Six million years is a mere blip on the screen of evolution, but a blip long enough to separate man from chimpanzee. A study headed by David Page of the Whitehead Institute has found that the key that separates men from chimps may be in the Y chromosome, at least partly. Dr. Page’s team found that man’s Y chromosome is evolving much faster than expected, refuting the belief that man and chimp Y chromosomes were very similar.

Painstaking sequencing of the human Y chromosome was completed in 2003 by the Genome Center at Washington University after 13 years of work, and Page’s team took eight years to sequence that of a 24-year-old chimp. They were then able to compare the structure and gene content of each, with unexpected results.

For half a century, it was thought that the Y was in a state of decline, or at least stagnation. Why? It is seen as a single genetic unit, rather than a series of blocks that can be interchanged. A mutated gene can be swapped for a good copy on its chromosomal pair, except between the X and Y. If this happened, it would create gender ambiguity. Because of this inability to swap, most of the genes on the Y chromosome have decayed. Two hundred million years ago, the Y had the same genetic material as the X, and most have now disappeared. Most scientists saw this as evidence that the Y chromosome was either headed for eventual extinction or was stagnant.

Page’s team found, instead, that the Y is constantly evolving—and faster than any other part of the genome. Most of man’s genome is 99 percent identical to chimpanzees, differing in some cases only by a single DNA unit. The Y chromosomes of the two species, however, differ by 30 percent, indicating rapid change in the time when man and chimp split from a common ancestor. Instead of decay, they found renewal. Dr. Page says, “Natural selection is shaping the Y and keeping it vital.”

How does the Y’s rapid change affect the human genome as a whole? Dr. Page says it is “hard to imagine that these dramatic changes in the Y don’t have broader consequences.” What those consequences are is as yet unseen, but one thing it does not mean is that men are evolving faster than women.