The most important aspect of evolution, from a philosophical viewpoint, is the rise of complex, advanced creatures from simple, primitive ones. This "vertical" dimension of evolution has been downplayed in both the specialist and popular literature on evolution, in large part because it was in the past associated with unsavory political views. The avoidance of evolution's vertical dimension has, however, left evolutionary biology open to the perception, from outside, that it deals merely with the diversification of rather similar creatures, all at the same level of "advancedness" from a common ancestor—for example, the classic case studies of finches with different beaks or moths of different colors.
The latest incarnation of creationism, dubbed intelligent design (or ID), has taken advantage of this situation. It portrays an evolutionary process that is constantly guided—especially in its upward direction—by the hand of an unseen Creator, who is able to ensure that it ends up producing humans. Creatures of Accident attacks the antiscience ID worldview, mainly by building a persuasive picture of how "unaided" evolution produces advanced creatures from simple ones by an essentially accidental process. Having built this picture, in the final chapter the book reflects on its religious implications.