http://www.nytimes.com/2003/07/25/science/25HUMA.html

"Scientists studying the genetic signatures of Siberians and American
Indians have found evidence that the first human migrations to the New World
from Siberia probably occurred no earlier than 18,000 years ago."

"The new estimate undermines arguments for colonization as far back as
30,000 years ago, but reinforces archaeological findings and a linguistic
theory that most American languages belong to a single family called
Amerind."

"The new date derived by Dr. Seielstad and Dr. Spencer may strengthen the
hand of linguists who argue that all American languages fall into three
groups, known as Amerind, Na-Dene and Eskimo-Aleut, with Amerind being by
far the largest."