“Coming into America” explores how prehistoric immigrants (host Alan Alda calls them “the true American pioneers”) might have arrived here---up to 20,000 years ago. Conventional wisdom has been that the “Clovis people” made their way down from Siberia about 13,000 years ago and spread out. But archaeologists interviewed point to artifacts in various places around the U.S. that are far older---and there's evidence that the ice sheets remained in place longer than previously thought---casting doubt on that theory.