The Northern Land Bridge
Much archaeological evidence supports the widely held view that the first people to migrate to the American continent entered through a land bridge, which at one time joined Eastern Asia with North America. This land bridge, known as Beringia, was a wide expanse of terrain that emerged from the ocean as the sea levels dropped due to widespread glaciation at the time of the last Ice Age. Groups of humans could have slowly migrated across the grassy tundra of Beringia, feeding on herds of animals or coastal marine life. At the time, much of North America including what is now Canada, would have been covered with thick ice sheets making further movement southward a physical impossibility.
At a later period, the ice receded and humans would have been able to traverse an ice-free corridor in a southerly direction, and gradually settle the vast uninhabited continent. The ending of the Ice Age also caused the ocean levels to rise, thus submerging Beringia and isolating the new migrants from peoples on the Asian side. During the next centuries the descendants of the pioneers who crossed Beringia gradually peopled the entire landmass of the Americas. This view of the origins of the first Americans is often known as the Clovis-first theory (named after the site in New Mexico where stone artifacts made by these settlers were first found). Many subsequent finds of Clovis tools demonstrate this culture's extensive penetration throughout the continent.