You may have learned in school that -- for a while -- there was a land bridge between Siberia and Alaska. That's how, the theory goes, people first arrived in North America. Did you ever consider what happened once they crossed? They were likely greeted by walls of ice that went on for miles, says Kirk Johnson, who directs The Smithsonian's National Museum of Natural History.
Johnson, who is the former chief curator at the Denver Museum of Nature and Science, stars in a new "Nova" series on PBS called "Making North America" in which he tells that story and others. -- including evidence that camels roamed modern-day Boulder long ago. He joined Colorado Matters host Ryan Warner for brief history of the continent.
More: An interactive map by Nova that takes you on Johnson's journey of North America though time
