Embryology

The new field of "evo-devo"–an integration of evolutionary biology with our growing understanding of embryonic development–is an exciting a fruitful area of intense scientific research. A book purporting to "explore evolution" would do well to address this exciting field, yet the discussion in Explore Evolution is mired in disputes about what Darwin thought about embryos 150 years ago, and the legitimacy of illustrations by Ernst Haeckel 100 years ago. Current work that shows how the developmental process can evolve, and how that understanding deepens our understanding of the common ancestry of modern species, goes unmentioned.

Instead, Explore Evolution obsesses over whether certain illustrations of embryos from textbooks are or are not legitimate, and whether actual photographs of embryos confirm or disconfirm comments made by Darwin or by Haeckel. Modern evolutionary biology does not stand or fall on the views of people a century ago, but on the current evidence and research, research and evidence the book omits.

p. 66: "Darwin thought… similarities in … embryos revealed what the[ir] ancestors would have looked like."

This view that "ontogeny recapitulates phylogeny" was stated by Haeckel, not Darwin, and historians generally agree that Darwin did not accept that view. Darwin, like modern evolutionary biologists, preferred a much broader view advanced by von Baer: that characters acquired earlier in a species' evolution tend to develop earlier in an embryo.

Yolked into it: Removing yolk from photographs of embryos makes them look much more like Haeckel's drawings.Yolked into it: Removing yolk from photographs of embryos makes them look much more like Haeckel's drawings.

p. 66: Haeckel … show[ed] that [vertebrate] embryos … were very similar during their earliest stages.

Historians have shown that Haeckel meant something different by "earliest stages" of an embryo than modern authors, but the book uses the confusing translation to create a straw man. Haeckel was aware of divergence in early stages of embryos, and like modern embryologists, was aware that these divergences are driven in part by factors like the amount of yolk, not by fundamentally different developmental processes.

p. 69: "[Haeckel's] pictures were faked and the facts were distorted."

Historians investigating Haeckel's drawings have shown that he used the best illustrations available at the time, and continued to update the illustrations in his books as better ones became available. Comparisons to Richardson's photographs are flawed because Richardson, unlike Haeckel, did not remove the yolk from all of his embryos. This distorts the shape and outline of the embryos and makes it difficult to compare lineages with the yolk removed and those without. As one prominent Haeckel biographer concludes: "fraud not proven."

p. 68: "In 1894, Adam Sedgwick…challenged Darwin's two claims…"

Sedgwick's essay is not a reply to Darwin, as shown by the title of the cited essay: "On the Law of Development commonly known as von Baer's Law." Darwin, who died 12 years before Sedgwick's essay was published, did accept von Baer's results and, like modern biologists, found them bourn out by his own experience. To attribute those laws to Darwin, or to claim Sedwick was responding to Darwin, is simply false.

p. 68: "chickens and ducks[] display specific differences very early in development."

Researchers find that, other than a small difference in the rate of development, early chicken and duck embryos are "nearly identical."

p. 68: Sedwick claims: "There is no stage of development in which the unaided eye would fail to distinguish [chicken and shark]."

Sedwick acknowledges that these embryos are "superficially" not similar, but notes "striking similarities" of embryos, shared traits "which the adults do not exhibit." Rather than relying on a single paper from over a century ago, a truly inquiry-based book would lay out those structures and encourage students to explore their evolutionary implications.

p. 69: "This error [in Haeckel's drawings] remains in many modern high school and college biology textbooks."

Most textbooks in use today do not show Haeckel's drawings. They either use photographs, or redraw the embryos to correct errors. Evolutionary biologists' agreement about evolution's impact on development is hardly driven by a reliance on century-old illustrations, but on their experience with actual embryos in the lab. Explore Evolution gives students no understanding of that research.

p. 71: "To explain … adaptations in embryos, … evolutionary biologists invoke 'macromutations'"

This is simply false. Richard Goldschmidt's ideas were never widely accepted when he proposed them in the 1940s, and play no role whatsoever in modern evolutionary developmental biology. Explore Evolution later acknowledges this rejection in a passage plagiarized from a creationist website.

p. 70: "Darwinists have effectively made it impossible to challenge the theory with counterevidence"

Embryological evidence can and does challenge specific hypotheses of common ancestry. They also know that explanations of embryonic morphology must incorporate our knowledge of common ancestry and the species' immediate adaptive history. Finding that embryos differ because of adaptions specific to different environments is a prediction of evolution, not a falsification of it.

Major Flaws:

History of embryology: Rather than focusing on modern evolutionary developmental biology, this chapter rehashes arguments from the 19th century. Darwin's own views are serially misrepresented. A single essay from 1894 is transformed into a scientific attack on Darwin and evolution, despite the original author's narrower focus and the subsequent research which has invalidated his claims. Errors in a century-old illustration are dolled up as a evidence that the illustrations, and all of evolutionary developmental biology since, are frauds. This despite extensive research by biologists and historians showing that the illustrations were the best available at the time, and that the evolution of development is well-documented.

Philosophy: The authors misrepresent science by comparing the scientific process to how a jury operates. Students cannot be a jury if they are not given adequate background in the modern state of the field. This book does not give that background, and neither do many high school biology texts. Students are thus not qualified to sit in judgment on evolution, and this book does not give them the investigative tools they would need to be qualified. The authors also misrepresent the way evolutionary developmental biology works by pretending that the only force operating on embryos ought to be common ancestry. A falsification of evolutionary biology must address its totality, including both the constraints of common ancestry and divergences driven by other evolutionary mechanisms.

