ONTOGENETIC DEPTH AS A COMPLEXITY METRIC FOR THE CAMBRIAN EXPLOSION

Various attempts have been made to quantify the increase in biological complexity exhibited by metazoans across the Neoproterozoic-Cambrian boundary. These include such metrics as genome size, cell type (Valentine et al. 1994), and a variety of complexity measures (e.g., McShea 1996). Here we develop a measure of ontogenetic depth – i.e., the distance, in terms of cell division and differentiation, between a unicellular condition and a macroscopic adult metazoan capable of reproduction (generation of gametes). We then apply this metric to the radiative events which occurred during the Cambrian Explosion, and assess the evolutionary mechanisms that may explain the increases in ontogenetic depth at the origin of the phyla.