I believe that, although humans have walked the earth for the last three million years, everybody who is alive today is descended from one man and one woman who lived somewhere around 50,000 to 100,000 years ago. In other words, I believe that all of us are more closely related to each other than almost everybody else seems to suspect. The differences between us, such as skin color, are really minor and trivial.
This has been my basic thesis for a long time. However, with modern DNA testing, there is more and more scientific evidence that I am right. Here is one recent study on this, entitled "The Human Family Tree: 10 Adams and 18 Eves". One interesting conclusion is that North and South America were likely originally colonized by four groups: Three groups came across the Baring Strait from Siberia to Alaska and the other came by boats from Polynesia to South America. Thus, the question of whether America was first reached via Alaska or Polynesia is answered that both of them are true.
It is possible but much less likely that America was also reached across the North Atlantic via Iceland and Greenland or directly across the Atlantic from West Africa to Brazil. Two native North American tribes have DNA otherwise only found in Europe, which suggests that some pre-history Europeans reached North America and merged in with the local population.
The point is that based on DNA testing, there are four types of original Native Americans. It so happens that Native Americans also fall into four distinct language groups.