The earliest immigrants to the Americas arrived thousands of years ago. There are no written records of their migrations or of the migrations of the later generations that settled throughout the Americas. (Migration differs from immigration in that it can take place within one country, while immigration involves moving across national borders.) From the bone remains and the artifacts (products made by the people of earlier periods) that the earliest Americans left behind, some facts are known about them, but in many important ways, these early Americans remain a mystery. Science shows that human beings did not originate on the American continents—no remains of early forms of humans have ever been found there as they have been found in Africa, Europe, and Asia. Thus, according to most scientific studies, the first Americans were immigrants to the...