There has always been consensus that mankind first evolved in Africa, migrated to Europe and Asia, and then after many millennia eventually crossed to the Americas - but there has been disagreement on when and how the latter event occurred.
Traditional anthropology and archaeology taught that early man walked to North America across a landbridge between Siberia and Alaska about 13,000 years ago, when the presence of glaciers on the continents used so much of the world's water that sea levels fell and exposed the seabed between the continents. According to this worldview man accessed lower North America via an "ice-free corridor" between glaciers in what is now Canada and eventually migrated down to South America.
Others (myself included) believe that early man was more adventurous and innovative, and was capable of voyaging by sea, which would have allowed passage along the coast at much earlier times, either following the Aleutians and Pacific Amerian coast, or even crossing the Atlantic or Pacific directly. Excavations at the Monte Verde site in Chile support the latter view, and even some North American sites are suggested to be much older than believed by conventional theory.
An article in this week's US News and World Report online discusses at length the emerging use of DNA to analyse this controversy. I won't belabor the details here; those with an interest would be better served going to the link.
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