Why do human embryos look like fish?

An organism’s development, or ontogeny, may contain clues about its history that biologists can use to build evolutionary trees.

Ancestral characters are often, but not always, preserved in an organism’s development. For example, both chick and human embryos go through a stage where they have slits and arches in their necks like the gill slits and gill arches of fish. These structures are not gills and do not develop into gills in chicks and humans, but the fact that they are so similar to gill structures in fish at this point in development supports the idea that chicks and humans share a common ancestor with fish. Thus, developmental characters, along with other lines of evidence, can be used for constructing phylogenies.

Not recapitulation

In the late 1800s some scientists felt that ontogeny not only could reveal something about evolutionary history, but that it also preserved a step-by-step record of that history. These scientists claimed that ontogeny recapitulates phylogeny (ORP). This phrase suggests that an organism’s development will take it through each of the adult stages of its evolutionary history, or its phylogeny. At the time, some scientists thought that evolution worked by adding new stages on to the end of an organism’s development. Thus its development would reiterate its evolutionary history — ontogeny recapitulating phylogeny.

This idea is an extreme one. If it were strictly true, it would predict, for example, that in the course of a chick’s development, it would go through the following stages: a single celled organism, a multi-celled invertebrate ancestor, a fish, a lizard-like reptile, an ancestral bird, and then finally, a baby chick.

This is clearly not the case — a fact recognized by many scientists even when the idea of ontogeny recapitulating phylogeny was introduced. If you observe a chick’s development, you will find that the chick embryo may resemble the embryos of reptiles and fish at points in its development, but it doesn’t recapitulate the forms of its adult ancestors.

Even on a smaller scale, ORP is often untrue. For example, the axolotl evolved from a salamander ancestor that had internal gills in the adult stage. However, the axolotl never develops through a stage with internal gills; its gills remain external in flagrant violation of ORP.

If ORP were completely true, it would certainly make constructing phylogenies a lot easier. We could study an organism’s development and read its history directly. Unfortunately, phylogeneticists are out of luck here.

Next

Constraints on evolutionary change

An international team of biologists has shed light on why all vertebrate animals look alike during the phylotypic stage of embryo development.

Led by The University of Western Australia (UWA) in collaboration with the Spanish National Research Council and Radboud University, the team’s discovery has been published in the journal Nature Genetics.

During the phylotypic stage, embryos of birds, fish and even humans start to look the same — before they diverge again and become very different looking animals. The similarity was first described by 19th-century embryologist Karl Ernst von Baer, who accidentally mixed up phylotypic-stage embryos of different vertebrate species and was unable to differentiate between them.

“If you were to put a human embryo next to a fish, a toad and a mouse at that stage, the human embryo would look very much like the others,” said lead author Dr Ozren Bogdanovic, from the ARC Centre of Excellence in Plant Energy Biology at UWA. This is despite the time taken to reach the phylotypic stage varying between species, occurring 1–2 days after fertilisation in fish and toads, 9.5 days after conception in mice and four weeks after conception in humans.

Examining mice from Madrid, fish from Seville and toads from Nijmegen, the team studied the epigenome — an extra layer of information present in cells that is made up of millions of miniscule chemical tags attached to the DNA — which can switch genes on or off to trigger the correct development of early embryonic structures.

The researchers used powerful genome analysis technologies to precisely map the location of these chemical signposts in order to better understand the epigenetic process of development. Dr Bogdanovic explained, “By looking at early-stage embryos of different species, we were able to find the existence of multiple epigenetic switches that appear to be critical for limb formation or brain development.

“The switches change similarly in all these different organisms, even though they’re separated by hundreds of millions of years of evolution.”

The researchers believe vertebrates have a similar type of epigenetic control during this period because that is when the fundamental structure of the body is being set up. According to Professor Ryan Lister, also from the ARC Centre, “Correct establishment of the body plan and organ formation at that early stage is so critical to life that the molecular processes underlying it have remained very similar despite millions of years of divergence between these species.”

The discovery is especially significant, noted Professor Lister, as it opens “a window onto the processes that likely occur during human embryo development” — processes that cannot be studied directly during such development.

Professor Lister added that the research could eventually be used to screen for potential epigenetic aberrances associated with early development.

“Investigating these processes is essential in order to understand the potential repercussions when they don’t take place correctly,” he said.

Image caption: Dr Ozren Bogdanovic and Professor Ryan Lister. Image credit: ARC CoE Plant Energy Biology.

Editor’s Note: First published in St. Louis MetroVoice 3, no. 12 (December 1993).

Almost from the beginning, evolutionists have attempted to equate the process of evolution with the progressive development of the embryo. During the famous Scopes “Monkey Trial” in 1925, for example, lawyers and expert witnesses defending teaching Darwinism in public schools, repeatedly confused evolution with embryology. The lawyers even insisted that evolution must be taught if physicians are to understand the development of babies in the womb! The very word “evolution” (which means “unfolding”) was taken from the name of an early theory of embryonic development which proposed that humans are completely preformed in miniature in the fertilized egg, simply “unfolding” during the development of the baby. Obviously, the blind chance process of Darwinian “evolution” has nothing whatever to do with the exquisitely controlled process of embryological development. Still, evolutionists have long attempted to relate embryology to evolution, presumably in an effort to extrapolate the readily observable process of embryonic development into the unobservable process of macroevolution. Embryology continues to play a role in current evolutionary dogma. Generations of students have been told, for example, that the human embryo developing in the womb passes through stages of its evolutionary ancestry—even at one point having gills like a fish!

