The Coming of the First Settlers
As there are no written records maintained from the era prior to the coming of Europeans to North America, there is no exact information on how the people now referred to as "Native Americans" came to the continent. There have been theories proposed to explain the coming of the people, with one being generally accepted as the most plausible; the Landbridge Theory.
During the most recent of the Ice Ages large portions of the northern hemisphere were coated in ice. This ice covered much of what is now Canada. As all of this ice was formed the water levels around the world lowered, causing land masses previously inaccessible to become available. One such landmass was located where the Bering Strait is today: Beringia which formed a link between what is now Siberia and Alaska. This newly formed landmass is estimated to have been 1000 kilometres wide and 90 kilometres long (the modern distance between the two continents).
With the increasing cold, plant life in the northern lands was dwindling and causing an exodus of wildlife. Animals began roaming farther in search for sustenance. As the people of the time were dependant on both, they were forced to follow or die. Those that moved across Beringia were a nomadic people which followed their prey into the previously untapped lands which now include Alberta and parts of the Yukon and British Columbia. This migration of people happened some time between 100,000 and 30,000 years ago.
About 17,000 years ago the temperatures in North America began to increase once more, and the ice, which is thought to have been upwards of 3 kilometres thick in locations over the Rocky Mountains and Hudson's Bay, began to melt off. With the receding of the ice at the end of this time, the scars in the land carved by the coming of the glaciers and their movement were filled with the water produced by the melting. This is used to explain the creation of what is now the Great Lakes of Ontario and the myriad of other small bodies of water through Canada.
Beringia, the Bering Strait landbridge, ceased to exist approximately 10,000 years ago with the rising of water levels.
A secondary theory that is accepted to a lesser degree is the Watercraft Theory, which states that Asian people could have created rafts in order to travel on the ice-free Pacific Ocean into what is now California.
The reason that these two theories are accepted over theories of Africans moving to South America and then into North America is the links between Asian and North American species and the physical resemblances between Asians and Native Americans. All three of these theories of American infusion have since been correlated via genetic sampling, as well as archaeological evidence. Genetically, Native North Americans are almost entirely Asian in origin, while Native South Americans are largely African in origin. The archaeological evidence is illustrated by technological jumps reached by the North American tribes and nations via the Pacific at various times.