanic islands that speckle the north Pacific.

The prevailing theory about how the people of the Americas came to those lands is via that bridge. We refer to it as a land bridge, though given its duration and size, it was simply continuous land, thousands of miles from north to south; it’s only a bridge if we view it in comparison to today’s straits. The area is called Beringia, and the first people across it the Beringians. These were harsh lands, sparse with shrubs and herbs; to the south, there were boreal woodlands, and where the land met the sea, kelp forests and seals.

Though these were still tough terrains, according to archaeological finds Western Beringians were living near the Yana River in Siberia by 30,000 BCE. There’s been plenty of debate over the years as to when exactly people reached the eastern side, and therefore at what point after the seas rose they became isolated as the founding peoples of the Americas. The questions that remain—and there are many—concern whether they came all at once or in dribs and drabs. Sites in the Yukon that straddle the U.S.-Alaskan border with Canada give us clues, such as the Bluefish Caves, 33 miles southwest of the village of Old Crow.

The latest radio-dating analysis of the remnants of lives in the Bluefish Caves indicates that people were there 24,000 years ago. These founding peoples spread over 12,000 years to every corner of the continents and formed the pool from which all Americans would be drawn until 1492. I will focus on North America here, and what we know so far, what we can know through genetics, and why we don’t know more.

Until Columbus, the Americas were populated by pockets of tribal groups distributed up and down both north and south continents. There are dozens of individual cultures that have been identified by age, location, and specific technologies—and via newer ways of knowing the past, including genetics and linguistics. Scholars have hypothesized various patterns of migration from Beringia into the Americas. Over time, it has been suggested that there were multiple waves, or that a certain people with particular technologies spread from north all the way south.