Embryology: Remarkably, this chapter simply does not discuss modern evolutionary developmental biology. While this means it is devoid of factual errors on that point, it also means that the chapter's title is inaccurate, and the content uninformative for students.

History of Embryology

Evolutionary developmental biology is a vital and active field of study. High school biology textbooks rarely cover it in detail, so Explore Evolution might have done a service by offering a brief exploration of that modern field. Instead, it focuses on creationist hobbyhorses from the history of biology. Most prominent of these historical arguments is a debate over illustrations by Ernst Haeckel.

Later illustrations from Haeckel: Haeckel continued to update his illustrations as better ones became available.Later illustrations from Haeckel: Haeckel continued to update his illustrations as better ones became available.

These century-old drawings, like any historical illustrations, have errors. Historians reviewing Haeckel's life and work are confident that he used the best illustrations available to him, and that he did not intend to mislead his readers. Contrary to claims in Explore Evolution, a comparison between that illustration and more recent photographs does not show a stark difference. Haeckel's drawings omit the yolk, while illustrations by Michael Richardson do not do so consistently. When the yolk is digitally removed from the images, the similarites become clear again. Nonetheless, Explore Evolution claims these illustrations are "possibly fraudulent," and implies that all of modern evolutionary developmental biology depends on those illustrations' fate. In fact, Haeckel's illustrations, like his idea of recapitulation (which the book wrongly claims Darwin accepted), have been superceded by more recent research.

This is also true of the book's discussion of Adam Sedgwick's writings against von Baer's law. That single essay from 1894 does not invalidate all the subsequent research of the last century. Recent research has specifically addressed many of the claims quoted from Sedgwick's essay, and it is foolish and inaccurate for students to be presented with Sedgwick's essay as if it were the last word on the matter. Inquiry-based eduation requires providing students with enough information that they can conduct their own inquiry, yet this chapter does not give even a basic understanding of modern embryology, and its account of the history of embryology is profoundly inaccurate.

Darwin on Dissimilarities

Explore Evolution focuses excessively on the details of what Darwin argued 150 years ago, rather than informing students about the dynamic field of evolutionary developmental biology. Whether or not Darwin argued that dissimilarities in early development do not cause a problem for evolution is less important than helping students understand how modern scientists view these issues. Explore Evolution fails to explain that the amount of yolk in an egg has adaptive value and is responsible for differences in embryogenesis. Instead of explaining a concrete example such as this, Explore Evolution makes a vague reference to Richard Goldschmidt's work from the 1930's and 1940's on macromutations – a hypothesis rejected by modern biologists.

From Explore Evolution:

Darwin was aware of these dissimilarities, but he argued that they do not disprove Common Descent. As Darwin explained, some groups of embryos have been "so greatly modified [by adaptations]* as no longer to be recognized."
Explore Evolution, p. 70

Darwin did argue that adaptations to embryonic life would result in the dissimilarities of embryos, as Jerry Coyne observes:

Darwin himself noted that embryos must adapt to the conditions of their existence, and the earliest stages of vertebrate embryos show adaptation to widely varying amounts of yolk in their eggs.
Jerry Coyne, 2001. "Creationism by Stealth," Nature, 410, p. 475-476

The early cleavage embryos of humans and chickens are quite different due to constraints imposed by the large yolk of chickens. The amount of yolk in an egg varies depending on how long the embryo will rely on the yolk for energy and nutrients. In amniotic mammals such as humans, relatively little yolk is needed because the developing embryo is able to get continual nutrition from maternal sources. Other organisms, such as chickens, have a large and yolk-rich egg as they lack constant maternal nutrition and lack feeding larval stages. Because yolk does not divide as the cells in the egg divide, the pattern of embryonic cell cleavage in chickens is necessarily different from the pattern in humans in order to accommodate their large amount of yolk.

Gastrulation is the process by which the embryo is converted from a single layer of cells to three different layers by a series of coordinated cellular movements. How these cell movements occur is dependent upon the earlier cleavage patterns. Therefore, embryos can differ significantly in their morphology at the gastrula stage. PZ Myers explains this relationship between yolk, cleavage patterns and gastrulation here..

Instead of explaining a concrete example of how the amount of yolk is an adaptation to embryonic life and how it affects the early development, students are referred to Richard Goldschmidt's work from the 1930's and 1940's.

*To explain what might cause these adaptations in embryos, some evolutionary biologists invoke "macromutations," large-scale changes in form that occur in one generation. One such biologist, the late University of California at Berkeley geneticist Richard Goldschmidt, believed that such macromutations could produce what he called "hopeful monsters."
Explore Evolution, p. 71

Notably, Richard Goldschmidt's macromutation hypothesis is not included in modern evolutionary biology. As Michael Dietrich notes:

Richard Goldschmidt is remembered today as one of the most controversial biologists of the twentieth century. Although his work on sex determination and physiological genetics earned him accolades from his peers, his rejection of the classical gene and his unpopular theories about evolution significantly damaged his scientific reputation.
Michael Dietrich, 2003. "Richard Goldschmidt: Hopeful Monsters and other heresies." Nature Review Genetics, 4, p. 71.

The mention of macromutations to explain adaptations to embryonic life by Explore Evolution is yet another failure to use "current evidence and arguments for and against the key ideas of modern Darwinian theory." The book is too focused on trying to attack Darwin to properly examine modern biology.

