The Ancestor of All Life
THE EVIDENCE that all known living organisms are descended from a single common ancestor is overwhelming. We cannot exclude the possibility that unknown or poorly known organisms of different origin exist in some remote environment that has remained isolated for a very long time. However, no discovery suggestive of a major break with "our" way of life has yet been made. Until proven otherwise, the hypothesis of single ancestry holds true.
In this chapter, I shall try to reconstruct the profile of our common ancestor and to retrace its emergence historically, paying special attention to the shape of its hidden roots. Did the universal ancestor arise as a single shoot along a highly deterministic pathway that left little to chance? Or was it one of many branches, a branch that simply happened to spread faster, smothered all others, and ended up filling the world with its progeny?
The universal ancestor is defined as the organism that existed just before the tree of life divided into two separate branches that have spread out extensions to the present day. This definition distinguishes the universal ancestor from the more primitive ancestral forms that came before; it also leaves open the possibility of earlier branchings that have left no extant progeny. In principle, drawing a portrait of this ancestral organism is simple: just put together all the properties that are shared by all living organisms. In practice, three caveats complicate matters. First, we must subtract from our picture such shared properties as could have been acquired separately in individual branches after the first forking of the tree of life. Second, we must also subtract properties that appeared solely in one branch and were later acquired by the other branch or branches by some mechanism of gene transfer.