For many millennia, humans have gazed up in wonder at the night-time sky. The full panoply of the Milky Way is an awesome sight. The scale of space is immense. Is there life out there somewhere? If so, where, and what form does it take? In the space of a couple of sentences, we’ve already gone from generalized wonder to specific questions. The next step is from questions to hypotheses, or, in other words, proposed answers. Here are two such hypotheses that I’ll flesh out as the book progresses: first, life exists on trillions of planets in the universe; second, it usually follows evolutionary pathways that are broadly similar to – though different in detail from – those taken on Earth.