ADNA sample from an African-American living in South Carolina revealed that the human Y chromosome may be much older than previously thought— pushing back the most recent common ancestor for the Y chromosome lineage tree by almost 70 percent to around 338,000 years ago. The Y chromosome is the hereditary factor determining the male sex; because it does not exchange genetic material with other chromosomes, it is easier to trace the ancestral relationships between lineages. After analysis, it was shown that this may be the oldest known branch of the human Y chromosome—a new divergent lineage that apparently branched from the Y chromosome tree even before the first appearance of anatomically modern humans in the fossil records.

The sex of a new organism is determined at the instant the egg is fertilized by the sperm. For example, in humans, the female’s egg cell contains a single X chromosome; the sperm cell may contain either an X or Y chromosome (all, of course, with the other twenty-two autosomes in the egg and sperm cells).