Introduction

The living world confronts us with wonder at every turn. Life exists in a staggering range of forms and thrives in the most extraordinary settings — icy mountain slopes, dense deciduous forests, the open ocean, freshwater lakes, arid deserts and even scalding hot springs. A horse breaking into a gallop, birds crossing continents on migration, a valley bursting into flower, or a shark closing in on its prey — each of these stirs a deep sense of awe. That same feeling arises closer to home too: in the constant give-and-take of conflict and cooperation among members of a population, among different populations sharing a community, and even in the ceaseless molecular activity taking place inside a single cell. All of this pushes us toward one basic question — what does it mean to be alive? Bound up in that question are really two separate questions: a technical one, asking what marks the living off from the non-living, and a philosophical one, asking what purpose life serves. Being a science, biology takes up only the first of these two questions.