Ontogeny & Phylogeny

There is no question that the study of development played an important role in Charles Darwin's thinking on evolution, and that it continues to play an important role in modern evolutionary biology. Ernst Haeckel's early suggestion that ontogeny (development) recapitulates phylogeny (evolution) was far less influential and is rejected by modern biologists. Alternative views on the relationship between development and evolution include Baer's Laws, which simply state that general characteristics of the group to which an organism belongs often develop earlier than special characters of a species, and that organisms tend to become more divergent through their development. This formulation remains valid today, and (unlike Haeckel's proposal) has a straightforward relationship with evolutionary mechanisms. Despite these simple facts, Explore Evolution invests substantial effort trying to tie Darwin to Haeckel's views, rather than exploring the modern state of evolutionary developmental biology. In doing so, the book mangles both history and biology.

Intelligent design proponents have a tradition of flogging embryology in order to attack common descent (Wells 2000, 2003, 2005). Explore Evolution continues this tradition of misrepresenting embryology and evolution. The first misrepresentation is the claim that Darwin accepted Haeckel's Biogenetic Law. In this case, Explore Evolution presents a minority position among scholars who have studied this question. This claim is the first move by Explore Evolution to link Darwin and Haeckel as closely as possible, so as to tarnish Darwin's arguments for common descent with the controversy about Haeckel's embryos. It is notable that the only embryological data Explore Evolution offers students in support of common descent is a modified diagram of Haeckel's embryos from 1894. In protecting their anti-evolution viewpoint, the authors omit recent research showing evolutionary conservation of the genetic pathways regulating animal development (e.g., Carroll et al. 2005, Davidson, 2005).

Intelligent design proponents have attacked the idea of common descent through promoting a misunderstanding of embryology and the new field of evolutionary developmental biology (evo-devo):

There is a whole stable of intelligent design creationist writers associated with the Discovery Institute, and we will see more slick books of bogus science produced to influence the teaching of biology, and even federal funding of research. Evo-devo data have become a part of the creationist rhetorical weaponry, and as evo-devo grows in prominence, the problem will grow in severity.
Rudolf Raff, 2001. "The Creationist Abuse of Evo-Devo," Evolution and Development, 3:6, p. 374

From Explore Evolution:

Darwin noticed certain similarities in the embryos of vertebrate animals, similarities he thought were especially great during the embryo's earliest stages of development.
Explore Evolution, p. 66

This short statement in Explore Evolution makes two significant mistakes, one of omission and one of commission. Darwin considered that similarity of early embryos provided very strong support for common descent. However, he never personally examined vertebrate embryos. Instead he relied upon Karl von Baer’s observation in 1828 that early vertebrate embryos were more similar to each other than when fully developed. Failing to distinguish von Baer from Haeckel later allows Explore Evolution to present Adam Sedgwick's erroneous criticism of Baer's Laws as if they were criticism of Haeckel.

In addition, Explore Evolution falsely asserts Darwin thought the similarities between embryos were greater at the earliest stages of development. This begins a relentless pattern of suggesting that common descent and Haeckel's Biogenetic Law require that the earliest stages of animal development are most similar. However, as Jerry Coyne notes, that is not what Darwin actually thought, embryos at the earliest stages of development can vary signficantly from one another depending upon the amount of yolk in their eggs.

Darwin himself noted that embryos must adapt to the conditions of their existence, and the earliest stages of vertebrate embryos show adaptation to widely varying amounts of yolk in their eggs.
Jerry Coyne (2001) "Creationism by Stealth," Nature, 410,p. 476

While Darwin accepted von Baer's Law, it is much less clear whether Darwin accepted Haeckel's Biogenetic Law, proposed in 1866, which claimed that embryonic development recapitulates the adult stages of their ancestors ("ontogeny recapitulates phylogeny"). Nonetheless, Explore Evolution claims:

Darwin thought that the observable similarities in different embryos revealed what the ancestors to these organisms would have looked like.
Explore Evolution, p. 66

This claim by Explore Evolution contradicts the majority view of prominent Darwin scholars (including Ernst Mayr, Stephen Jay Gould, David Hull, and Peter Bowler) who have argued that Darwin did not accept the Biogenetic Law.

most authors (including Darwin) rejected the claim that ontogeny is the recapitulation of the adult stages of the ancestors.
Ernst Mayr (1982), Growth of Biological Thought, , p. 475.
Darwin saw that ancestral groups in an established community of descent would differ least in their adult form from the embryonic state common to all members of the community. The gill slits of the human fetus represent no ancestral adult fish; we see no repetition of adult stages, no recapitulation.
Stephen Jay Gould, Ontogeny and Phylogeny, p. 72
belief that ontogeny (individual growth) recapitulates phylogeny (the history of the type) thus owes little to Darwinism and is more characteristic of the non-Darwinian, or developmental, view of evolution.
Peter Bowler (1988), The Non-Darwian Revolution: Reinterpreting a Historical Myth, , p. 11.

Not all historians of science who have studied embryology and evolution are in agreement that Darwin was not a recapitulationist (Roberts, 1990). However, this fails to warrant the claim by Explore Evolution that Darwin accepted Haeckel's Biogenetic Law.

Haeckel's Drawings

Explore Evolution incorrectly asserts that Haeckel’s Biogenetic Law claims that the earliest stage of embryos are most similar. Haeckel's concept of caenogenesis fully acknowledged that there can be signficant differences between embryos including at the earliest stages of development. This misrepresentation of the Biogenetic Law allows Explore Evolution to set up erroneous claims that Haeckel committed fraud and that the distinctions between the early stages of development of different classes of vertebrates argue against common descent.

