The atmosphere of the earth differs greatly from that of the other terrestrial planets with respect to composition, acidity, redox potential and temperature history predicted from solar luminosity. From the fossil record it can be deduced that stable optimal conditions for the biosphere have prevailed for thousands of millions of years. We believe that these properties of the terrestrial atmosphere are best interpreted as evidence of homeostasis on a planetary scale maintained by life on the surface. Some possible mechanisms of this biological homeostasis have been noted and the implications of this concept for experimental studies pointed out.