[W]e can paraphrase the Italian geneticist Guido Barbujani: imagine that at some time in the future Indian astronauts colonise Mars, and geneticists then type their Y chromosomes. We may well find that their lineages date back to 9,000–20,000 years ago. But we would not be wise to infer that they have been living on Mars for 9,000 years.
From DENISE R CARVALHO-SILVA, TATIANA ZERJAL, CHRIS TYLER-SMITH, Ancient Indian Roots? J. Biosci. 31(1), March 2006.