Honestly, no one knows. There are several competing theories, and it is possible that each is correct to some degree.
We suspect that about 30,000 years ago, groups crossed Beringia (the land bridge that existed across the Bering Straits. They followed herds possibly, down through present day Canada into the center of North America, and eventually spread to the rest of the western hemisphere.
The problem with this are the sites in lower North America (the 15,000 year old Buttermilk Creek complex is one) and down the west coast of South America that appear to refute this. Most recently, human remains were located in a sea cave off the coast of Chile. Resting among the tool marked remains of mammoth, the location refutes more than one commonly accepted tenet of mainstream archaeology.
