Haeckel had a point

In 1874, a German comparative embryologist and Darwin enthusiast, Ernst Haeckel, began publishing his drawings of vertebrate embryos in various stages of development to explain common ancestry and support Darwin’s theory of evolution. Haeckel drew the first phylogenies and coined the term “tree of life.” His skillful and detailed drawings clearly showed that the early-stage embryos of vertebrates are nearly if not wholly identical. With his drawings and accompanying descriptions, Haeckel promoted the idea that “ontogeny recapitulates phylogeny,” also called the biogenic law. The biogenic law states that the path of an organism during its embryological development (ontogeny) is a summary of its evolutionary history (phylogeny). For example, a human embryo has structures that resemble gill slits, just like a fish. The human embryo then loses the gill slits and grows a tail and four limbs, resembling a reptile. The tail disappears, and the embryo begins resembling a primate. Haeckel tirelessly promoted his biogenic law, and many biologists in the nineteenth and into the twentieth centuries embraced it as a simple explanation of evolution. How convenient that all we had to do was look at embryological development to understand an organism’s evolutionary history!

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, yet few rejected the biogenic law outright. One of the first written criticisms of the biogenic law appeared in an 1894 article by zoologist Adam Sedgwick in the Quarterly Journal of Microscopic Science. Sedgwick argued that the biogenic law conflicted with a principle known as von Baer’s law, after Karl Ernst von Baer, one of the founders of embryology. Von Baer had noted, contrary to Haeckel, that the embryos of higher animal forms resembled the embryos, not the adults, of earlier forms. Thus, for example, a human embryo may pass through a stage where it resembles a fish embryo, but not an adult fish. Sedgwick noted, “Embryos of different members of the same group are more alike than the adults, and the resemblances are greater the younger the embryos examined.” He continued that when the actual embryos were examined “a blind man could distinguish between them.”

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.

Haeckel was not wholly wrong. As we will see below, 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. Whether Haeckel deliberately fudged his drawings remains a matter of debate. Unfortunately, the creationist Jonathan Wells, in the book Icons of Evolution [discussed in Chapter 3], 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

In its original form, the idea that “ontogeny recapitulates phylogeny” is not an accurate evolutionary statement. 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 (see below) 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. For example, the notochord is a character that provides internal structural support and unites all members of the phylum Chordata (or chordates, animals with notochords and pharyngeal arches, among other characteristics). At some point in development, every species within Chordata, from sea squirts to sea lions, possesses a notochord. Most members of the chordate phylum, those in the subphylum Vertebrata, replace the notochord with vertebrae (the backbone) later in development to support the pelvic and pectoral girdles (to which the front and hind limbs are attached) and to protect the spinal cord. Yet in vertebrates, as the notochord is forming (during a developmental process called gastrulation), it first functions to establish the midline of the embryo. Once the notochord forms, its cells send molecular signals to neighboring cells, inducing those cells to begin developing divisions of the central nervous system and eventually the brain and spinal cord. Thus, without the notochord functioning as an organizer in an early vertebrate embryo, the development of the entire central nervous system of the individual is compromised. In the two other subphyla of the chordates, Cephalochordata and Urochordata, the notochord is either maintained in the adult (Cephalochordata such as Lancelets) or present in the larval form and lost during metamorphosis into the adult form (Urochordata such as Tunicates).

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. Proper interaction between the arches and pouches is required for normal embryological development. For example, a mutation on chromosome 22 in humans allows an improper association between these early structures and results in the thymus failing to develop. The resulting syndrome, DiGeorge syndrome, causes recurrent infections, heart defects, and facial bone malformation.

Conclusion

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.

In the next chapter, we will introduce and describe a new field of biology that shows, through the biology of genes, how an individual passes from a single-celled egg to a multi-celled adult and how a fish journeys through evolutionary time to eventually arrive as a human. This is the field of evolutionary developmental biology, or Evo Devo.

I am very sorry, but if you want to read the next chapter, you will have to buy the book.

A similar argument was presented briefly here in the text and the comments.

Chapter 11 of Why Evolution Works (and Creationism Fails), by Matt Young and Paul K. Strode. Copyright © 2009 by Matt Young and Paul K. Strode. All rights reserved. Reprinted with permission.