Figure 1.

Four possibilities for the relationship between habitable environments (bottom of each subfigure) and types of lyfe (top of each subfigure). (a) A one-to-one correspondence: every habitable environment produces a unique form of lyfe. (b) A one-to-many correspondence: there is just one single class of habitable environment, but it is capable of generating and supporting myriad different kinds of lyfe forms. (c) A many-to-one correspondence: there are many different classes of habitable environments, but only one kind of lyfe (life) in the universe. (d) A many-to-many correspondence: there are many different classes of habitable environments, each of which is capable of generating and supporting myriad different kinds of lyfe forms. (e) A many-to-many correspondence but not every environment is capable of supporting every form of lyfe. If scenario (e) represents reality, then on other worlds—even Earth-like worlds—lyfe as we do not know it may emerge, and the development of agnostic biosignatures will be key to the search for life in the universe.