Recapitulation theory
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The theory of recapitulation, also called the biogenetic law or embryological parallelism and often expressed as "ontogeny recapitulates phylogeny" is a largely discredited biological hypothesis.
The general agreement among historians his that the concept originated in the 1790s among the German Natural philosophers. The first formal formulation was proposed by Étienne Serres in 1824–26 as what became known as the "Meckel-Serres Law", it 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.
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 arose from Haeckel's attempt to synthesise the ideas of Lamarckism and Goethe's Naturphilosophie with Charles Darwin's concepts. While Haeckel is often seen as rejecting Darwin's theory of branching evolution for a more linear Lamarckian "biogenic law" of progressive evolution, this is not accurate. Rather Haeckel used the Lamarckian picture to describe the ontogenic and phylogenic history of the individual species, but agreed with Darwin about the branching nature of all species from one, or a few, original ancestors. Since around the start of the twentieth century, Haeckel's "biogenic law" has been refuted on many fronts.
Darwin's more sophisticated view that early embryonic stages are similar to the same embryonic stage of related species, but not to the adult stages of these species, has been confirmed by modern evolutionary developmental biology.
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Haeckel's theory
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. 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. 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.
Rejection
Modern biology rejects the literal and universal form of Haeckel's theory. Although humans 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).
However, as will be shown below, there are numerous connections between ontogeny and phylogeny, which can be explained using evolutionary theory without recourse to Haeckel's specific views.
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, published in 1861, five years before Haeckel first published on the subject:
- "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."
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.Template:Citation needed
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.
- In bird embryos, very briefly fingers start to develop. After a short time, they partly disappear again, partly are fused with the handbones to form the carpometacarpus.
But this rule-of-thumb is not universal. Modern birds, for example, though descended from toothed animals, never grow teeth even as embryos. However, they still possess the genes required to do so, and tooth formation has been experimentally induced in chickens.
