Biogeography is the study of flora and fauna and how they got where they are today. It also applies to humans. We are pretty sure that Asians began to settle North America not quite 20,000 years ago when glaciers covered half of the northern hemisphere and sea level was 100 feet or so lower than today. Many, if not all, came by way of the “land bridge,” now submerged, between Siberia and Alaska. Many mammals and other vertebrates came to the Americas by the same route.