HIERARCHICAL INTEGRATION OF MODULAR STRUCTURES IN THE EMERGENCE OF COMPLEXITY AMONG EARLY METAZOAN SKELETONS

Living organisms grow by development of hierarchically organized, modular structures with non-arbitrary dimensions. In the evolution of hard-part skeletons of metazoans, five stages are recognized in the development and permutation of skeletal elements: (1) appearance of simple spicules, scales and cones, with little or no direct linkage between them; (2) elaboration of form by the fusion, differentiation, or serial duplication of structures, facilitating active modes of life at increased body size; (3) development of biomechanically advantageous jointed-lever skeletons, initially by proterstomes and significantly later by deuterostomes with internal skeletons; (4) reduction in the disparity of elements within individual skeletons, often with increasingly consistent patterns of symmetry or regular departures from it; (5) reduction in the total numbers of skeletal elements as these become increasingly well integrated, morphologically and physiologically, and are in some cases lost in specialized taxa.

Empirical data document a rapid logistic expansion and subsequent leveling off in the exploitation of basic skeletal design options by latest Proterozoic and Cambrian metazoans. Metabolic processes evolved at lower structural levels, within cells, were essential precursors of developments in which they would be coopted to secrete hard parts. Skeletal elements themselves coevolved with the soft tissues by which they were formed. Subsequently, feedback from physiological processes and behavior that emerged at higher structural levels was more likely to lead to a reduction than a further increase in the number and variety of hard skeletal elements employed. The rate of exploitation of potential designs to form animal skeletons lies at an intermediate point on a spectrum of acceleration in evolution, driven by increasingly efficient harnessing of energy resources, from the gradual diversification of cell types among unicellular organisms during much of the Precambrian to the explosive proliferation of cultural artifacts unleashed by human behavior.