Following Darwin's lead, Haeckel tried to discover the evolutionary history of various animals by studying their embryos. He produced a set of influential drawings showing that the embryos of various classes of vertebrates were very similar during their earliest stages of development.
Explore Evolution, p. 66. Emphasis added.

This claim is false. Haeckel was aware of significant departures in the morphology of early embryos from what would be predicted by his Biogenetic Law. These differences in order, timing and pattern of embryogenesis were termed as caenogenesis by Haeckel. He also understood that a prominent reason for why embryos showed a signficant difference in their cleavage and gastrulation patterns was due to the amount of yolk in their eggs. As Michael Richardson (the developmental biologist whose work re-ignited the interest in Haeckel's embryos) and Gerhard Keuck observe:

The early divergence, which violates some of von Baer's laws, is due to differences in egg size and patterns of cleavage and gastrulation among species. Recent explanations of the conservation of midembryonic stages, despite variations in early development, include the idea that they are subject to strong stabilizing forces (e.g. selection, pleiotropy; see Wagner & Misof, 1993; Raff, 1996; Wagner, 1996). Haeckel was aware of these early differences, and they were included among his caenogenetic exceptions.
Richardson and Keuck, 2002. "Haeckel's ABC of Evolution and Development," Biol. Rev. 77 , p. 507

Why are Haeckel's embryos focused upon in this section? Most likely because this diagram was found in many high school biology textbooks, and that Michael Richardson's research on vertebrate embryos rekindled an old controversy about whether the early embryos in the diagram are accurate. However, as Richardson and colleagues note, this hardly undermines the strong support for common descent from embryology, despite the claims of Creationists and ID proponents.

Data from embryology are fully consistent with Darwinian evolution. Haeckel’s famous drawings are a Creationist cause célèbre (3). Early versions show young embryos looking virtually identical in different vertebrate species. On a fundamental level, Haeckel was correct: All vertebrates develop a similar body plan (consisting of notochord, body segments, pharyngeal pouches, and so forth). This shared developmental program reflects shared evolutionary history. It also fits with overwhelming recent evidence that development in different animals is controlled by common genetic mechanisms. (4)
Richardson et al. (1998) "Haeckel, Embryos and Evolution." Science, 280:983

Finally, given that Explore Evolution claims:

This book is one of the first textbooks ever to use the inquiry based approach to teach modern evolutionary theory. It does so by examining the current evidence and arguments for and against the key ideas of modern Darwinian theory.
Explore Evolution, preface

it is remarkable that Explore Evolution fails to include recent work in evo-devo in the Embryology: Case For section. We now know there is an evolutionarily conserved "genetic toolkit" - a set of genes responsible for constructing all animals, from sea anemones to fruit flies to humans (Carroll et al. 2005, Davidson 2005). The only mention of the genetic regulation of development refers to the outdated macromutation theory from the 1940's by Richard Goldschmidt. So while Explore Evolution generally acknowledges that modern biologists consider embryology to provide strong support for common descent, students are at a high risk of incorrectly concluding that this support is from the diagram of Haeckel's embryos.

Accuracy in embryo illustrations

It has been widely noted that a number of the embryos in top row of the Tables 6 and 7 from Haeckel's Anthropogenie (1874) are not realistic representations. However, the assertion by Explore Evolution that Haeckel claimed that top row represented earliest embryos is false. Nor are the book's claims that Haeckel engaged in fraud justified by current historical scholarship.

Haeckel's Diagrams: embryonic development, as drawn by Ernst HaeckelHaeckel's Diagrams: embryonic development, as drawn by Ernst HaeckelIllustrations from Ernst Haeckel, Anthropogenie, 4th ed. (1891): Reproduced from Richards (2009)Illustrations from Ernst Haeckel, Anthropogenie, 4th ed. (1891): Reproduced from Richards (2009)

When considering Haeckel's embryos, it is important to note that Haeckel compared embryos of different groups in several publications. For example, in Naturliche Schopfungsgeschichte (1868), Haeckel compared vertebrate embryos including human and dog. But his most famous diagram are Plates 4 and 5 from Anthropogenie (1874). Explore Evolution uses a version of this diagram from Romanes (1894) that is not completely identical to Haeckel's original (1874) diagram. Haeckel himself updated his illustrations over several editions (compare figures at right).

A diagram showing real vertebrate embryos by Michael Richardson and colleagues (used in Figure 4.2 of Explore Evolution) suggest that Haeckel took considerable license in portraying the earlier embryos in the series, particularly in the top row. Scholars who have studied Haeckel generally think the reason why Haeckel exaggerated similarities of early embryos was not to mislead his readers. A key difference between Richardson's figures and Haeckel's is that Richardson did not remove the yolk from his embryos. The yolk distorts the comparative outline of the embryos and alters the posture of the embryo in ways with no evolutionary or developmental significance.

Explore Evolution also claims that Haeckel misled his readers into thinking that the top row of embryos is the earliest embryonic stage. This charge about mislabeling the top row embryos as earliest was first made by Jonathan Wells in 2000 and has been refuted by Alan Gishlick at the National Center for Science Education. Nonetheless, Explore Evolution continues this false charge.

The stage that Haeckel labeled first is actually midway, through development. Embryologists such as Sedgwick knew this, but their publications challenging the Darwinian interpretation were lost beneath the popularity of Haeckel's inaccurate (and possibly fraudulent) drawings.
Explore Evolution, p. 68

It is notable that none of Haeckel's contemporary critics, including Sedgwick, accused him of claiming that the embryos in his famous diagram were at the earliest stage. (Richardson and Keuck, 2002; Sedgwick, 1894)

Nick Hopwood, an historian of science, has comprehensively explored the production of Haeckel's embryos diagram (Hopwood, 2006) and shows that Haeckel's own writings would not have been taken by contemporaries to represent the earliest stages of development. Hopwood describes the original diagram with the following figure legend from Hopwood's paper in Isis. Quotation marks below are in the original and represent the English translation of Haeckel's description of the diagram.

