Nomothetism and Understanding the Cambrian "Explosion"

Abstract

Steve Gould (1980), in his paper on the promise of paleobiology as a nomothetic evolutionary discipline, sets the nomothetic, the search for general rules or laws, against the idiographic, the detailed description of unique events. Gould (1980) notes that all idiographic events have nomothetic undertones. I agree, but go further. For me, unique events in the history of life not only have nomothetic undertones, but have nomothetic underpinnings. Here I outline these underpinnings for perhaps the most dramatic event seen in the fossil record, the Cambrian “explosion” of bilaterian animals. I am most interested in the rapid increases in disparity (e.g., the origin of the bauplane) and diversity, as well as why they started when they did. Before outlining the principles necessary for an understanding of the Cambrian “explosion” I briefly describe the classes of explanation typically invoked to account for the event. Most explanations center on one primary cause or trigger, usually a developmental, environmental, or ecological event or innovation. #### Developmental Explanations These point to the origin of the bilaterian developmental system, including the origin of the Hox genes, etc., as the primary cause of the “explosion” (Peterson and Davidson, 2000; Erwin and Davidson, 2002). While there is little doubt that the origin of the bilaterian developmental system was critical to the Cambrian radiation, this class of explanation doesn’t address the question of why the origin of such a system should, ipso facto, lead to increased diversity or disparity. In fact, if at least one Ediacaran is a bilaterian (e.g., Kimberella , Spriggina , Dickinsonia , or Arkarua ), then the bilaterian developmental system existed at least a few tens of millions of years prior to the Cambrian “explosion,” suggesting something more than a new developmental system is needed to account for the “explosion.” #### Environmental Explanations There has been a resurgence in …