3. Lessons from the History of Life

Abstract

The discussion of life on other worlds is inevitably qualified by the phrase, “life as we know it.” This customary and appropriate caution among scientists serves to (1) admit that all our speculations and extrapolations are based on a known sample size of only one, and (2) imply that the one form of life we know may be peculiar to the physical conditions under which it exists. While these constraints do place boundaries on the scientific latitude we should allow ourselves in speculating about unknown forms of life, the sample with which we are familiar does constitute a specific and robust example that has persisted through numerous crises in variable, changing, and often extreme environments. Assuming that the laws of physics and chemistry are universally operative, then life elsewhere might be expected to follow the same evolutionary and ecological principles that have characterized its history on Earth. Thus, rather than thinking of “life as we know it” in terms of constraints, this chapter explores the insights to be gained by regarding the one life we know as a harbinger and example of the life we can reasonably expect to exist on other worlds.