Recapitulation theory

The theory of recapitulation, also called the biogenetic law or ontogeny recapitulates phylogeny, is a hypothesis 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, although recognized as being partly accurate. In biology, ontogeny is the embryonal development process of a certain species, and phylogeny a species' evolutionary history. Observers have noted various connections between phylogeny and ontogeny, explained them with evolutionary theory and taken them as supporting evidence for that theory.

Haeckel's theory


Ontogeny is the development of the embryos of a given species; phylogeny is the evolutionary history of a species. The theory claims that the development of the embryo of every species repeats the evolutionary development of that species fully. Or 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.

Haeckel produced several embryo drawings which overemphasized similarities between embryos of related species and found their way into many biology textbooks.

For example, the human embryo with gill slits in the neck was believed by Haeckel to not only signify a fishlike ancestor, but it represented a total fishlike stage in development. Gill slits are not the same as gills and are not functional. Gill slits 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.

Modern biology rejects the literal form of Haeckel's theory. While for instance the phylogeny of humans as having evolved from fish through reptiles to mammals is accepted (though 'reptiles' are now known to be a composite group and the ancestors of mammals split off before todays' reptiles evolved), no cleanly defined "fish", "reptile" and "mammal" stages of human embryonal development can be discerned. There is no linearity in the development. For instance in kidney development, at one given time, the anterior region of the kidney is less developed (nephridium) than the posterior region (nephron).

The fact that the literal form of recapitulation theory is rejected by modern biologists has sometimes been used as an argument against evolution by creationists. The argument is: "Haeckel's theory was presented as supporting evidence for evolution, Haeckel's theory is wrong, therefore evolution has less support". This argument is not only an oversimplification but misleading because modern biology does recognize numerous connections between ontogeny and phylogeny, explains them using evolutionary theory without recourse to Haeckel's specific views, and considers them as supporting evidence for that theory.

Historical impact
Although Haeckel's specific form of recapitulation theory is now discredited among biologists, it did have a strong impact in social and educational theories of the late 19th century. 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 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.
 * All land vertebrates, which have evolved from fish, show gill pouches at one stage of their embryonal development.
 * 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 the connection to the gut later disappears.

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 modifications in all later steps and are therefore less likely to succeed. Most of the successful changes will thus affect the latest stages of the program, and the program will retain the earlier steps. Occasionally however, a modification of an earlier step in the program does succeed: for this reason a strict correspondence between ontogeny and phylogeny, as expressed in Ernst Haeckel's discredited recapitulation law, fails.