ABSTRACT
Although much can be said about the behavior of single-cell organisms and plants, comparative psychology is mostly about the evolution and development of the brain and behavior of animals. Just as humans need to be seen in the context of animal evolution, animals are part of the larger context of living organisms, all sharing some fundamental properties, such as the genetic code. By contrast, sponges and mesozoans are generally agreed to be the most ancient animal phyla. The rapid evolution of animals is consistent with the relatively low resolution of phylogenetic trees derived from sequential DNA analyses. The key innovation achieved by Cambrian animals could be the evolution of a skeleton likely to mineralize. Information from living species is also relevant to understanding the origin of animals. These animals were generally small, with hollow bones, and lived in forested areas, not necessarily in close association with water, all features that do not promote fossilization.
