We analyzed the allelic polymorphisms in seven Y-specific microsatellite loci and a Y-specific alphoid system with 27 variants (“hl-XXVII). A total of 53 Y chromosomes carrying the DYS 199T allele and belonging to Amerindian (51) and Na-Dene (2) linguistic groups were studied. The information gathered allowed us to identify the ancestral founder haplotype (0A) and to recognize 7 derived haplogroups diverging from 0A by 1-7 mutational steps. The 0A haplotype was the most frequent and had the following allele constitution: DYS199T; “hll; DYS19/13; DYS389A/10; DYS389b/27; DYS390/24; DYS39I 10; DYS392/ 14; DYS393/13 (microsatellite alleles are indicated as number of repeats). All native Americans had the DYS 199T allele and the “hll form. Since there are no indications of recurrency for the DYSI99C>T transition, we concluded that all DYS199T haplotypes were derived from a single individual in which the C>T mutation occurred for the first time; we call this individual a “New World Adam”. We analyzed the Y-specific microsatellite mutation rate in 1743 father-son transmissions and we pooled our data with equivalent information in literature to obtain an average rate of 0.0018. We could estimate that the 0A haplotype has an average age of 33,750 years (minimum 20,250 and maximum 88,050 years). DYS 199T allele is found in 85-90% of Amerindian chromosomes indicating that 0A haplotype is the most prevalent or perhaps the only founder paternal lineage of New World aborigines.Instituto Multidisciplinario de Biología Celular