A body plan, or body form, 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.
Body plan is the basis for phylum, and there are 35 different basic animal body plans, corresponding to different phyla.
The evolution of body plans became inevitable with the emergence of differentiated multicellular life in the Ediacaran Era. The most basic and successful structure, for free-moving organisms, is the "pipe". This is common even to organisms as diverse as humans and earthworms. It is essentially a mouth linked to an anus by a gut. The simple process of nutrient capture, digestion, and waste disposal is fundamental to the body plan of advanced, free-moving animals. Vertebra, limbs, even brains are supplementary to the pipe. Natural selection has spun off an enormous range of variations on this basic theme, but the pipe model itself remains. The basic symmetry and organization of this body plan apparently gave an ancient organism an enormous advantage at survival and reproduction, and it has been preserved in most animals ever since.
Bauplan (German for building plan, blueprint; plural: baupläne or bauplaene) is a closely related term in biology referring to the common new and original (homologous) properties of the members of a systematic group (taxon). It is not necessary that a bauplan precisely describes any one particular species of that group.
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.
A particular subgroup of homeobox genes are the Hox genes, which are found in a special gene cluster, the Hox cluster (also called the Hox complex). Hox genes function in patterning the body axis. Thus, by providing the identity of particular body regions, Hox genes determine where limbs and other body segments will grow in a developing fetus or larva. Mutations in any one of these genes can lead to the growth of extra, typically non-functional body parts in invertebrates, for example aristapaedia complex in Drosophila, which results in a leg growing from the head in place of an antenna and is due to a defect in a single gene (this mutation is also known as Antennapedia). Mutation in vertebrate Hox genes usually results in spontaneous abortion.
It is possible that the preservation of rare benign or beneficial mutations in these genes may be one source of variation in body plans.
One of the more surprising and, perhaps, counter-intuitive results of such research in evolutionary developmental biology done in this period is that both the diversity of body plans and morphology in organisms across many phyla is not necessarily reflected in similar diversity at the level of the genetic sequences controlling development. Indeed, as Gerhart and Kirschner (1997) have noted, there is an apparent paradox: "where we most expect to find variation, we find conservation, a lack of change".
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.
One common theme in science fiction is the appearance of extraterrestrial beings, descriptions of which have ranged from being simple variants on human anatomy to beings with body plans wildly different from any found on Earth. The field of exobiology attempts to bring these and similar speculations into the realm of serious scientific investigation.