Biology Textbooks Misuse Embryology to Argue for Evolution

Casey Luskin

In August 2008, The New York Times reprinted material from the NCSE claiming that the 19th century embryologist Ernst Haeckel's "longdiscredited drawings" of vertebrate embryos have not been used in textbooks since "20 years ago."²⁶¹ That Haeckel's drawings were fraudulent and have been used in textbooks is essentially beyond dispute,²⁶² but the reality is that multiple biology textbooks have been used within the past 20 years that still use Haeckel's drawings to promote evolution.²⁶³ In a 2000 article in Natural History, Stephen Jay Gould recognized that Haeckel's drawings not only fraudulently obscured the differences between the early stages of vertebrate embryos, but that they were used inappropriately in textbooks: Haeckel had exaggerated the similarities by idealizations and omissions. He also, in some cases--in a procedure that can only be called fraudulent--simply copied the same figure over and over again. At certain stages in early development, vertebrate embryos do look more alike, at least in gross anatomical features easily observed with the human eye, than do the adult tortoises, chickens, cows, and humans that will develop from them. But these early embryos also differ far more substantially, one from the other, than Haeckel's figures show. Moreover, Haeckel's drawings never fooled expert embryologists, who recognized his fudgings right from the start. At this point, a relatively straightforward factual story, blessed with a simple moral story as well, becomes considerably more complex, given the foils and practices of the oddest primate of all. Haeckel's drawings, despite their noted inaccuracies, entered into the most impenetrable and permanent of all quasi-scientific literatures: standard student textbooks of biology. . . .We should therefore not be surprised that Haeckel's drawings entered nineteenth-century textbooks. But we do, I think, have the right to be both astonished and ashamed by the century of mindless recycling that has led to the persistence of these drawings in a large number, if not a majority, of modern textbooks!²⁶⁴ Gould also quotes embryologist Michael K. Richardson, acknowledging the widespread use of Haeckel's drawings in textbooks: If so many historians knew about the old controversy [over Haeckel's falsified drawings], then why did they not communicate this information to numerous contemporary authors who use the Haeckel drawings in their books? I know of at least fifty recent biology textbooks which use the drawings uncritically. I think this is the most important question to come out of the whole story.²⁶⁵ Likewise, in an article titled Haeckel's Embryos: Fraud Rediscovered, the journal Science recognized that "[g]enerations of biology students may have been misled by a famous set of drawings of embryos published 123 years ago by the German biologist Ernst Haeckel. They show vertebrate embryos of different animals passing through identical stages of development. But the impression they give, that the embryos are exactly alike, is wrong."²⁶⁶ The article quotes Richardson by stating that "[i]t looks like it's turning out to be one of the most famous fakes in biology."²⁶⁷ Elsewhere, in the journal Anatomy and Embyrology, Richardson and other embryologists acknowledge that Haeckel's fraud has had a non-trivial influence on both evolutionary thought and evolution education:Haeckel's ideas soon came in for strong criticism. His drawings are also highly inaccurate, exaggerating the similarities among embryos, while failing to show the differences (Sedgwick 1894; Richardson 1995; Raff 1996). Sedgwick (1894) argued that even closely related species of vertebrates can be told apart at all stages of development, but that the distinguishing characters are not necessarily the same as those used to distinguish among adults. . . . . Another point to emerge from this study is the considerable inaccuracy of Haeckel's famous figures. These drawings are still widely reproduced in textbooks and review articles, and continue to exert a significant influence on the development of ideas in this field.²⁶⁸

Sylvia Mader's 2010 textbook, Biology, uses colorized versions of Haeckel's embryo drawings with only a few small modifications. As seen in the side-by-side comparison above, the black and white drawings are Haeckel's original drawings and the colored drawings are from Mader's 2010 textbook. Just like Haeckel's original drawings, Mader's colorized drawings obscure the differences between the early stages of vertebrate development in order to give students the following misleading caption: "At these comparable developmental stages, vertebrate embryos have many features in common, which suggests they evolved from a common ancestor. (These embryos are not drawn to scale.)" (Sylvia S. Mader, Biology, p. 278 (McGraw Hill, 2010).) Click the graphic for the full picture.

