12,000 years ago, the ancestors of present day Native Americans crossed a land bridge that connected Asia and the North American continent across the Bering strait from present day Russia to Alaska. This land bridge was formed by low sea levels caused by glaciers freezing up vast amounts of ocean water and exposing a path for the pre Columbian Native Americans to cross into the "New World" thousands of years before Christopher Columbus "discovered" it. When the early immigrants came to North America, they dispersed across the North American continent, and moved into Central and South America.
To lump all Natve Americans in to one cultural pattern is a mistake. For Native Americans Dwelling in the Arctic, woodlands, rain forest, plains, and deserts of the Western Hemisphere developed cultures and languages that were as different from each other as Chinese culture and language is different from German culture and language. These cultures developed without European enfluence, growing into a cornucopia of different religions and lifestyles determined by various ideologies and environments that surrounded them. To each of these cultures, their way of life was the right path, and European beliefs and cultural practices were not necessary to find the path to God and right living.
It is impossible to establish a true number of the Native American population of the Western Hemisphere at the time of Christopher Columbus's arrival in the Carribian in 1492. Population counts of forty ninety million are estimated. Guessing how many tribal groupings existed is difficult because most tribes lacked the formal politcal organizations that leave records that historians can use as evidence to state in any concrete manner the number of tribes in the Western Hemisphere. The one statement that can be made with any real accuracy is that there were dramatically fewer Native Americans after Europeans arrived.