© 1999 by The Society for Integrative and Comparative Biology
The Origin of Bodyplans1
Department of Paleobiology, MRC-121, National Museum of Natural History Washington, D.C. 20560
Correspondence: 2E-mail: Erwin.Doug{at}NMNH.SI.EDU
Paleontologists have documented the progressive origination of metazoan bodyplans beginning about 610 million years ago (Ma) with the major period of innovation occurring from 570 Ma to about 525 Ma. The breadth of this event is now well documented among soft-bodied, skeletonized and trace fossils. Changes in both the phytoplankton and in geochemical markers suggest pervasive transformation of the environment during this interval, implicating an ecological role in triggering this innovative burst. The extent to which developmental innovations contributed to this evolutionary burst requires placement of the protostome-deuterostome ancestor (PDA) in time: a PDA close to the radiation could indicate a greater role for developmental innovation. Molecular evidence points to a maximum age of about 670 Ma. Uncertainties over the extent of functional homologies among the widely conserved developmental control genes, and gene pathways, bracket plausible dates for such an ancestor: the maximally complex ancestor could not predate about 545 Ma; the least complex alternative could date to 575 Ma or even earlier. The nested hierarchical structure of developmental control genes and bodyplan originations suggests certain temporal inhomogeneities to the evolutionary process: as certain developmental patterns are established they limit subsequent evolutionary trajectories.