I lost an interesting article I was going to post this morning from the MSN.com Science page.
Someone had the extremely clever idea to search for early North American DNA in Siberia and the Pacific Rim to see if the Bering Strait was a two way land bridge that enticed ancient North Americans to return to Asia.
The scientists found clear DNA evidence that people in northern Japan, northern China, and Siberia had returned between 10,000-20,000 years ago. The North American DNA slowly stopped appearing around 5,000 years ago, when crossing the Bering Sea would have been a life threatening challenge.
I think this evidence is also helpful in figuring out when large scale migration was taking place.
If there is no trace of North American DNA in Asia before 20,000 years ago, that would suggest that very few Asians were heading for Alaska and Canada before 20,000 years ago.
That figures!