A couple of reponses to items earlier in this thread:

1. Dr Silvia Gonzalez of Liverpool John Moores University, and colleagues, found 38,000 year old footprints 130 kms south of Mexico City (http://news.bbc.co.uk/2/hi/science/nature/4650307.stm). These have been linked to long faced people "similar to southern Asian, Australians and populations of the South Pacific Rim" dated to at least 12,700 years ago and who may have survived till the 18th century as the now extinct Pericues people .(http://www.eurekalert.org/pub_releases/2004-09/bis-ant091004.php)

2. "Genetic evidence for different male and female roles during cultural transitions in the British Isles" (Proceedings of the National Academy of Sciences, Wilson et al, April 2001) finds Irish and Welsh Celtic speakers share Y chromosomes (male DNA) with Basques to such a marked extent that their common signature is called the Atlantic Modal Haplotype (or AMH). David Goldstein, a team member, described Basques and these Celts as "statistically indistinguishable".
"The Longue Duree of Genetic Ancestry: multiple Genetic Marker Systems and Celtic origins on the Atlantic Facade of Europe" (American Journal of Human Genetics, McEvoy et al, 2004) concludes that Celts originate in Atlantic Europe "in the processes at the end of the last Ice Age".
Just in case, English Brits are feeling left out, "A Y Chromsome Census of the British Isles" (Current Biology, Capelli et al, May 2003) concludes that there is "limited continental input in southern England, which appears to be predominantly indigenous". (Indigenous, as established above = AMH = Atlantic Modal Haplotype = Celt)