Embryos Tell a Different Story — Often Not Mentioned by Science
- What is the idea behind saying that "ontogeny recapitulates phylogeny"?
- Does "ontogeny recapitulate phylogeny" ... does it really? Or is this simply a misguided slogan? [And if misleading, why is this concept still taught to biology students?]
- What examples are there where evidence from embryology counters this popular conception of evolution?
This feature article really needs a plain language 'short answer' because it gets into a technical topic. So take note, as elsewhere within the WindowView, we are taking a first glimpse at a topic that has broader implications. Consider this short answer a starting point and that a problem uncovered here cuts through long-standing assumptions. Additional explorations help to reveal how this further supports themes presented in other feature articles within the WindowView.
Does "ontogeny recapitulate phylogeny" ... does it really?
The concept that embryology can illustrate an evolutionary history in the developing stages of many (vertebrate) embryos is not new. We are told human embryos recapitulate or follow the stages of development in a sequence covering all the evolutionary steps of animals preceding our species. A dictionary definition directly reflects this notion [recapitulate: "Biology. To appear to repeat (the evolutionary stages of the species) during the embryonic development of the individual organism." American Heritage Dictionary, 1994]. So, for decades, biology instructors and professors have been telling students: "ontogeny recapitulates phylogeny." [ontogeny: origin and development of an individual organism from embryo to adult; and phylogeny: evolutionary development and history of a species or higher taxonomic grouping of organisms]
The short answer is: This concept seems logical but it is incorrect. The 'logical part' goes to the credit of assumption. A closer look at embryos of a number of organisms and the illustrations used to promote the concept fail the slogan that's been taught for so many decades. We'll not delve into every detail, but references given below lead to detailed descriptions of why and how this all came about. Without this slogan, one of evolution's supporting icons or cornerstones is removed.
Our main purpose in visiting embryology is to expand the view to yet another area where a closer examination is warranted. If assumptions are unsupported, then it's time to recognize new questions that surface ... especially as errant concepts are eliminated.
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This story really starts with comparative embryology. And Charles Darwin is at least one source behind the slogan that 'ontogeny recapitulates phylogeny.' Let's take a look at how that happened:
Darwin considered some of the best evidence for his theory to be the striking resemblance of vertebrate embryos at an early stage of their development. He wrote ... "the embryos of mammals, birds, fishes, and reptiles" are "closely similar, but become, when fully developed, widely dissimilar." He argued that the past explanation for their embryonic similarity was that such animals "are the modified descendants of some ancient progenitor." According to Darwin, "the embryonic or larval stages show us, more or less completely, the condition of the progenitor of the whole group in its adult state" (Darwin 1936, 338, 345).
Darwin believed that evolutionary changes tend to occur in the later stages of development and are gradually pushed back into embryogenesis, with the result that the embryonic development bears the imprint of past evolution (in Ernst Heackel's words, "ontogeny recapitulates phylogeny"). The doctrine of recapitulation fits so nicely with Darwin's theory that it has endured to the present and can be found in many modern biology textbooks. But it was clear to embryologists even during Darwin's lifetime that it did not fit the facts. Nineteenth-century embryologist Karl Ernst von Baer pointed out that although vertebrate embryos resemble each other at one point in their development, they never resemble the adult of any species, present or past. The most that can be said is that embryos in the same major group (such as the vertebrates, which include fishes, reptiles, birds and mammals) tend to resemble each other at a certain stage before they develop the distinguishing characteristics of their class, genus and species (Gould 1977; Hall 1992; Raff 1996). Wells (MC) Page 58
The following diagram is found in a 1924 edition of a biology textbook. This is the widely published Haeckel illustration that appears to align embryos at parallel points in a particular species embryological development with other similar, parallel, stages in other vertebrate species. What would you think of evolution theory if this illustration were to amount to a slight of hand ... like pulling a rabbit out of a hat? If illusory, then at the very least we'd ask for a more objective assessment. What does the embryological evidence tell us when taken at face value, without assumption, and without any preconceived notion of how evolution might be revealed?
The diagram above incorporates images that are made to look more alike than biological reality. For this, Haeckel and his illustration received criticisms during his professional career. There are various aspects to the misrepresentation he has assembled. For example, half of the embryos illustrated are mammals from one order, yet other distinct vertebrate orders are clearly omitted. Might we say Haeckel dealt from a stacked deck?
Again, there is something of a debate that whirls about this diagram and evidence or views that both support and undercut the validity of the concept built into the illustration. A considerate treatment of this topic appears in Dr. Wells' book Icons of Evolution (< bookstore link). To read this account in its entirety adds both context and sufficient detail to reveal the serious nature of the issues raised to our attention here. And be aware, the focus is on embryology that in a most objective sense is now revealing scientific evidence that is powerful enough to correct misguided assumptions concerning Darwin's theory. And if embryology tends to indicate we are not descended from a common ancestor, then let future research investigate where that leads us!