"Comparison of the embryos” of various vertebrates “at three different stages of development.” This expanded double plate shows fish (F), salamander (A), turtle (T), chick (H), pig (S), cow (R), rabbit (K), and human (M) embryos at “very early” (I), “somewhat later” (II), and “still later” (III) stages… Lithograph by J. G. Bach of Leipzig after drawings by Haeckel from his Anthropogenie (Leipzig: Engelmann, 1874), Plates IV–V.
Nick Hopwood, 2006. "Pictures of Evolution and Charges of Fraud: Haeckel's Embryological Illustrations"Isis: 97:292

What did Haeckel mean by "very early" embryos? The definition of embryos has changed over the last 100 years. Unlike the modern definition of embryos, which refers to any stage after the fertilized egg, the 19th century definition of embryo was more restricted.

In present English usage, the term "embryo" includes even the earliest stages. The German tradition, however, largely established by von Baer, restricts the term "embryo" to the basic rudiment of the body or its later stages (the "embryo proper" in English usage). This is evident from many remarks by von Baer, for instance: "The germ ["Keim", blastodisc] during its growth transforms into two parts; [… ] the middle forms the embryo, the much wider periphery the Keimhaut [extraembryonic blastoderm]" (von Baer, 1828, p.44).
Klaus Sander and Urs Schmidt-Ott, 2004. "Evo-Devo aspects of classical and molecular data in a historical perspective," J. Exp. Zool. (Mol. Dev. Evo) 302B:69–91.

Thus, Haeckel's "very early" embryos are midway through development and were not meant to represent the earliest stages of development.

When Explore Evolution claims that "the pictures were fabricated and the facts were distorted" (p. 69), the authors are ignoring recent scholarship. In addition to Hopwood's reassessment of Haeckel's original writings, historian Robert Richards has reexamined the drawings and their sources, finding no support for the claim of intentional fraud. Richards notes that Anthropogenie was a popular work based on a series of lectures Haeckel presented. The figures began as visual aides for his lecture, and represented the best illustrations available at the time. In Haeckel's subsequent writings, he replaced those illustrations as newer and better illustrations became available.

Richardson and his colleagues selected images from the first edition of Haeckel’s Anthropogenie, which was hastily drawn together from his lectures. The book, though, went through five further editions. With each new edition the text grew fatter as Haeckel deployed more evidence; and the illustration in question expanded the comparison from 8 species of embryo to 20 by the 5th edition (1905). In the subsequent editions, the images grew ever more refined, so that even by the 4th edition (1891), the differences among them became more pronounced. The refinements were a function of more material available and better instrumentation (embryos at the earliest stages are invisible to the naked eye). Had the Science article compared Richardson’s photos with illustrations from Haeckel’s later editions, the argument for fraud would have withered.
Robert J. Richards (2009) "Haeckel's embryos: fraud not proven" Biology and Philosophy 23:147-154.

This level of historical detail would be irrelevant to a class covering modern biology, but if Explore Evolution is to use this obscure debate to attack evolution, it behooves them to accurately describe the history. The best available historical research shows that Haeckel's drawings of embryos represented the best available illustrations, and were not a fraud.

The Earliest Stages

Explore Evolution claims:

both Darwin's and Haeckel's comparisons left out the earliest stages of development.*

Whether this omission was intentional is a matter of some debate.

Explore Evolution, p. 68
Eggs, cleavage stages, and gastrula stage embryos Eggs, cleavage stages, and gastrula stage embryos

Vertebrate embryos (in the modern definition - after fertilization) have a number of stages such as the cleavage and gastrula stages which precede the embryos shown in Haeckel's diagram. Darwin's Use of Embryonic DrawingsDarwin's Use of Embryonic Drawings Nick Matzke at Panda's Thumb has pointed out that comparisons of these earlier stage embryos are actually shown in Anthropogenie. Indeed, Anthropogenie has over 20 figures showing gastrula embryos from different groups. This does not reconcile with Explore Evolution's implication that Haeckel intended to deceive his readers.

Shown below are Plates 2 and 3 from Anthropogenie comparing eggs, cleavage stages, and gastrula stage embryos from different animals. As discussed earlier, Haeckel used von Baer's convention for embryo as the stage in which a body form is first apparent. Instead of describing these earliest stages of development as "embryos", Haeckel uses terms which include "keim" (meaning "germ"). For example, morula embryos are called "maulbeerkeim", and blastula embryos are referred as "blasenkeim".

In Descent of Man (1871), Darwin compared drawings of a human and dog embryo at the same stage (originally from Bischoff and from Ecker). However, nowhere does Darwin imply that this is the earliest stage of development. In fact, because dogs and humans are mammals and have very small eggs, their earliest stages of development are extremely similar. So it is absurd to suggest that Darwin purposefully left out the earliest embryonic stages of humans and dogs in order to mislead his readers.

Sedgwick's Two Challenges

Explore Evolution asserts that in 1894, Adam Sedgwick challenged Darwin's two claims about embryos, 1) early embryos of related organisms are more similar than adults and 2) the younger the embryos, the greater the resemblance. A comprehensive comparison of vertebrate embryos does not support Sedgwick's challenges about the similarity of embryos. Explore Evolution also neglects to mention that the section of Sedgwick's paper quoted by Explore Evolution is actually challenging von Baer's Law (that development proceeds from a more general to a more specific morphology such that taxa-specific features are added later in development), and not Darwin or Haeckel.

