1. Introduction

Abstract

Astrobiology studies the origin, evolution, distribution, and fate of life throughout the universe, with no direct evidence that life exists anywhere in the universe other than on Earth. But there are compelling reasons to assume that life exists pervasively throughout the cosmos. That assumption is derived from empirical observations on the nature of the universe and the natural laws that govern it, from analysis of the history and properties of the one case of life that we do know, and on a logical integration of fact and theory. The science of astrobiology is thus as strong, if not as revolutionary, as Darwin’s theory of evolution before fossil humans were found to prove our animal origins; as firm, if not as precise, as the astronomical predictions that Neptune must exist before it was detected; and, in our view, as certain as the conclusion that the world was a sphere before Magellan sailed around it.