Look at what Darwin did with this type of 'evidence,' and see where it ends up. The initial idea is ...
... similarities in early embryos not only demonstrate that they are descended from a common ancestor, but also reveal what that ancestor looked like. Darwin considered this "by far the strongest single class of facts in favor of" his theory. Wells (IC) Page 81-82.(See reference palette listing of sources)
Haeckel's embryos seem to provide such powerful evidence for Darwin's theory that some version of them can be found in almost every modern textbook dealing with evolution. Yet biologists have known for over a century that Haeckel faked his drawings; vertebrate embryos never look as similar as he made them out to be. Furthermore, the stage Haeckel labeled the "first" is actually midway through development; the similarities he exaggerated are preceded by striking differences in earlier stages of development. Although you might never know it from reading biology textbooks, Darwin's "strongest single class of facts" is a classic example of how evidence can be twisted to fit a theory. Wells (IC) Page 82-83.
Whether or not Haeckel was guilty of fraud—that is, deliberate deception—there is no doubt that his drawings misrepresent vertebrate embryos. First, he chose only those embryos that came closest to fitting his theory. ... Wells (IC) Page 91
One more comment on Dr. Wells' book on icons. In his Chapter 5 (Haeckel's Embryos) you'll not only find the critical points noted here, but also a more even handed and broader treatment of the topic. For example, in place of the diagram above, Dr. Wells includes another diagram to illustrate what the real "first" stage looks like. This revels a more diverse array of forms in place of Haeckel's set of rather similar looking embryos. Furthermore, Dr. Wells adds a diagram that goes from fertilized egg to adult organism. Four stages in-between reveal markedly different developmental sequences. Finally, Dr. Wells illustrates what he calls the developmental hourglass and in the discussion notes how "one would presumably conclude that the various classes of vertebrates are not descended from a common ancestor, but had separate origins." (Wells (IC) Page 101).
Embryology starts with studies from the unfertilized egg cell all the way to the full development of the adult form of each species. It's time to remember to include the beginning of the story as well as the stages down the line ...
As you read here, keep in mind that there are other interesting problems that arise from a closer look at data from embryology. Although not examined on this web page, recent symposia and related publications challenge the concept that random genetic mutations occurring during early embryo development lead to evolutionary innovations. While some scientists say this must be the case, the intricate nature of an embryo's early development illustrates how random mutations end up with lethal results. So where is the advancement in evolution? Further discussion of this aspect of embryology will be addressed in another feature article [A link will be added here in the future] based on comments made by Drs. Meyer and Nelson at the Yale Symposium held November 2000.
Why Not Start With A Look Earlier In The Embryological Sequence?
How often are we given a description of the evolution of species from a starting point long after life's origin. As we've already noted in the Science Area narrative, starting at the beginning—before life's appearance on earth—creates all kinds of problems for evolution theory. In fact, it's difficult to conceive of biological evolution even 'getting off the ground.' So, often the story starts with life as a given. Well, a similar problem in building an objective viewpoint exists for the world of embryology. If we look at the evidence from embryology we don't get the smooth transition into evolution's assumed mainstream. The idea of recapitulation jumps past an entire field of evidence:
Darwin and his followers ignored these difficulties, however, and the modern synthesis excluded embryology entirely. Only in the past twenty years, with the rise of developmental genetics, has comparative embryology attracted significant interest from evolutionary biologists. One result of this renewed interest has been the recognition that patterns of early development of do not fit the Procrustean bed of recapitulationism. Wells (MC) Page 58
We've noted previously that advances in science fill in gaps from Darwin's day. Again, as stated in another article within WindowView, were Charles Darwin alive to consider present day evidence, his perspectives and theory would differ. He might be the first to set aside much of what he assumed previously. Microscopy and genetics were advanced much further in the century to follow Darwin's theory. The evidence of today is clearly graphic to the extent of drawing very different conclusions ... in spite of the illustrations and apparent similarities.
After fertilization, animal embryos first undergo a process called cleavage, ...
Each major group of animals follows a distinctive cleavage pattern; among vertebrates, for example, mammals, birds, fishes and reptiles cleave very differently.
Animal embryos then enter the gastrulation stage, during which their cells move relative to each other, rearranging themselves to generate basic tissue types and establish the general layout of the animal's body.
Like cleavage patterns, gastrulation patterns vary markedly among the major groups of animals, including the different classes of vertebrates (Elinson 1987).
Only after gastrulation do the embryos of mammals, birds, fishes and reptiles begin to resemble each other.
