The North American continent was first colonized by Asian nomads that crossed the frozen Bering Strait sometime around 20,000 BC. These tribes quickly spread out, reaching Cape Horn, which is located at the Southern tip of South America, roughly 10,000 years later. This is the major theory although recent archeological finds suggest multiple immigrations and different time scales but meaning of this evidence is widely debated.
Although several large, centralized civilizations developed in the western hemisphere (e.g., the Inca in the Andes, the Aztecs and the Maya in Central America), the major North American mound building civilizations like the Cahokians had very few major population centers. The capital of the Cahokians, Cahokia -located near modern East St. Louis, Illinois may have reached a population of over 20,000. At its peak, between the 12th and 13th centuries Cahokia was the most populous city in North America. Monks Mound, the major ceremonial center of Cahokia remains the largest earthen construction of the prehistoric New World.
By the 15th century AD, corn had been transmitted from Mexico and was being farmed in the Mississippi Valley, but further developments were cut short by the arrival of Europeans. Potatoes were utilized by the Inca and chocolate by the Aztec.
The continent was rediscovered by Europeans later. Initially the Vikings established a short-lived settlement in Newfoundland. Theories exist about earlier and later Old World discoveries of the east coast (or of the west coast by the Chinese), but none of these are considered proven. It was the later voyage of Christopher Columbus that led to extensive European colonization of the Americas. Direct control from Europe began to unravel on July 4, 1776 with the United States Declaration of Independence which was followed in the early 1800's by the independence of Haiti and several South American countries.
Vast immigration from Europe along with smaller immigration from Asia and forced movement of African slaves led to population growth throughout the Americas after the population of Native Americans collapsed from war, slavery and foreign diseases. In many countries of the Americas, Native Americans became marginalized politically and economically and in several countries, such as the Canada, the United States and the Caribbean countries, Native American no longer form a significant portion of the population.