Key discoveries and sites associated with the story of man's settlement of prehistoric North America range from the earliest human fossil found on the continent, Los Angeles man of 21,600 BC, to the great Indian metropolis of Cahokia, Illinois, which sprawled over six square miles, contained more than a hundred ceremonial mounds and may have had as 20,000 inhabitants in AD 1100.
Today, with the possibility that the first American settlers arrived no less than 25000 years ago, and perhaps more than 30000 years ago, the time element is no longer cogent. In 250 centuries, even people of common ancestry can develop marked cultural and physical differences. Thus while it is possible that the Indians are the descendants of successive waves of immigrants, each introducing new ideas, it is just as possible that they spring from a relatively small number of very ancient first settlers who arrived during a fairly brief span of time.
Because the ancient Indians were so completely dependent on the land around them, their surroundings shaped their lives. Environment determined culture more than location, chronology or even tradition. Tribes may be widely separated by time; geography and genetic relationship yet share similar cultures if the ecologies of their homelands were similar. But of course this environmental influence was affected by the gradually developing sophistication of these early Americans. In the beginning they lived primarily by hunting large mammals; following this hunting, or "Palaeo-Indian", period came a time when a greater variety of resources, both plant and animal, were exploited. In this "Foraging" or "Archaic" period, man's increasingly intensive and efficient exploitation of relatively restricted regions made possible the first semi permanent settlements. Finally, in the "Formative" period, the beginning of agriculture turned some settlements into more or less year-round villages and eventually cities.