The embryos of mammals, birds and reptiles never possess gills. Wells (MC) Page 59
Let's review a moment by being admittedly simplistic. If a logical sequence of developments were to describe evolution's course through time, then the embryos of each type of organism—from each new phyla that appeared over time—would come from some unifying characteristics. The patterns in embryo development would serve like a trail of crumbs. Follow the trail back and you get to the earlier and logical ancestral organisms ... and thus the earlier phyla that appeared on earth.
For example, the way an egg cell starts to divide shortly after being fertilized might change somewhat over time as more complex phyla arise, but something in all the patterns of the way all embryos develop would still serve as evidence for evolution's trail. So, is the trail there to be found by scientists? As cells divide, some will eventually turn into key parts of the organism, such as a fore or hind limb. In the different phyla, from the earlier to latter appearing groups, one might expect forelimbs to be homologous structures. That is, their embryological origin ought to be from the same source within the developmental sequence. But is that true?
Homologous structures are often specified by non-homologous genetic systems and the concept of homology can seldom be extended back into embryology. The failure to find a genetic and embryological basis for homology was discussed by Sir Gavin de Beer, British embryologist and past Director of the British Museum of Natural History, in a succinct monograph Homology, an Unresolved Problem.
In some ways the egg cell, blastula and gastrula stages in the different vertebrate classes are so dissimilar that, were it not for the close resemblance in the basic body plan of all adult vertebrates, it seems unlikely that they would have been classed as belonging to the same phylum. There is no question that because of the great dissimilarity of the early stages of embryogenesis in the different vertebrate classes, organs and structures considered homologous in adult vertebrates cannot be traced back to homologous cells or regions in the earliest stages of embryogenesis. In other words, homologous structures are arrived at by different routes. Denton (ETC) Page 145
Well then, can we at least say in some way genetic information reveals a common root for homologous structures? Perhaps the source information holds to some unity while the developmental paths to get to the structures has simply changed.
The evolutionary basis of homology is perhaps even more severely damaged by the discovery that apparently homologous structures are specified in quite different genes in different species. The effects of genes on development are often surprisingly diverse. Denton (ETC) Page 149
Levels of complexity within biological beings seem to wave a big flag and say 'Evolution isn't a simple one gene one result process.' Embryology presents a matrix of genetic and structural relationships. How can one little adjustment in the matrix of related genetic relationships be correspondingly accommodated by all other parts of the matrix without actual changes throughout the entire matrix—through to the endpoint of the final development of form and structure. This question alone suggests we just can't arrive at complexity of this sort in a simple stepwise fashion. A coordinated multiple gene change would be required for any one genetic mutation—yet multiple 'coordinated' random mutations would be the rarer case.
Almost every gene that has been studied in higher organisms has been found to affect more than one organ system, a multiple effect which is known as pleiotropy. Denton (ETC) Page 149
What options are their for a response to embryological evidence against the standard assumptions supporting evolution theory?
Since 1980, with the rapprochement between developmental and evolutionary biology, there has been a growing recognition that the early stages of embryogenesis contradict Darwin's view. Rather than seeing a embryological evidence as a threat to evolutionary theory, however, Darwinian biologists now welcome it as a new source of support. Their reasoning goes something like this: major evolutionary changes require major changes in development; if similar morphologies (such as pharyngula) can be reached by very different developmental pathways, as in the vertebrates, then early development must be more changeable than we thought; since early development is so changeable, then large-scale evolution must be relatively easy to achieve. Wells (MC) Page 60
Yet the difficulties are hard to ignore, especially where the problems facing the evolutionists run deep.
On turning to the observations, however, we find striking differences in early development (see figure 6.2 for some examples strictly within the vertebrates). Eric Davidson (1990, 365) remarks on these differences, calling them "anything but trivial and superficial," ... Nelson (MC) Page 154
And again (Davidson 1994, 604):
The initial embryonic specification strategies employed by organisms of different phylogenetic groups actually differ from one another to a surprising extend, however. In some embryos, specification depends on intracellular interaction during cleavage, while in others this cannot be so since specification occurs while the nuclei are syncytial; some rely on invariant cell lineages, while others develop from populations of migratory cells of no fixed of lineage; some generate autonomously specified founder cells, while others have none; and so forth. Nelson (MC) Page 155
Note that there are differences in embryos to be observed concerning the role of different groups of cells. Some stay relatively fixed in place while others move or migrate about to a place for their continued and fateful biological development. Embryologists have made studies in great detail to follow each new cell division and to track the destiny of each and every cell as it gives rise to tissues, then organs or appendages. If there are differences in the lineages or 'cellular pathways,' so to speak, then the avenues of development reveal a corresponding dissimilarity. If not, then what continuity is here to speak of a common ancestry?
