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Ontogeny and Phylogeny

Haeckel’s “Law”: Ontogeny recapitulates phylogeny

An example of the scientific method in action

In the 19th century, the German Biologist Ernst Haeckel (1834-1919) popularized what he called the “law of recapitulation” Many scientists at the time noted that the embryos of different animals all looked very much alike. The younger the embryo, the more they had in common with each other. This surely could not be a coincidence.

Haeckel made observations of embryos at various stages. He then drew idealized versions of them (these were not photographs.)

Like any good scientist he proposed a hypothesis to explain these observations. Unfortunately called his hypothesis a “law” (the law of recapitulation)

Because of this poor choice of words, later writers have attacked his work in recent years. Some have used errors in his work, or misunderstandings of it, to try and disprove the usefulness of science, and all of Evolution.

To be clear, it is always fair to make observations, come up with a hypothesis, and share one’s ideas. Then, other scientists can make their own observations, and see if they agree or disagree. Others can make predictions based on this hypothesis, and test them.

That is how science works, and that is what Haeckel did in this case. As we will see, in some ways he turned out to be correct, while in other ways he turned out to be wrong.

His hypothesis was that “ontogeny recapitulates phylogeny.”
This means that the ontogeny (development of an animal, from embryo to baby) repeats the phylogeny (evolutionary history) of the animal.

“…that if we evolved from a fish that evolved into a reptile that evolved into us, [then] our embryos physically echo that history, passing through a fish-like stage and then into a reptile-like stage. How could this happen? Haeckel argued that evolutionary history was literally the driving force behind development, and that the experiences of our ancestors were physically written into our hereditary material…. This was an extremely attractive idea to scientists; it’s as if development were a time machine that allowed them to look back into the distant past, just by studying early stages of development.”

– from Wells and Haeckel’s Embryos: A Review of Chapter 5 of Icons of Evolution, by P.Z. Myers

Idea rejected by creationists and some scientists

Excerpted from “Haeckel had a point” By Matt Young

While Haeckel had many supporters, he was not without his early critics. Almost immediately, many of Haeckel’s fellow embryologists noticed that he had taken artistic liberties in his drawings to support his ideas…

As the twentieth century unfolded and empirical embryology and genetics emerged, it became clear that Haeckel had emphasized similarities between the embryos of various vertebrate classes in his drawings and neglected the differences. It is not clear whether he purposely altered his drawings to better fit his ideas. Even so, the drawings fascinated lay people and scientists outside the fields of embryology and evolution. Biology textbook authors -looking for images to illustrate their chapter pages – happily included the drawings.

Due to the cost-effective practice of recycling images and accompanying explanations, as well as the paucity or even lack of evolutionary biologists on the editorial staffs of textbook companies, the drawings remained in many textbooks until the 1970s. During this time, many teachers who had little or no background in evolution taught their students that the drawings were evidence of evolution, and the students were encouraged to understand and memorize the catchy phrase, “ontogeny recapitulates phylogeny.”

Finally, in 1977, in his technical book Ontogeny and Phylogeny, Stephen Jay Gould carefully dissected Haeckel’s drawings and disproved the general ideas behind the biogenic law. Later, in one of his last essays in Natural History magazine (March 2000), Gould explained that while Ernst Haeckel was regarded among his contemporaries as a master naturalist, he often “took systematic license in ‘improving’ his specimens to make them more symmetrical or more beautiful.” Most likely as a result of Gould’s careful critique of Haeckel, Haeckel’s drawings are no longer found in today’s biology textbooks.

Criticism of his work often incorrect and exaggerated

His work was more nuanced and accurate than his critics gave him credit for.

See these essays from the National Center for Science Education.

http://ncse.com/creationism/analysis/haeckels-drawings

http://ncse.com/creationism/analysis/exaggerations

Modern view of his work

Excerpt from “Haeckel had a point” By Matt Young

Haeckel was not wholly wrong. Phylogeny is to some extent recapitulated in embryological development – but it does not recapitulate the adult forms. Rather, embryological development of today’s vertebrates summarizes the evolution of past embryos, not past adults.

Haeckel’s conception that evolution is inherently progressive, however, is no longer accepted.

Unfortunately, the creationist Jonathan Wells, in the book Icons of Evolution, exaggerates the importance of Haeckel and the biogenic law, and purports to show that the entire theory of evolution is founded on a handful of errors such as the biogenic law. In reality, as we have seen, biologists disputed the biogenic law early on and eventually replaced it with the more-realistic von Baer’s law.

The eventual discovery of Haeckel’s misstep is an excellent example of how science self-corrects. That such mistakes are uncovered is not a weakness of science, but a strength. Haeckel’s drawings were mere bumps on the road to a comprehensive theory of evolution.

Recapitulation nevertheless provides helpful insight into evolutionary relationships and ancestry.

Harvard University zoologist Ernst Mayr described recapitulation in embryological development as the appearance of an ancestral structure that is found in two different lineages: Say, for example, the pharyngeal arches and pouches that are found in the embryos of both fish and mammals. The structure then disappears from the embryo (or provides the organizational foundation for future structures) in one lineage, mammals, but is maintained in the adult form of the other lineage, fish.

Mayr argued that this structure provides evidence that these two lineages are, in fact, connected to a common ancestor, that is, a group of organisms, not a single individual, from which two lineages most likely descended.

But does not an organism waste energy on a structure that will disappear later during embryological development?

To the contrary, these ephemeral ancestral features provide a framework upon which the successful development of future structures depends.

In other words, these ancestral characteristics are now playing new roles in embryological development, like organizing tissues that will eventually become bones in the skeleton of an individual.

Another character that unites the chordates is the pharyngeal arches and pouches (sometimes inaccurately called gill arches and gill slits). It is not advantageous for terrestrial animals with lungs to retain the gills that develop from the pharyngeal arches and pouches in fish species. The successful embryological development of terrestrial vertebrates, however, requires the organizing presence of these early structures.

For example, among other things, the arches give rise to facial bones, parts of the inner and outer ears, and the cartilage of the larynx.

The pouches give rise to the Eustachian tubes, and the thyroid and thymus glands.

Whereas Haeckel’s recapitulation theory was wrong in detail, recapitulation, as we now understand it, provides powerful support for Darwin’s ideas of common ancestry and descent with modification.

Because development is genetically controlled, small changes in developmental genes can have substantial implications for an individual’s juvenile and adult form and function, and on its eventual ability to survive and produce offspring of its own.
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Here is one of Haeckel’s summary diagrams, showing the similarity of how embryos develop, in fish, salamanders, tortoises, chickens, pigs, calves, rabbits and humans.

Haeckel_drawings

External resources

Ontogeny and phylogeny, Understanding Evolution

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