How do you write a book about things, life and language on planets outside our own Solar System, when you do not even know if they exist or not? The only sensible answer is by looking at how life developed on Earth1 and then considering how this could manifest itself on exoplanets, those beyond the planets which orbit our star, the Sun. This is because when considering possible Earth-like planets (see Section 8.8 for 10 criteria) we have, at our present state of knowledge, a set consisting of only one member, our Earth; this is the ‘set of one’ issue (Figure 1.1).