Recall also, as we have just noted, that early development is strikingly diverse in the animals. How then did these remarkable differences evolve from a common ancestor? Nelson (MC) Page 156
To add another voice to the discussion on embryological recapitulation, points made by Dr. Wise restate as well as add to what we've noted thus far:
One evidence of macroevolution popularly heralded in the latter part of the nineteenth century was embryological recapitulation. According to this hypothesis, as organisms develop from a fertilized egg, they pass through stages very similar to the evolutionary stages of their ancestors. Each human, for example, starts as a single cell—as all life supposedly did—then develops through a wormlike stage, then a fishlike stage (complete with gill slits), then a froglike staged, then a stage in which there is a tale, and finely this stage of a human child. In this way and organism's embryology (development) recapitulates (briefly repictures) its phylogeny (evolutionary history). Wise (CH) Page 215
Third, although developmental stages appeared to be broadly similar to earlier evolutionary stages, when examined closely the similarities break down. In human development, for example, the fertilized egg is a diploid eukaryotic cell with twenty-three chromosome pairs—not a haploid prokaryotic cell with a single strand of DNA from which we supposedly evolved. In a like manner, this similarity between human developmental stages and worms and frogs breaks down very quickly upon close examination. The so-called gill slits in human development are not gill slits, and did the "tail" in human development is not actually a tail. These things only bear superficial resemblance to those structures.
Fourth, in many cases development and runs through stages in "incorrect" order for phylogeny. In sum, embryological recapitulation suffers from too many difficulties to be considered a viable theory. It has been rejected by a number of evolutionary biologists and has been expunged from the textbooks over the years. Wise (CH) Page 216
There is a lot more to the storyline emerging here. This in part is why we've recommended a book for further reading and drawn comments from several other sources. And further, as noted in the side bar above, t here are other interesting problems that arise from a closer look at data from embryology. Again there appear to be serious problems related to assuming that random mutations occurring during embryo development lead to evolutionary innovations. The intricate nature of an embryo's early development illustrates how random mutations end up with lethal results. Further discussion of this aspect of embryology will appear in a future feature article here within the WindowView [When ready, a link will be added here]. And as noted above, this article will be based on comments made by Drs. Meyer and Nelson at the Yale Symposium held November 2000.
What else is to be made of embryological evidence. The various articles within the WindowView have done more than to simply hint that more is afoot in nature than chance events. The specificity we see in biological systems in drawing more and more attention to the design that is inherent in these natural systems. And if the design reflects an intelligence and is observed in more and more unique locations throughout the natural world, then design becomes an ever larger possibility. That this has been repeatedly ignored is insufficient as a disproof for the existence of design. Again, evolution theory is eloquent in its simple explanations, until the complex and specific examples unveil elements of intelligent design. One is explained by chance, the other stands where chance events fail as an explanation. So, we will leave you with a hint of the what comes by building the window's larger view. A natural perspective is the norm. Opening the floor to discussing design is a growing part of a new dialog.
From a naturalistic perspective, development of supposedly proceeds mechanistically, like a ball rolling down a hill. Changes in developmental pathways are likely to produce alterations in the final outcome, and the earlier the changes, the more drastic the alterations.
From a design perspective, however, it is possible to regard development as an end-directed process. If organisms are designed, which is to say produced according to a preconceived plan, then in some sense their final form precedes their embryonic development. Wells (MC) Page 61
If you are new to the Science Area feature articles within WindowView, then we respectfully ask that you keep in mind two very different perspectives—one based on naturalism and the other on design. And then read on to see what happens with these different vantage points.
If you have been reviewing the Science Area narrative, skim reading or exploring the details in the feature articles that precede this one on embryology, then a broader picture is already in view. It's not entirely important to declare design is wholly a valid concept or fact at this stage of the game—not needed to get the main point concerning assumptions rooted in the work of Haeckel and Darwin. However, based on the information above and by reading the source material upon which this article is based, there is indeed a consistency with the issues discussed throughout the entire Science Area. Design may be a new term in the biological vocabulary to many of our readers, but by all appearances, this is a term that's here to stay.
Quotations from Dr. Michael Denton's "Evolution: A Theory in Crisis" are used by permission of Adler and Adler Publishers Inc., 5530 Wisconsin Ave, Suite 1460, Chevy Chase, MD 20815
Quotations from "Mere Creation" (MC) edited by William A. Dembski are used by permission of InterVarsity Press, P.O. Box 1400, Downers Grove, IL 60515. www.ivpress.com All rights reserved. No portion of this material may be used without permission from InterVarsity Press.
Quotations from "The Creation Hypothesis" (CH) edited by J. P. Moreland and "Mere Creation" (MC) edited by William A. Dembski are used by permission of InterVarsity Press, P.O. Box 1400, Downers Grove, IL 60515. www.ivpress.com All rights reserved. No portion of this material may be used without permission from InterVarsity Press.
Writer / Editor: Dr. T. Peterson, Director, WindowView.org