From Explore Evolution:

In 1894, Adam Sedgwick, an embryologist at Cambridge University, challenged Darwin's two claims: 1) that vertebrate embryos were more alike than the vertebrate adults, and 2) that the younger the embryos, the greater the resemblance.
Explore Evolution, p. 68

Adam Sedgwick's critique was directed not against Charles Darwin, but Karl von Baer:

The generalization commonly referred to as v. Baer’s law is usually stated as follows: - Embryos of different members of the same group are more alike than the adults, and the resemblances are greater the younger the embryos are examined.
Adam Sedgwick, 1894. "On the Law of Development commonly known as von Baer’s Law; and on the Significance of Ancestral Rudiments in Embryonic Development," Quart. J. Microscopy , 36, p. 35

Darwin did think that the Karl von Baer's observations provided strong support for descent with modification. However, this does not merit the inaccurate statement that von Baer's claims can be attributed to Darwin.

Explore Evolution cites Sedgwick to challenge the first of von Baer's claims:

Even the embryos of "closely allied animals," such as chickens and ducks, display specific differences very early in development. "I can distinguish a [chicken] and a duck embryo on the second day," he wrote.
Explore Evolution, p. 68

A more recent and comprehensive analysis of embryogenesis of poultry shows that morphological differences between two day duck and chick embryos are only because duck embryonic development is slower than chick embryonic development. Otherwise, their development is "nearly identical". Hence, Sedgwick's first challenge to von Baer is not supported.

Chicken, turkey, Japanese quail, and Pekin duck blastoderms from oviductal eggs showed differences in the rate of development that were inversely correlated with egg size. … Although it is recognized that the temporal rate of development will differ between different species and strains, the external features of any embryo in any given stage will be nearly identical.
Sellier, et al., 2006. "Comparative Staging of Embryo Development in Chicken, Turkey, Duck, Goose, Guinea Fowl, and Japanese Quail Assessed from Five Hours After Fertilization Through Seventy-Two Hours of Incubation." J. Appl. Poult. Res., 15, p. 219

Explore Evolution also cites Sedgwick to challenge the second claim of von Baer, that younger embryos of different groups have a greater resemblance than older embryos.

Comparing the embryos of a chicken and shark, he continued, "There is no stage of development in which the unaided eye would fail to distinguish them with ease."
Explore Evolution, p. 68

Sedgwick does observe clear differences between these dogfish and chick embryos.

According to law of v. Baer these embryos (fowl and dogfish) ought to be closely similar in the younger stage. Do these embryos, developing under similar conditions, conform to the law? Superficially, clearly not. There is no stage of development in which the unaided eye would fail to distinguish between them with ease- the green yolk of one, the yellow yolk of the other; the embryonic rim and blastopore of the fish, the absence of these in the chick; the six large gill-slits bearing gills on the one hand, the four rudimentary clefts on the other; the small head, straight body and long tail, as opposed to the enormous head, cerebral curvature , short tail, and so on.
Adam Sedgwick, 1894. "On the Law of Development commonly known as von Baer’s Law; and on the Significance of Ancestral Rudiments in Embryonic Development." Quart. J. Microscopy, 36, p. 36

However, Sedgwick also admits that there are "striking similarities" between these embryos which are not found in adults.

These embryos are not closely similar, but it is maintained that the law is justified by certain remarkable features of embryonic similarity which the adults do not exhibit, and of which the most important are the presence in the chick of pharyngeal clefts, a tubular piscine heart and a similarity in the arrangement of the cardiac arterial system, a cartilaginous endo-skeleton, oro-nasal grooves and a notochord. Now I freely admit that these are striking similarities, but I question whether they are sufficient to justify the law of v. Baer.
Adam Sedgwick, 1894. "On the Law of Development commonly known as von Baer’s Law; and on the Significance of Ancestral Rudiments in Embryonic Development." Quart. J. Microscopy, 36, p. 36

Explore Evolution asserts that Sedgwick had successfully disproved von Baer's second claim that younger embryos show a greater resemblance to each other than older embryos. However, when considering the major differences between the fully developed dogfish and chick embryos (presence or absence of limbs, feathers, teeth etc.) as well as the shared features of these embryos at earlier stages that are not found in adults (heart structure, cardiac arterial system, notochord etc.), it is apparent why biologists consider the early embryos to be more similar than later embryos. Interestingly, Explore Evolution does not ask students to specifically examine the early and late dogfish and chick embryos shown in Figure 4.2.

Michael Richardson's photographs

Michael Richardson and colleagues in 1997 were instrumental in pointing out the discrepancies between Haeckel's popular diagram and genuine embryos. However, Explore Evolution simply accepts a flawed creationist interpretation of Richardson's work. The claim that the common ancestry of vertebrates requires that embryos be most similar at the earliest embryonic stage was not accepted by Darwin nor Haeckel nor any modern evolutionary or developmental biologist. This is simply a straw man argument that was set up by the misrepresentations of Haeckel's and Darwin's view of embryology and development discussed above.

