It is likely that environmental, developmental and ecological factors all played a role in the Cambrian explosion of complex animals. Less clear is the nature of the interplay between these factors, or whether one was most important. The fossil record shows that large organisms were already extant tens of millions of years prior to the Cambrian explosion, suggesting that the physical environment was already suitable for complex animals well before the Cambrian. Similarly, under the assumption that at least some Ediacaran organisms were bilaterian animals, it appears that the basic genetic tool-kit necessary to produce a diverse array of complex organisms was also in place well before the Cambrian (based on the phylogenetic distribution of key developmental genes among living animals). So if the environmental conditions and genetic tool-kit necessary for building complex animals were already present in the late Neoproterozoic, why was there a lag of tens of millions years before the Cambrian explosion actually occurred? Drawing inspiration from Karl Niklas' computer simulations of early land plant evolution, I argue that it is only with the evolution of behaviorally sophisticated interactions between animal species that the unrealized potential for building complex organisms already present in the Neoproterozoic genome could be extracted. Predation was probably the most important factor. The core of the argument is that with the increased number of selective pressures caused by the evolution predation (due to the many different ways of pursuing prey and avoiding predators), both the number of species and complexity of their morphologies dramatically increased. Under this hypothesis there is no need to invoke any fundamental changes in the genetic tool-kit used by complex Cambrian animals compared with that used by Ediacaran bilaterians, although specific genetic changes needed to develop predation were required to initiate the ecological interactions that drove the Cambrian explosion. This hypothesis suggests that we need to focus our attention not only on the genes that establish animal body plans but also on those genes crucial to ecological interactions, such as behavioral capacities, etc., and that paleoecological analysis is crucial to understanding the Cambrian explosion.