North America, before the arrival of humans, had some crazy critters. Among the cast of giants was the wooly mammoth and its cousin the mastodon, as well as beavers that could stand two meters tall. But if you could go back just a little bit further, you’d find that one North American icon is missing—the bison.

Like humans, bison are relatively new on the North American scene. They made their way from Asia during a time when a low sea level left a bridge of land between Siberia and Alaska. Exactly when they took that trip has been an open question, though. It definitely occurred during the ice ages of the last 3 million years, and some studies constrain their crossing to the last 640,000 years.