http://www.pbs.org/wgbh/nova/transcripts/3116_stoneage.html

"There was a fifth source of DNA of mysterious origin. They called it X, and unlike A, B, C and D, they couldn't find it anywhere in Siberia or eastern Asia. But it was similar to an uncommon lineage in European populations today. At first, they thought it must be the result of interracial breeding within the last 500 years, sometime after Columbus."

: But that assumption proved wrong. When they looked at the amount of variation in the X lineage, it pointed to an origin long before Columbus, in fact, to at least 15,000 years ago. It appeared to be evidence of Ice Age Europeans in America."