From Explore Evolution:

Critics of the argument from embryology agree that common descent might be a reasonable inference to draw from the similarity of embryos - if embryos really were similar in their earliest stages of development. But they're not, say most embryologists.
Explore Evolution, p. 68

The inference that earliest embryos of various groups must be similar if they shared a common ancestry is a claim of intelligent design proponents such as Jonathan Wells in Icons of Evolution. Darwin certainly never made such a claim. In his review of Icons of Evolution Jerry Coyne addresses this particular fallacy:

Wells also notes that the earliest vertebrate embryos (mere balls of cells) are often less similar to one another than they are at subsequent stages when they possess more complex features. According to Wells, this counts as evidence against biological evolution, which supposedly predicts that the similarities among groups will be strongest at the very first stages of development. But Darwinism makes no such prediction. Darwin himself noted that embryos must adapt to the conditions of their existence, and the earliest stages of vertebrate embryos show adaptation to widely varying amounts of yolk in their eggs.
Jerry Coyne, (2001) "Creationism by Stealth." Nature, 410, p. 475-476

Michael Richardson and Gerhard Keuck find that Haeckel also did not imply that the earliest stages of embryos must be similar if they share a common ancestor. His concept of caenogenesis, the adaptations to embryonic life which blur recapitulation, also applied to early stages of development, particularly with respect to the size of eggs.

Haeckel was aware of these early differences, and they were included among his caenogenetic exceptions. With regard to egg size for example, he noted that ova of different species look very similar at early stages of maturation (although he did acknowledge that they must show molecular differences; see Haeckel, 1896b: 1, p. 137).
Richardson and Keuck, 2002. "Haeckels ABC of Evolution and Development." Biol. Rev., 77, p. 507

Explore Evolution repeats another false claim from Wells.

This error even crept into the Encyclopedia Britannica, and remains in many modern high school and college biology textbooks.
Explore Evolution, p. 69

This is incorrect. A recent survey of 36 biology textbooks, dating from 1980 to the present and covering high school biology, college introductory biology, advanced college biology, and developmental biology books, found that only 8 of these textbooks mentioned Haeckel or the biogenetic law. Two of these 8 were creationist/ID books (Of Pandas and People, and Biology for Christian Schools from Bob Jones University Press). Of the 6 mainstream textbooks that mentioned Haeckel or the biogenetic law, two are advanced college-level books. In all cases where Haeckel is mentioned (except for the creationist/ID books), the text discussion does not reproduce Haeckel's mistakes.

Explore Evolution emphasizes data from the 1997 paper by Michael Richardson and colleagues that strongly challenged a literal interpretation of Haeckel's diagram.

In 1997, an international team of scientists, led by the embryologist Michael Richardson, compared Haeckel's drawings to photographs of actual embryos at various developmental stages. They found that Haeckel had distorted the evidence at every turn, leading Richardson to tell Science that "it looks like it's turning out to be one of the most famous fakes in biology."
Explore Evolution, p. 69

However, the claim that Haeckel "distorted the evidence at every turn" is untrue. As Michael Richardson and colleagues also point out, there is, in fact, compelling similarity of early embryos which provides strong support for common descent.

Data from embryology are fully consistent with Darwinian evolution. Haeckel’s famous drawings are a Creationist cause célèbre (3). Early versions show young embryos looking virtually identical in different vertebrate species. On a fundamental level, Haeckel was correct: All vertebrates develop a similar body plan (consisting of notochord, body segments, pharyngeal pouches, and so forth). This shared developmental program reflects shared evolutionary history. It also fits with overwhelming recent evidence that development in different animals is controlled by common genetic mechanisms (4)
Richardson, et al., 1998."Haeckel, Embryos and Evolution." Science, 280, p. 983

Furthermore, historians comparing Richardson's images to Haeckel's find other problems. Robert Richards observes:

Richardson and his colleagues selected images from the first edition of Haeckel’s Anthropogenie, which was hastily drawn together from his lectures. The book, though, went through five further editions. With each new edition the text grew fatter as Haeckel deployed more evidence; and the illustration in question expanded the comparison from 8 species of embryo to 20 by the 5th edition (1905). In the subsequent editions, the images grew ever more refined, so that even by the 4th edition (1891), the differences among them became more pronounced. The refinements were a function of more material available and better instrumentation (embryos at the earliest stages are invisible to the naked eye). Had the Science article compared Richardson’s photos with illustrations from Haeckel’s later editions, the argument for fraud would have withered.
Robert J. Richards (2009) "Haeckel's embryos: fraud not proven" Biology and Philosophy 23:147-154.
Richardson vs. Haeckel: Reengineered photographs of embryos with yolk material removed, comparable scaling, and orientationRichardson vs. Haeckel: Reengineered photographs of embryos with yolk material removed, comparable scaling, and orientation

Furthermore, Richardson's own images display disturbing inaccuracies:

several (but not all) of the photographed embryos retain the attached yolk sack and other maternal material; this exaggerates their differences from Haeckel’s images. Haeckel explicitly indicated that he pictured his specimens without yolk, allantois, and amnion (Haeckel 1874, p. 256). The bulge of the salamander is not part of the embryo; rather, it is the yolk sack, as is the case for the fish and the human embryos (though not for the chick and the rabbit, from which the yolk sacks have been removed); moreover the salamander photo is obviously not reduced to the same scale as the others (despite the assertion in the caption for the figure in Science). The chick was photographed in a highly circumflex orientation, which occurs at a somewhat later stage of development than that represented by Haeckel. Again, Haeckel expressly stated that he oriented his embryos all in the same way for ease of comparison. I have used a computer program to remove the yolks in the photographs, scale back the salamander, and straighten out the chick. The result is a bit crude, but one can clearly see that the differences between photograph and illustration are not nearly as great as presented in the Science article. Shorn of yolk, the photographed embryos would not have provided the kind of graphic evidence upon which the Science article was premised.
Robert J. Richards (2009) "Haeckel's embryos: fraud not proven" Biology and Philosophy 23:147-154.

