It is the first time all known American Indian mitochondrial DNA (mtDNA) sequences and lineages have been compiled, corrected and organized into a single tree with branches dated, the Utah-based Sorenson Molecular Genealogy Foundation and Italy's University of Pavia said in a release.

The study released identifies the six surviving Native American mtDNA lineages that are dated to approximately 20,000 years ago, designated as A2, B2, C1b, C1c, C1d and D1. The study also confirms the presence of five geographically limited genetic groups: X2a, D2, D3, C4c and D4h3.

The findings are published online by the Public Library of Science.

Lead author Dr. Alessandro Achilli of the University of Pavia, said the five more rare genetic groups will help researchers isolate branches within the pan-American groups. "For example, we learned one branch is only found among Aleuts and Eskimos," he said. "The presence of these additional subgroups suggests different migratory events from Asia or the Bering Straits.