Recapitulation theory

From Wikipedia, the free encyclopedia

The theory of recapitulation, also called the biogenetic law or embryological parallelism, and often expressed as "ontogeny recapitulates phylogeny", was put forward by Étienne Serres in 1824–26 as what became known as the "Meckel-Serres Law" which attempted to provide a link between comparative embryology and a "pattern of unification" in the organic world. It was supported by Étienne Geoffroy Saint-Hilaire and became a prominent part of his ideas which suggested that past transformations of life could have had environmental causes working on the embryo, rather than on the adult as in Lamarckism. These naturalistic ideas led to disagreements with Georges Cuvier. It was widely supported in the Edinburgh and London schools of higher anatomy around 1830, notably by Robert Edmond Grant, but was opposed by Karl Ernst von Baer's ideas of divergence, and attacked by Richard Owen in the 1830s.^[1]

In 1866, the German zoologist Ernst Haeckel proposed that the embryonal development of an individual organism (its ontogeny) followed the same path as the evolutionary history of its species (its phylogeny). This theory, in the highly elaborate and deterministic form advanced by Haeckel, has, since the early twentieth century, been refuted on many fronts.^[2] Haeckel's drawings used artistic licence, his theory was associated with Lamarckism, it was quite clearly wrong in supposing that embryos passed through the adult stages of more primitive life-forms, it ignored organs such as teeth which are "held over" to a late developmental stage, and it was used by Haeckel to promote the supremacy of the white European male. However, the basic idea of recapitulation is still widespread - Stephen Jay Gould's first book (Ontogeny and Phylogeny) begins by declaring that many scientific professionals believe, privately and informally, that there is "something in" the notion. ^[3] (See Evolutionary developmental biology)

[edit] Haeckel's theory

Romanes's 1892 copy of Ernst Haeckel's controversial embryo drawings (this version of the figure is often attributed incorrectly to Haeckel).^[4]

Haeckel formulated his theory as "Ontogeny recapitulates phylogeny". The notion later became simply known as the recapitulation (OED: 'a summing up or brief repetition') theory. Ontogeny is the growth (size change) and development (shape change) of an individual organism; phylogeny is the evolutionary history of a species. Haeckel's recapitulation theory claims that the development of advanced species passes through stages represented by adult organisms of more primitive species.^[2] Otherwise put, each successive stage in the development of an individual represents one of the adult forms that appeared in its evolutionary history.

For example, Haeckel proposed that the gill slits (pharyngeal arches) in the neck of the human embryo represented an adult "fishlike" developmental stage as well as signifying a fishlike ancestor. Embryonic pharyngeal arches, the invaginations between the gill pouches or pharyngeal pouches, open the pharynx to the outside. Such gill pouches appear in all tetrapod animal embryos: in mammals, the first gill bar (in the first gill pouch) develops into the lower jaw (Meckel's cartilage), the malleus and the stapes. At a later stage, all gill slits close, only the ear remaining open.^[5] But these embryonic pharyngeal arches could not at any stage carry out the same function as the gills of an adult fish.

Haeckel produced several embryo drawings that often overemphasized similarities between embryos of related species. These found their ways into many biology textbooks, and into popular knowledge.

[edit] Rejection

Modern biology rejects the literal and universal form of Haeckel's theory.^[6] Although humans are generally understood to share ancestors with other taxa, stages of human embryonic development are not functionally equivalent to the adults of these shared common ancestors. In other words, no cleanly defined and functional "fish", "reptile" and "mammal" stages of human embryonal development can be discerned. Moreover, development is nonlinear. For example, during kidney development, at one given time, the anterior region of the kidney is less developed (nephridium) than the posterior region (nephron).

Modern biology does recognize numerous connections between ontogeny and phylogeny^{[citation needed]}, and explains them using evolutionary theory without recourse to Haeckel's specific views, and considers them as supporting evidence for that theory.

[edit] Historical influence

Although Haeckel's specific form of recapitulation theory is now discredited among biologists, it had a strong influence on social and educational theories of the late 19th century.

English philosopher Herbert Spencer was one of the most energetic promoters of evolutionary ideas to explain many phenomena. He compactly expressed the basis for a cultural recapitulation theory of education in the following claim:^[7]

If there be an order in which the human race has mastered its various kinds of knowledge, there will arise in every child an aptitude to acquire these kinds of knowledge in the same order.... Education is a repetition of civilization in little.^[8]

– Herbert Spencer

The maturationist theory of G. Stanley Hall was based on the premise that growing children would recapitulate evolutionary stages of development as they grew up and that there was a one-to-one correspondence between childhood stages and evolutionary history, and that it was counterproductive to push a child ahead of its development stage. The whole notion fit nicely with other social Darwinist concepts, such as the idea that "primitive" societies needed guidance by more advanced societies, i.e. Europe and North America, which were considered by social Darwinists as the pinnacle of evolution.^{[citation needed]} An early form of the law was devised by the 19th-century Estonian zoologist Karl Ernst von Baer, who observed that embryos resemble the embryos, but not the adults, of other species.

[edit] Modern observations

Generally, if a structure pre-dates another structure in evolutionary terms, then it also appears earlier than the other in the embryo. Species which have an evolutionary relationship typically share the early stages of embryonal development and differ in later stages. Examples include:

• The backbone, the common structure among all vertebrates such as fish, reptiles and mammals, appears as one of the earliest structures laid out in all vertebrate embryos.
• The cerebrum in humans, the most sophisticated part of the brain, develops last.

If a structure vanished in an evolutionary sequence, then one can often observe a corresponding structure appearing at one stage during embryonic development, only to disappear or become modified in a later stage. Examples include:

• Whales, which have evolved from land mammals, don't have legs, but tiny remnant leg bones lie buried deep in their bodies. During embryonal development, leg extremities first occur, then recede. Similarly, whale embryos have hair at one stage (like all mammalian embryos), but lose most of it later.
• The common ancestor of humans and monkeys had a tail, and human embryos also have a tail at one point; it later recedes to form the coccyx.
• The swim bladder in fish presumably evolved from a sac connected to the gut, allowing the fish to gulp air. In most modern fish, this connection to the gut has disappeared. In the embryonal development of these fish, the swim bladder originates as an outpocketing of the gut, and is later disconnected from the gut.

[edit] Notes

[edit] References

• Division of Biology and Medicine, Brown University. "Evolution and Development I: Size and shape". http://biomed.brown.edu/Courses/BIO48/30.S&S.HTML. 
• Haeckel, E (1899). "Riddle of the Universe at the Close of the Nineteenth Century". http://www.ucmp.berkeley.edu/history/haeckel.html. 
• Rebecca Irwin. "Ontogeny and Phylogeny". http://www.utm.edu/~rirwin/391OntogPhylog.htm. 
• Richardson, M., et al. (1997). "There is no highly conserved stage in the vertebrates: implications for current theories of evolution and development". Anatomy and Embryology 196 (2): 91–106. doi:10.1007/s004290050082. 
• Gould, Stephen Jay (1977). Ontogeny and Phylogeny. Cambridge Mass.: Belknap Press of Harvard University Press. ISBN 0-674-63941-3. 
• Desmond, Adrian J. (1989). The politics of evolution: morphology, medicine, and reform in radical London. Chicago: University of Chicago Press. ISBN 0-226-14374-0. 

[edit] External links

Look up ontogeny recapitulates phylogeny in Wiktionary, the free dictionary.

• Of Parts and Wholes: Self-similarity and Synecdoche in Science, Culture and Literature
• "Ontology Recapitulates Philology", a parody phrase