The evolution and development of segmented body plans

Segmentation is both a morphological phenomenon and a developmental process occurring in bilaterally symmetrical animals. A segmented body plan is one in which repeated body units are arranged along the anterior--posterior axis, each unit containing elements from a number of organ systems. Segmentation is found in three phyla: the arthropods, annelids, and chordates. There is some debate over whether the segmented body plan in these three phyla is homologous. However, despite many similarities, when using multiple data sources, the bulk of the evidence points towards a convergent evolution of the segmented body plan and the segmentation process in these three phyla. Segmentation probably arose first as an efficient mode for repeating units of different organ systems along the body axis. It then provided an improved mode of locomotion. Once segmentation became fixed in several lineages, these lineages diversified dramatically because of the enhanced evolvability and modularity that is thought to be conferred by the segmented body plan and its development.