This chapter explores the possibility of a shadow biosphere, that is, a form of microbial Earth life descended from an alternative abiogenesis.1 It is widely assumed that all life on Earth shares a common origin. Yet there is surprisingly little theoretical or empirical support for this belief, although it is true that all known life is so related. As Section 9.2 explains, the possibility that more than one form of life arose on Earth is consistent with (i) prevailing models of the origin of life (the RNA and SM (Small Molecule) Worlds, discussed in Section 5.4) and (ii) our current understanding of molecular biology and geochemical conditions on the early Earth. While the possibility that our planet hosted more than one abiogenesis is often conceded, many scientists nonetheless insist that any descendants would have been eliminated long ago by our microbial ancestors in a Darwinian competition for vital resources. As we shall see, this theoretical argument is undermined by what has been learned in recent years about the structure and dynamics of microbial communities.