This error is significant, and demolishes Explore Evolution's claim that Haeckel's drawings are inherently unreliable. The inconsistency among Richardson's own images, however, makes them unreliable for use with students, and nothing in this historical dispute offered by Explore Evolution undermines modern evolutionary developmental biology.

Philosophy

Rather than presenting an account of how embryology is studied in the 21st century, the "Embryology" chapter concludes by exhorting students to pretend they are jurors in a court case against evolution. It's unclear what the charges might be, but it is certain that the students would be diverted from a fair verdict by the chapter's studious avoidance of current science in the field.

Rather than a scientific argument, the book offers a philosophical charge, that evolution has been rendered unfalsifiable. This charge rests on a misrepresentation of how science works, and what evolutionary developmental biologists actually claim. It is one thing to argue that there is no evidence which currently falsifies common ancestry, and quite another to say that no such evidence could ever exist. Scientists agree to the former statement, but not the latter.

Evolutionary Predictions of Common Ancestry

Explore Evolution argues that if common descent predicts both similarity and dissimilarity of embryos, it is impossible to challenge the theory. The similarity of embryos is best explained by common descent. The dissimilarity of embryos can be explained by environmental adaptations under natural selection, and while it can cause scientists to re-evaluate particular claims of common ancestry, no embryological evidence now available would cause scientists to reject universal common ancestry.

Explore Evolution insists

By arguing that common descent predicts both embryonic similarity and dissimilarity, Darwinists have effectively made it impossible to challenge the theory with counterevidence. When the case is stated this way, common descent would be consistent with whatever we observe in embryos.
Explore Evolution, p. 70

In this claim, Explore Evolution fails to distinguish two main threads of modern evolutionary biology; common descent and the role of natural selection in adaptation. As noted earlier, Darwin realized that embryos would show differences if they had adaptations to different environmental circumstances. Perhaps the most important adaptation is the amount of yolk, which affects the egg size and the amount of stored nutrition available for the developing embryo. One of the most striking differences is between animals that undergo metamorphosis from a feeding larval stage and animals that undergo direct development, bypassing the larval stage. The comparative analyses of sea urchin embryos and larvae has shed light upon the adaptive strategies of these different modes of development.

Observations of a [sic] sea urchin larvae show that most species adopt one of two life history strategies. One strategy is to make numerous small eggs, which develop into a larva with a required feeding period in the water column before metamorphosis. In contrast, the second strategy is to make fewer large eggs with a larva that does not feed, which reduces the time to metamorphosis and thus the time spent in the water column. The larvae associated with each strategy have distinct morphologies and developmental processes that reflect their feeding requirements, so that those that feed exhibit indirect development with a complex larva, and those that do not feed form a morphologically simplified larva and exhibit direct development.
Smith, Zigler and Raff, 2007. "Evolution of direct-developing larvae: selection vs loss." Bioessays, 29:6, p. 566

Significantly, these different modes of development are found in closely related species who diverged relatively recently, about 4 million years ago, and have occurred in multiple instances in other echinoderm lineages. The comparison of indirect and direct development has demonstrated that the earliest stages of development are relatively plastic (Raff ref).

Shared features of organisms are normally most parsimoniously explained by inferring common descent. The support for common descent, which is disputed by Explore Evolution but accepted by the vast majority of biologists, arises from the independent convergence of evidence from a wide variety of fields including biogeography, biochemistry, molecular biology, and embryology. One such shared feature of primate embryos, including human, is a tail. Primates which lack a tail as adults, such as humans and chimps, resorb the tail during later embryogenesis. Primates which have a tail as adults, do not resorb the embryonic tail. Phylogenetic analysis have unambiguously demonstrated that the ancestors to primates were tailed. The vast majority of biologists would consider human embryonic tails to be best explained by common descent. How would Explore Evolution explain it to students?

Students as Jurors

Explore Evolution claims that many scientists who criticized Haeckel's embryos still support common ancestry; but students, as good jurors, should keep an open mind. Students are learners, not jurors. Their science class is a chance to gain enough context to continue their science education in college and graduate programs, where they will get the background necessary to challenge well-established science. To suggest that students should reject such science at the beginning of their scientific careers is irresponsible.

Keeping an open mind is a virtue when coupled with skepticism and critical analysis. Such skepticism and analysis requires accurate information, which Explore Evolution fails to offer.

Asking students, at the very start of their biological studies, to keep an open mind about the views of an extremely small minority of biologists has questionable pedagogical value. This is particularly problematic because Explore Evolution suffers from so many distortions.

In Explore Evolution, students are exposed to a set of arguments from authority presented in a courtroom-like "he said/she said" fashion. This is not "inquiry-based" education, but rather a rhetorical exercise.

In science, there are not always two diametrically-opposed sides to every issue. When measuring the velocity of objects falling in gravity fields, there is no "dissenting view" or other side that can claim equal time. In areas of active scientific research, there are often more than two sides or interpretations. The courtroom-like approach of this book is an inaccurate representation of how science works.

References

References

Carroll, S. (2005) Endless Forms Most Beautiful.

Davidson, E. (2005). The Regulatory Genome.

Richards, RJ. (1990) The Meaning of Evolution.

Richards, RJ. (2008) The Tragic Sense of Life.

Wells, J. (2000) Icons of Evolution.

Wells, J. (2003) "Survival of the Fakest." American Spectator.

Wells, J. (2005) The Politically Incorrect Guide to Intelligent Design and Evolution.