The most recent online issue of Current Biology has an article describing the research which reveals evidence that the first Americans immigrated via two distinct populations at around the same time. One population is comprised of haplogroup D4h3, which took the Pacific coastal route; the second is made up of haplogroup X2a and they migrated through the land corridor between the Laurentide and Cordilleran Ice Sheets.
Both haplogroups migrated Siberia between 15,000 and 17,000 years ago just after the Last Glacial Maximum with D4h3 migrating the Pacific coast all the way to Tierra del Fuego at the tip of South America. X2a stayed in North America.
The data comes from the studies in molecular genetics by Antonio Torroni, Ugo Perego, and Alessandro Achilli at the UniversitÃ di Pavia in Italy. They examined the mitochondrial DNA the two rare haplogroups above. mtDNA is passed through the generations from mother to child and is very useful in studying the phylogeny of organisms including humans.