Body plan
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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.
Origin
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 animals. Vertebra, limbs, even brains are supplementary to the pipe. The arms-race of natural selection develops more and more advanced biological components but the pipe model itself remains, and therefore today the scope for radical body plan change is very limited.
Control mechanism
Body plans in most species appear to be controlled by a set of genes known as the homeobox genes, which are involved in the regulation of development (morphogenesis) of animals, fungi and plants.
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".
Examples
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, in which a landslide trapped many living creatures in a Cambrian sea, showing all the different body forms that came to rise (only to later fall extinct) during the Cambrian explosion.
Fictional
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.