Many people believe that an embryo, while developing into a new individual, will retrace (recapitulate) the evolutionary history of that kind of creature. The idea is called recapitulation theory in which “Ontogeny [an individual’s development] recapitulates [retraces] phylogeny [evolutionary history].” This idea is appealing to evolutionists because it is visually so easy to communicate. A biologist and ardent supporter of Charles Darwin, Ernst Haeckel (1834-1919), drew various embryo stages of a number of organisms to illustrate his idea of a “biogenic law” or the recapitulation theory.
There were a few problems with this illustration however. Firstly, Haeckel chose organisms that best fit what he wanted to illustrate. This is called cherry picking. Secondly, he drew the embryo stages more similar to each other than they really are (some people call this a fake illustration). And thirdly he omitted earlier stages of development that are much different from each other (more cherry picking!) Also, in his selection of specimens, four out of eight are mammals which would be expected to be very similar in any case (more cherry picking).
Support for this idea of the recapitulation theory comes from people who assume that evolution has occurred, and then read that idea back into the embryological evidence. One of the claims of this theory that impresses many people, is the idea that early-stage human and other embryos display gill slits like a fish. This would represent an early fish-like stage in human evolution, for example. The situation however is that half way through development, all vertebrate embryos [with a backbone] display a series of folds in the neck region. The proper term is pharyngeal arches or ridges. Many interpret these as gills. But they are not even gills in fish that later develop gills! The folds are simply part of the process of bulking up tissue in the neck region. There is no embryological reason to call them gill-like.
A modern version of the recapitulation theory and Haeckel’s work, has served to keep most textbooks today still insisting that Haeckel was basically right. The texts still declare that the embryos of diverse vertebrates develop in similar ways and that this is clear evidence that they all evolved from a common vertebrate [with a backbone] ancestor. But the initial version needs updating.
Since each multicellular animal develops from a fertilized egg, it is evident that the early stages of development are an extremely important part of animal life. The recapitulation theory focuses on these early stages of each organism, from fertilized egg, through various stages towards the adult form. This theory holds that as evolution progressed and new body plans and lifestyles appeared, the early embryological processes stayed much the same. It was the later stages that showed greater change leading to the different vertebrate classes such as fish, amphibians, reptiles, birds and mammals.
Haeckel declared that the similarities in vertebrate embryos were the result of common descent with change (evolution). As such, the various parts of the embryos were all derived from similar parts in vertebrate ancestors. Thus the nervous system, for example, in any extant vertebrate, is homologous [evolutionarily related] to the nervous system of other vertebrate classes since they are all (according to the theory) the result of descent with change from the same common ancestor.
But today there is a new modern version of Haeckel’s theory. The problem was that the public were coming to realize that the embryos from the various vertebrate classes did not look much alike. This modern approach focuses on the “phylotypic stage,” a point in development in which the embryos from various classes look more closely alike than at other stages. Apparently in the 1990s, one specialist proposed that developing embryos from various classes converge on a point of similarity (the phylotypic stage) and then diverge again to their destined end body plans. It is the phylotypic stage, say the experts, that demonstrates the common ancestry of these groups. Thus, David Swift in a review paper on the topic (Biocomplexity) declared: “Contemporary evolutionary texts continue to present the similarities of the phylotypic stage as evidence of common ancestry, with no mention of the diversity preceding this stage.” [p. 2]
In keeping with the new interpretation, embryological development today is considered to display an hourglass pattern. First there is considerable diversity in the early stages which moderates to a narrow “waist” of similar structured shapes, followed by increasing differences again in the later stages of development. The key question then is what is the significance of the waist line stage of the pattern? Does it reflect true similarities or is this pattern merely of superficial similarities. Are these embryos really similar and thus indicative of common descent?
The definition of homologous in an evolutionary sense, is that a given feature in various organisms is derived from the same kind of tissue in a common ancestor. Based on this definition, we quickly see a problem here for modern evolutionary theory. The diversity in the early embryonic stages is highly significant. There is no common process of change. Thus, as David Swift describes the situation: “the diversity extends from the earliest stages: the very first cell divisions (cleavage) of the zygote (fertilized egg) occur in significantly different ways. And at later stages, key structures such as the neural tube (the beginning of the nervous system), gut, and even vertebrae (arguably the defining feature of vertebrates) form in substantially different ways.” [p. 2]
Specifically, we see this situation in the early embryonic stage, the blastula. Thus “the wide variety of the blastulas of different classes of vertebrate challenges the view that the resultant embryonic tissues can be considered equivalent or homologous.” [p. 7] This is also true for the next stage, the gastrula: “for all the major classes of vertebrates: the mechanism of gastrulation is significantly different from any of the others, even the source tissues of the germ layers are different.” [p. 3]
The definition of homology includes the idea of progressive change of one tissue in an ancestor into similar tissues with similar functions in subsequent generations. The existence of such homologous features is supposed to point to a common ancestor in the past. But in the case of vertebrate embryos, the body parts in the various classes are derived from vastly different initial tissues which follow different trajectories of development. Thus, as far as the idea for a phylotypic stage is concerned: “This apparent homology is refuted by more detailed embryological evidences, despite their similarities, the phylotypic stages are formed embryologically in profoundly different ways.” [p. 9] And “there is no doubt that the tissues that become the embryo are not equivalent, and hence are far from being homologous across the various vertebrate classes.” [p. 7]
So, what is the significance of the term “phylotypic stage”? It is a biased term used to suggest the reality of an evolutionary process. The term is meant to rescue the recapitulation theory from the obvious fact that vertebrate embryos are not at all alike. The evidence does not support a conclusion of descent with modification but rather points to separate body plans, separate designs, and separate creations.
It is evident that Haeckel’s biogenic law is without foundation and wishful thinking. Nevertheless for various reasons, the overall idea is still appealing and popular. These are not good reasons to promote a false view of origins. The textbooks therefore are communicating false information and need to be revised.