Human Y chromosome is one for the ages.

• Position: Evolutionary Biology

The discovery and analysis of an extremely rare African-American Y chromosome pushes back the time of the most recent common ancestor for the Y chromosome lineage tree to 338,000 years ago. This time predates the age of the oldest known anatomically modern human fossils.

Geneticists from the University of Arizona, Tucson, have discovered the oldest known genetic branch of the human Y chromosome--the hereditary factor determining male sex. The new divergent lineage--which was found in an individual who submitted his DNA to Family Tree DNA, a company specializing in tracing family roots--branched from the Y chromosome tree before the first appearance of anatomically modern humans in the fossil record. The results appear in the American Journal of Human Genetics.

"Our analysis indicates this lineage diverged from previously known Y chromosomes about 338,000 ago, a time when anatomically modern humans had not yet evolved," reports Michael Hammer, associate professor in the Department of Ecology and Evolutionary Biology. "This pushes back the time the last common Y chromosome ancestor lived by almost 70%."

Unlike the other human chromosomes, the majority of the Y chromosome does not exchange genetic material with other chromosomes, which makes it simpler to trace ancestral relationships among contemporary lineages. If two Y chromosomes carry the same mutation, it is because they share a common paternal ancestor at some point in the past. The more mutations that differ between two Y chromosomes, the further back in time the common ancestor lived.

Originally, a DNA sample obtained from an African-American living in South Carolina was submitted to the National Geographic Genographic Project. When none of the genetic markers used to assign lineages to known Y chromosome groupings were found, the DNA sample was sent to Family Tree DNA for sequencing. Fernando Mendez, a postdoctoral researcher in Hammer's lab, led the effort to analyze the DNA sequence, which included more than 240,000 base pairs of the Y chromosome.

"The most striking...