Is a General Theory of Life Possible? Seeking the Nature of Life in the Context of a Single Example

Is one of the roles of theory in biology answering the question “What is life?” This is true of theory in many other fields of science. So why should not it be the case for biology? Yet efforts to identify unifying concepts and principles of life have been disappointing, leading some (pluralists) to conclude that life is not a natural kind. In this essay I argue that such judgments are premature. Life as we know it on Earth today represents a single example and moreover there is positive evidence that it may be unrepresentative of life considered generally. Furthermore, as I discuss, the prototype for theorizing about life has traditionally been based on multicellular plants and animals. Yet biologists have discovered that the latter represent a rare, exotic, and fairly recent form of Earth life. By far the oldest, toughest, most extensive, and diverse form of life on our planet is unicellular, prokaryotic microbes, and there are reasons to suppose that this is almost certainly true elsewhere in the universe as well. If there are explanatorily and predictively powerful, biologically distinctive principles for life that can be gleaned from our insular example of life it is more likely that they will be found among the microbes. I discuss some provocative ways in which unicellular microbes differ from multicellular eukaryotes and argue that some of them just might provide us with key insights into the nature of life.