Recapitulation theory

The theory of recapitulation, also called the biogenetic law, embryological parallelism or ontogeny recapitulates phylogeny, is a theory in biology which attempts to explain apparent similarities between humans and other animals. First espoused in 1866 by German zoologist Ernst Haeckel, a contemporary of Charles Darwin, the theory has been discredited in its absolute form ("strong recapitulation"), although recognized as being perhaps partly fruitful. In biology, ontogeny is the embryonal development process of a certain species, and phylogeny a species' evolutionary history.

Haeckel's 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 was seen to pass 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. Haeckel formulated his theory as such: "Ontogeny recapitulates phylogeny". This notion later became simply known as recapitulation (OED: 'a summing up or brief repetition').

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.

For example, Haeckel believed that the human embryo with gill slits (pharyngeal arches) in the neck not only signified a fishlike ancestor, but represented an adult "fishlike" developmental stage. Embryonic pharyngeal arches are not gills and do not carry out the same function. They are the invaginations between the gill pouches or pharyngeal pouches, and they open the pharynx to the outside. 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. In a later stage, all gill slits close, with only the ear opening remaining open. For a technical discussion on the topic, see.

Rejection
Modern biology rejects the literal and universal form of Haeckel's theory. Although humans share ancestors with many other taxa (roughly, fish through reptiles to mammals), 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, and explains them using evolutionary theory without recourse to Haeckel's specific views, and considers them as supporting evidence for that theory.

Historical influence
Although Haeckel's specific form of recapitulation theory is now discredited among biologists, it did have 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 pretty well everything in sight; He compactly expressed the basis for a cultural recapitulation theory of education in the following claim:

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. 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.

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 (like all mammalian embryos) have hair at one stage, 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.

Modern theory
One can explain connections between phylogeny and ontogeny if one assumes that one species changes into another by a sequence of small modifications to its developmental program (specified by the genome). Modifications that affect early steps of this program will usually require the right modifications in later steps in order to produce an individual that survives and reproduces. Therefore, such successful combinations of changes are less likely to occur and most of the successful changes affect the latest stages of the program, and the earlier steps are retained. But occasionally, a modification of an earlier step in the program does succeed: for this reason ontogeny and phylogeny do not strictly correspond, contrary to Haeckel's original theory.