We saw in the last chapter that evolutionary theory places hypotheses about the causes of trait evolution within the framework of a phylogenetic tree. These hypotheses, whether they say that the trait of interest evolved by natural selection or by some other process, make claims about what happened in lineages, and different lineages stem from common ancestors. For example, different extant species have different kinds of eyes, and some have no eyes at all. The fact of common ancestry places a constraint on how the present distribution of trait values must be explained. If all these species have a common ancestor, the lineages descending from that common ancestor had to start with the same trait value. It follows that the task of explaining why vertebrates have camera eyes is essentially connected to the task of explaining why other groups have other kinds of eyes while still others have none at all.

Given how central the thesis of common ancestry is to evolutionary reasoning, one might expect there to be a vast literature in which the evidence for that claim is amassed. In fact, the question is discussed, but the literature on it is hardly vast. For most evolutionists, the similarities that different species share make it obvious that they have common ancestors, and there is no reason to puzzle further over the question. The kind of genealogical question that attracts far more attention in evolutionary biology concerns how various species are related to each other, not whether they are.