Abody plan,Bauplan (pl. German:Baupläne), orground plan is a set ofmorphologicalfeatures common to many members of aphylum ofanimals.[1] Thevertebrates share one body plan, whileinvertebrates have many.
This term, usually applied to animals, envisages a "blueprint" encompassing aspects such assymmetry,layers,segmentation,nerve,limb, andgut disposition.Evolutionary developmental biology seeks to explain the origins of diverse body plans.
Body plans have historically been considered to have evolved in a flash in theEdiacaran biota; filling theCambrian explosion with the results, and a more nuanced understanding of animal evolution suggests gradual development of body plans throughout the earlyPalaeozoic. Recent studies inanimals andplants started to investigate whether evolutionary constraints on body plan structures can explain the presence of developmental constraints duringembryogenesis such as the phenomenon referred to asphylotypic stage.
Among the pioneeringzoologists, Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognised by modern zoologists has risen to 36.[1]
In his 1735 bookSystema Naturæ, SwedishbotanistLinnaeus grouped the animals intoquadrupeds,birds, "amphibians" (includingtortoises,lizards andsnakes),fish, "insects" (Insecta, in which he includedarachnids,crustaceans andcentipedes) and "worms" (Vermes). Linnaeus's Vermes included effectively all other groups of animals, not onlytapeworms,earthworms andleeches butmolluscs,sea urchins andstarfish,jellyfish,squid andcuttlefish.[2]
In his 1817 work,Le Règne Animal, French zoologistGeorges Cuvier combined evidence from comparative anatomy andpalaeontology[3] to divide the animal kingdom into four body plans. Taking thecentral nervous system as the main organ system which controlled all the others, such as the circulatory and digestive systems, Cuvier distinguished four body plans orembranchements:[4]
Grouping animals with these body plans resulted in four branches:vertebrates,molluscs,articulata (includinginsects andannelids) and zoophytes orRadiata.
Ernst Haeckel, in his 1866Generelle Morphologie der Organismen, asserted that all living things weremonophyletic (had a single evolutionary origin), being divided into plants, protista, and animals. His protista were divided into moneres, protoplasts, flagellates, diatoms, myxomycetes, myxocystodes, rhizopods, and sponges. His animals were divided into groups with distinct body plans: he named thesephyla. Haeckel's animal phyla werecoelenterates,echinoderms, and (following Cuvier) articulates, molluscs, and vertebrates.[5]
Stephen J. Gould explored the idea that the different phyla could be perceived in terms of a Bauplan, illustrating their fixity. However, he later abandoned this idea in favor ofpunctuated equilibrium.[6]
20 out of the 36 body plans originated in theCambrian period,[7] in the "Cambrian explosion".[8] However, complete body plans of manyphyla emerged much later, in thePalaeozoic or beyond.[9]
The current range of body plans is far from exhaustive of the possible patterns for life: thePrecambrianEdiacaran biota includes body plans that differ from any found in currently living organisms, even though the overall arrangement of unrelated modern taxa is quite similar.[10] Thus the Cambrian explosion appears to have more or less completely replaced the earlier range of body plans.[7]
Genes,embryos and development together determine the form of an adult organism's body, through the complex switching processes involved inmorphogenesis.
Developmental biologists seek to understand how genes control the development of structural features through a cascade of processes in which key genes producemorphogens, 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 ofhomeobox genes, which function as switches responsible for laying down the basic body plan in animals. The homeobox genes are remarkably conserved between species as diverse as the fruit fly and humans, the basic segmented pattern of the worm or fruit fly being the origin of the segmented spine in humans. The field of animalevolutionary developmental biology ('Evo Devo'), which studies the genetics ofmorphology in detail, is rapidly expanding[11] with many of the developmental genetic cascades, particularly in the fruit flyDrosophila, catalogued in considerable detail.[12]
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