Up to the 1960s, geology was the only source of information about the times when common ancestors lived. Since then, molecular genetics has become a major contributor to knowledge of these times. By comparing genetic macromolecules from any two living cells or species, one can now tell roughly when they had a common ancestor. Our contribution aims to trace this development and assess its implications. It is a suitable occasion to offer this perspective because 1987 was the 25th anniversary of the first inkling that genes and their protein products behave as evolutionary clocks (Zuckerkandl and Pauling, 1962). It should be noted that the term “molecular clock” was intended to imply that base substitutions accumulate on surviving molecular lineages at a fairly steady rate, not as steady as the ticking of a metronome but nearly as steady as the process of radioactive decay.