When the first edition of this book was published some 15 years ago, astrobiology was not recognized as a separate academic discipline, and few universities and colleges offered courses in the subject per se. But the question of what makes a planet capable of sustaining life, and whether inhabited planets exist in large numbers in the cosmos, was long a popular draw for courses in planetary science, geology, and astronomy. I wrote Earth: Evolution of a Habitable World so as to encourage instructors of freshmen and sophomore non science majors to take a consciously planetary bent in covering how our home planet came to be, its place in the overall evolution of the cosmos, how it became habitable and inhabited, and how life and the environment evolved together (sometimes coupled, sometimes not) to the present day. And in closing with chapters on human-induced global warming and depletion of resources, I wished to provide a “cosmic perspective” via the rest of the book to some very down-to-Earth problems. In the breadth of topics and perspective I took in writing it, Earth was alone in its chosen subject area, with only a few notable exceptions.

Today astrobiology is a thriving academic field with a daunting number and variety of textbooks on the subject. In preparing a revised edition I considered making the book more consciously astrobiological, either by aligning the contents more closely with the typical survey treatment - or by simply adding the word “Astrobiology” to the title.