Ontogeny recapitulates phylogeny

The theory that the stages in an organism's embryonic development and differentiation correspond to the stages of evolutionary development characteristic of the species. Also called Haeckel's law, recapitulation theory.

Biogenetic law, in biology, a law stating that the earlier stages of embryos of species advanced in the evolutionary process, such as humans, resemble the embryos of ancestral species, such as fish. The law refers only to embryonic development and not to adult stages; as development proceeds, the embryos of different species become more and more dissimilar. An early form of the law was devised by the 19th-century Estonian zoologist K. E. von Baer, who observed that embryos resemble the embryos, but not the adults, of other species. A later, but incorrect, theory of the 19th-century German zoologist Ernst Heinrich Haeckel states that the embryonic development (ontogeny) of an animal recapitulates the evolutionary development of the animal's ancestors (phylogeny).

Ernst Haeckel, German scientist, a contemporary and supporter of Charles Darwin, used this phrase to describe his embryological observation, which says basically that the development of the individual retraces the evolutionary steps of the species --- from its conception to its birth (or hatching, as the case may be), every animal passes through the evolutionary phases identical to the general process of evolution, from one-celled animals to advanced life-forms, over eons of time. In other words, every animal embryo "evolves" from a microscopic mass of cells to a fish, then to an amphibian, then to a reptile, and so on.

In terms of the human brain:

* At 25 days - the embryonic brain resembles the brain of a worm
* At 40 days - the embryonic brain resembles the brain of a vertebrate (fish)
* At 100 days - the embryonic brain resembles a mammalian brain
* At 5 months - the embryonic brain resembles the brains of other primates
The statement "ontogeny recapitulates phylogeny" is credited to Ernst Haeckel, and was the credo and motivation for much embrological research in the 19th and early 20th century. Despite the fact that this is still taught to high school and university students, it has been thoroughly disproven and discredited. That is not to say, however, that the essential idea that an organism's evolutionary history is not reflected in its embryological development. In fact, the modern credo is instead "phylogeny recapitulates ontogeny", the reverse of Haeckel's idea and in fact the hypothesis originally put forth by Karl Ernst von Baer, Haeckel's predecessor.
The latter statement means, and what modern biologists believe, that the embryonic stages of an organism or species should resemble the embryonic (and not adult) stages of its ancestors. In other words, the embryonic development of any species is constrained to resemble (closely) the embryonic development of its ancestors. In other words, by examining the ontogeny of an individual species, we can infer certain aspects of their evolutionary history.

Consider the two following cases as evidence supporting the modern hypothesis. First, snakes and legless lizards develop 'leg buds' as embryos, only to have them re-absorbed prior to hatching. This same pattern is observed in whales and dolphins. In both cases, independent evidence shows that snakes evolved from legged ancestors, as did the whale and dolphin. A second example of the same type can be found in the development of the mammalian inner ear. In reptile embryos, two bones develop into the articular bones of the hinge of the jaw, while these two same bones become the hammer and anvil of the inner ear in marsupials. This suggests that, evolutionarily, the inner ear of mammals developed from bones originally found in the jaw of a common ancestor to the reptiles, and the fossil evidence clearly shows this to be the case.
In biology, ontogeny refers to the embryonal development process of a certain species, and phylogeny to 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.

Observed connections

Generally, if a structure pre-dates another structure in evolutionarily 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, believed to 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 mammal embryos) have hair at one stage, but lose most of it later.
* All vertebrates, believed to 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.

Explanation

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.

"Ontogeny recapitulates phylogeny", also called the "biogenetic law" or the "theory of recapitulation", is a now discredited hypothesis in biology first espoused in 1866 by Ernst Haeckel. 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.
In order to support his theory, Haeckel produced several embryo drawings which overemphasized similarities between embryos of related species and found their way into many biology textbooks.
Modern biology rejects Haeckel's theory. While for instance the phylogeny of humans as having evolved from fish through reptiles to mammals is generally accepted, no cleanly defined "fish", "reptile" and "mammal" stages of human embryonal development can be discerned.

The fact that the strict 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. See: ontogeny and phylogeny.

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.

he theory of recapitulation, also called the biogenetic law or embryological parallelism and often expressed as "ontogeny recapitulates phylogeny" is a discredited biological theory. First 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, in the highly elaborate and deterministic form advanced by Haeckel, has, since the early twentieth century, been refuted on many fronts. Haeckel's drawings used artistic licence, his theory was associated with Lamarckism[citation needed], it was 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 to" the notion.

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 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.

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:
he 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.

Modern observations

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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.



Students of biology who have gone to the trouble to memorize this impressive sounding phrase will be disheartened to learn that it has been known to be untrue since it was first proposed as "fact" by Ernst Haeckel nearly 100 years ago! The recapitulation myth, better known as the biogenetic "law", claims that each embryo in its development passes through abbreviated stages that resemble developmental stages of its evolutionary ancestors. The fictitious "gill slits" of human embryos discussed in Myth # 1, for example, are supposed to represent the "fish" or "amphibian" stage of man's evolutionary ancestors. Most professional evolutionists no longer believe this myth. The famous evolutionist Dr. Paul Ehrlich, for example, said: "this interpretation of embryological sequences will not stand close examination. Its' shortcomings have been almost universally pointed out by modern authors, but the idea still has a prominent place in biological mythology." ('The Process of Evolution' 1963, p.66). In his book 'The Beginnings of Life' (1977, p. 32), embryologist Dr. E. Blechschmidt reveals some of his frustration with the persistence of this myth: "The so-called basic law of biogenetics is wrong. No buts or ifs can mitigate this fact. It is not even a tiny bit correct or correct in a different form. It is totally wrong." Yet in a recent (1980) survey of 15 high school biology text books, 9 offered embryological recapitulation as evidence for evolution!