Graphic by Leigh Coriale Design and Illustration, adapted from From So Simple a Beginning, Macmillan Publishing Company, New York, New York.

Organisms ranging from yeast to humans use an enzymeknown as cytochrome C to produce high-energy molecules as part of their metabolism. The gene that codes for cytochrome C gradually has changed over the course of evolution. The greater the differences in the DNA bases that code for the enzyme, the longer the time since two organisms shared a common ancestor. This DNA evidence for evolution has confirmed evolutionary relationships derived from other observations.