Examine evolution over the course of years or centuries, and you’ll find that it progresses much more quickly than it does over geologic time. Now the oldest viruses on the planet are enabling scientists to calibrate this evolutionary clock.
In the 1950s, the Finnish biologist Björn Kurtén noticed something unusual in the fossilized horses he was studying. When he compared the shapes of the bones of species separated by only a few generations, he could detect lots of small but significant changes. Horse species separated by millions of years, however, showed far fewer differences in their morphology. Subsequent studies over the next half century found similar effects — organisms appeared to evolve more quickly when biologists tracked them over shorter timescales.