Biogeography, Ecology, and Evolution of Deep Life

When we ponder the existence of life extending deep into Earth, a phrase from the movie Jurassic Park is often used: that “life finds a way.” Numerous investigations into the continental and marine subsurface have shown that life indeed finds a way to exist deep into the subsurface, provided that physical influences, particularly heat, allow for the existence of biomolecules. In this chapter, we will review what is known about the biogeography, ecology, and evolution of deep life, acknowledging along the way that this field is rapidly developing with every new set of experiments and continued exploration. The subsurface biosphere is loosely defined as the habitable region beneath the soil and sediments where the limits of habitability are typically defined by some physical process (also see Chapter 19, this volume). Current estimates of the habitable volume of the subsurface range from ~2.0 to 2.3 10 km, or roughly twice the volume of our oceans (Table 17.1). This large biosphere is estimated to hold ~70% of all bacterial and archaeal cells (Figures 17.1 and 17.2) and potentially over 80% all bacterial and archaeal species (for a review, see 1). A variety of habitats and sampling techniques to study the subsurface biosphere have been explored by scientists for nearly a century and are further described throughout this chapter (Sections 17.1.1–17.1.5; also see Figure 16.1 in Chapter 16, this volume).