Embryology continues to play a role in current evolutionary dogma.

Like most students of biology, I was required to memorize the “biogenetic law” which states that “ontogeny recapitulates phylogeny.” This means that the developing embryo (ontogeny) of each vertebrate species retraces (recapitulates) its evolutionary history (phylogeny). Specifically, each embryo in the course of its development, is said to pass through a progression of abbreviated stages that resemble the main evolutionary stages of its presumed ancestors.

Thus, in the case of the human embryo, recapitulation scenario goes something like this: 1) The fertilized egg starts as a single cell (just like our first living evolutionary “ancestor”). 2) As the fertilized egg repeatedly divides, it develops into an embryo with a segmented arrangement (the “worm” stage). 3) These segments develop into vertebrae, muscles and something that sort of looks like gills (the “fish” stage). 4) Limb buds develop with paddle-like hands and feet, and there appears to be a “tail” (the “amphibian” stage). 5) By about the eighth week of development, most organs are nearly complete, the limbs develop fingers and toes, and the “tail” disappears (the human stage). Now the mother can finally claim the baby as her own, or at least one of her own species. This ludicrous scenario has actually been used as a justification for abortion—after all you are only killing lower animals!

The “biogenetic law” was first promulgated in the late 1800s by the German biologist Ernst Haeckel, a committed disciple of Darwin. Impressed by the general similarity among vertebrate embryos, Haeckel chose to ignore their differences. (Haeckel was a scientific charlatan who even stooped to publishing two copies of the same woodcut side by side to demonstrate the “remarkable similarity” between human and dog embryos!) Haeckel’s “law” was shown to be unsound by many of the most distinguished embryologists of his own day, but its appeal to evolutionists was so great that it remained impervious to scientific criticism. In her book Essays in the History of Embryology and Biology (MIT Press, 1967, p. 150), Jane Oppenheimer said that the work of Haeckel “was the culmination of the extremes of exaggeration which followed Darwin.” She lamented that “Haeckel’s doctrines were blindly and uncritically accepted” and “delayed the course of embryological progress.” Embryologist Erich Blechschmidt considered Haeckel’s biogenetic “law” to be one of the most serious errors in the history of biology. In his book The Beginnings of Human Life (Springer-Verlag, Inc., 1977, p. 32), Blechschmidt minced no words in repudiating Haeckel’s “law”:

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.

We could ignore this whole sorry chapter in the history of evolutionism, were it not for the fact that the biogenetic “law” is still being taught as a fact in our public schools! Of 15 high school biology textbooks being considered for adoption by the Indiana State Board of Education in 1980, nine offered embryological recapitulation as evidence for evolution.

Evolutionists themselves have conceded that the biogenetic “law” has become so deeply rooted in evolutionary dogma that it cannot be weeded out. For example, Paul Ehrlich said “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). Even Dr. Benjamin Spock saw fit to perpetuate Haeckel’s recapitulation myth in his well-known book, Baby and Child Care (Cardinal Giant Edit, 1957, p. 223). Spock confidently assured expectant mothers that:

Each child as he develops is retracing the whole history of man-kind, physically and spiritually [sic], step by step. A baby starts off in the womb as a single tiny cell, just the way the first living thing appeared in the ocean. Weeks later, as he lies in the amniotic fluid of the womb, he has gills like a fish.

It is a well-established fact that the human embryo (like all mammalian embryos) never has gills in any sense of the word. The fanciful notion of gills is based upon the presence of four alternating ridges and grooves in the neck region of the human embryo (called pharyngeal arches and pouches) that bear a superficial resemblance to gills. While similar arches do give rise to gills in certain aquatic vertebrates such as fish, their developmental fate in mammals has nothing to do with gills or even breathing. In man and other mammals, these arches and pouches develop into part of the face, muscles of mastication and facial expression, bones of the middle ear, and endocrine glands.

No false biological statement has had a longer or more popular life than the one about the ontogeny of the four-chambered heart.

The embryological development of the heart has been another popular textbook example of embryonic recapitulation, and thus “proof” of evolution. Evolutionists argue that as the human heart develops, it goes from a two-chambered “fish heart” to a three-chambered “amphibian heart” and, finally, forms the four-chambered mammalian heart. In his book Comparative Anatomy and Embryology (Ronald Press, 1964, p. 509), William Ballard said, “No false biological statement has had a longer or more popular life than the one about the ontogeny of the four-chambered heart.” Ballard pointed out that “in real life, all vertebrate hearts are composed of the same four chambers at the pharyngula stage.” As the heart develops, these four chambers become specialized in different ways that are uniquely suited to the demands of aquatic, amphibious, or terrestrial life.

Embryologists are now aware that the embryos of each species of animal are unique and dynamically functional systems. The human embryo does not become human at some point during its development; rather it is uniquely human at every stage of its development. While scientists continue to learn much about the marvelous process of development in the embryo, the inspired words of King Solomon (Ecclesiastes 11:5, NIV) remain true:

As you do not know the path of the wind, or how the body is formed in a mother’s womb, so you cannot understand the work of God,

the Maker of all things.