"Time estimates for the arrival of X in North America are 12,000–36,000 years ago, depending on the number of assumed founders, thus supporting the conclusion that the peoples harboring haplogroup X were among the original founders of Native American populations. To date, haplogroup X has not been unambiguously identified in Asia, raising the possibility that some Native American founders were of Caucasian ancestry. An ancient arrival of haplogroup X in the Americas could be corroborated by the presence of haplogroup X in pre-Columbian human remains. Two studies on mtDNA variation in pre-Columbian samples have reported partial CR sequences that include the 16223T-and-16278T motif (Hauswirth et al. 1994; Ribeiro-Dos-Santos et al. 1996)."