TUCSON, ARIZONA—Geneticists from the University of Arizona have identified an extremely rare Y chromosome that they say is the oldest-known branch of the human Y chromosome lineage tree. The discovery pushes back the most recent common ancestor for the lineage tree to 338,000 years ago, before the appearance of modern humans in the fossil record. This particular Y chromosome came from an African-American man living in South Carolina who had sent a DNA sample to a consumer genetic testing company. His Y chromosome was eventually matched with 11 men from western Cameroon. “And the sequences of those individuals are variable, so it’s not like they all descended from the same grandfather,” said Michael Hammer of the University of Arizona. “It is likely that other divergent lineages will be found, whether in Africa or among African-Americans in the U.S. and that some of these may further increase the age of the Y chromosome tree,” he added.