A body plan, or bauplan, is essentially the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan. One of the key issues of developmental biology is the evolution of body plans as different as those of a starfish, a fern, or a mammal, from a common biological heritage, and in particular how radical changes in body plans have occurred over geological time. The body plan is a key feature of an organism's morphology, and since the discovery of DNA developmental biologists have been able to learn a lot about how genes control the development of structural features through a cascade of processes in which key genes produce morphogens, chemicals that diffuse through the body to produce a gradient that acts as a position indicator for cells, turning on other genes, some of which in turn produce other morphogens. A key discovery was the existence of groups of homeobox genes which are responsible for laying down the basic body plan in organisms. The homeobox genes are remarkably conserved between species as diverse as the fruitfly and man, the basic segmented pattern of the worm or fruitfly being the origin of the segmented spine in man. The field of evolutionary developmental biology, which studies the genetics of morphology in detail is now a rapidly expanding one , with many of the developmental genetic cascades, particularly in the fruitfly drosophila, now catalogued in considerable detail . .
Body plan is the basis for phylum, and there are 35 different basic animal body plans, corresponding to different phyla.
The Cambrian explosion refers to the massive increase in different body plans that took place around 530 million years ago. Fossils from this era show all sorts of weird and wonderful shapes, many quite unlike anything found today. At that time it was possible for organisms to survive and make a living even though they were unrefined and unlikely, because predation had yet to evolve, along with arms races that would optimise and streamline them to occupy a particular ecological niche.
The concept of bauplan is employed in the studies of morphology, taxonomy, comparative physiology and, most notably, phylogenetics and evolution. Before the advent of genetic sequencing, the analysis of the bauplan of fossils was an important method to devise hypothetical relationships and lineages of species, both living and extinct. The idea is, that species that are closely related share more common properties, hence a more detailed bauplan. Small differences of bauplan are indicative of species belonging to a parent, child or sibling taxon.
The current range of body plans is far from exhausting the possible patterns for terrestrial life: the Ediacaran biota appears to contain numerous species and taxa with body plans quite different from any found in currently living organisms.
The most commonly seen body plan amongst vertebrates is that of the tetrapod, which include all mammals, birds, amphibians and reptiles. Some animal groups, such as the cetaceans, bats and most birds have been modified (e.g. front limbs become wings or flippers) but nevertheless, they are still tetrapods.
The invertebrates employ a much more diverse array of body plans, such as seen in insects (six legs, three body parts and an exoskeleton), cephalopods (no skeleton, hydrostatically stiffened tentacles, primary propulsion by squeezing water out of a mantle cavity), echinoderms (five-fold radial symmetry, external skeleton, movement by hydrostatically operated tube feet) and various phyla of "worms" (tube-shaped, movement by expanding and contracting parts of the body).
The most varied collection of body forms known is found in the Burgess Shale, where fossils from a Cambrian sea show a tremendous variety of body forms that came to rise (only to later fall extinct) during the Cambrian explosion.