Although I've linked to National Geographic on the sidebar, I think it is worth noting a feature of their site. This interactive feature, here, takes one on an exploration from the dawn of man in Africa to the distribution of humanity throughout the planet. Certainly there are more mysteries to be solved as more evidence presents itself (if and when it does). But with the help of genetic data, as well as a combination of climatology, geology, archeology, and more, this page highlights the generally accepted model of human migration.
My recent trip to Chichen Itza has prompted me to explore more ancient civilizations and how they had come to be. While the Maya date back nearly 4,000 years, undoubtedly there were modern humans before them. To what time span is unclear, but the vastness of distance and time is quite a revelation when considering the dangers, uncertainty, and logistics to populate the continental world.
I am dumbfounded at the incredible plight of ancient humans. While today we are accustomed to driving our climate controlled SUV to work, stopping at a Burger King for lunch, and awaiting our favorite programs digitally recorded on our HD TV's at home it is a wonder, literally a stupifying wonder, to humbly grasp the daily peril that faced our ancestors in getting here.