When Did Humans Reach the Americas?

Until now, the best available evidence had indicated that humans probably reached the Americas about 20,000 years ago.  But now a new paper published in Nature raises the possibility that humans reached the Americas as early as 130,000 years ago.  The new findings could force a re-thinking of when, how, and by whom the Americas were colonized.

The new paper focuses on an archaeological site in California, where smashed and broken bones of a mastodon were found intermingled with what are reported to be crude stone tools.  Uranium/thorium dating shows that the bones are a whopping 130,000 years old.  The authors propose that the bones were deliberately smashed, leading to the conclusion that humans must have been in California at least as early as 130,000 years ago.

The findings are intriguing, but we'll probably have to wait for corroborating evidence before everyone accepts the new date.  For starters, it would help if we could find human bones that old; so far, no human bones older than about 20,000 years have ever been found in the Americas.  And then there's the question of how they could have gotten to the Americas in the first place.  The humans who colonized the Americas about 20,000 years ago crossed over from Asia via a land bridge that existed at the time between current-day Russia and Alaska.  That land bridge probably didn't exist 130,000 years ago.  They could have arrived by boat, but that would presume a higher degree of sophistication than implied by primitive stone tools.

Finally, there's the question of who these people were, if indeed they ever existed.  DNA evidence indicates that Native Americans can trace their ancestry back to a common ancestor who lived about 20,000 years ago.  If a band of humans arrived more than 110,000 years before that, they must have died out without leaving a continuous line of descendants.