Haeckel's long-discredited recapitulation theory is not necessarily the bedrock of evolutionary thinking today, yet as documented yesterday, leading authorities acknowledge that his drawings persist in textbooks today. While recapitulation theory may be largely out of current textbooks, current textbooks, still use the drawings to illustrate an allegedly high degree of similarity between embryos at the earliest stages of vertebrate embryonic development. Many textbooks cite such similarities in the earliest stages of vertebrate embryos as evidence for common ancestry. For example, Miller and Levine's Biology states that "[i]n their early stages of development, chickens, turtles, and rats look similar, providing evidence that they shared a common ancestry."²⁶⁹ Likewise Belk and Borden's Biology: Science for Life includes this caption to a picture of a vertebrate embryo: "Similarity among chordate embryos. Vertebrate embryos are very similar in the first stage of their development, shown here in the top row, evidence that they share a common ancestor that developed along the same pathway."²⁷⁰

These particular texts commendably do not use Haeckel's drawings, but instead use photographs of embryos. However, there are textbooks in use today, such as Mader's 2010 edition of Biology, which continue to use Haeckel's drawings (in Mader's case, essentially a colorized and slightly altered version of Haeckel's drawings) and state, "At these comparable developmental stages, vertebrate embryos have many features in common which suggests they evolved from a common ancestor."²⁷¹ Indeed, a textbook submitted by the J.M. Lebel publishing company for adoption in Texas in 2003, stated, "All vertebrate embryos closely resemble one another in early development" and used a slightly simplified version of Haeckel's original fraudulent drawings.²⁷² Thus, Haeckel's embryo drawings are still used to illustrate a purportedly valid point, namely that vertebrate embryos share early developmental pathways, and that this provides evidence for their shared ancestry. But the evidence shows that the earliest stages of vertebrate embryo development often have important differences that are left out of textbooks.

Some leading embryologists argue that the earliest stages of vertebrate embryo development are very different, and embryos start developing very differently, temporarily converge at a conserved stage midway through development, and then diverge again. Images depicting this conserved stage -- called the "tailbud," "phylotipic," or "phyarngular" stage -- are cherry-picked in textbooks to show similarities between vertebrates, even though the embryos are actually more divergent at earlier stages. As one paper in the journal Systematic Biology explains:Recent workers have shown that early development can vary quite extensively, even within closely related species, such as sea urchins, amphibians, and vertebrates in general. By early development, I refer to those stages from fertilization through neurolation (gastrulation for such taxa as sea urchins, which do not undergo neurulation). Elinson (1987) has shown how such early stages as initial cleavages and gastrula can vary quite extensively across vertebrates.²⁷³Likewise, Richardson and other embryologists explain that vertebrate embryos start their development looking quite different and become similar only at a middle stage of development:According to recent models, not only is the putative conserved stage followed by divergence, but it is preceded by variation at earlier stages, including gastrulation and neurulation. This is seen for example in squamata, where variations in patterns of gastrulation and neurulation may be followed by a rather similar somite stage. Thus the relationship between evolution and development has come to be modelled as an "evolutionary hourglass."²⁷⁴

The "hourglass" model of development is illustrated below, where it shows that vertebrate embryos are actually quite different in their earliest stages of development:²⁷⁵



Copyright Jody F. Sjogren 2000

Textbooks thus typically cherry pick the encircled stage as the alleged "earliest stage" of vertebrate development, when in fact vertebrate embryos at their earliest stages have significant non-trivial differences. Indeed, Richardson and other leading embryologists have called into question the very existence of a conserved "phylotopic" (or "pharyngular" or "tailbud") stage--commonly portrayed in textbooks as evidence for evolution. In a paper titled, There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development, Richardson et al. write the following:We find that embryos at the tailbud stage -- thought to correspond to a conserved stage -- show variations in form due to allometry, heterochrony, and differences in body plan and somite number. These variations foreshadow important differences in adult body form. Contrary to recent claims that all vertebrate embryos pass through a stage when they are the same size, we find a greater than 10-fold variation in greatest length at the tailbud stage. Our survey seriously undermines the credibility of Haeckel's drawings, which depict not a conserved stage for vertebrates, but a stylised amniote embryo. In fact, the taxonomic level of greatest resemblance among vertebrate embryos is below the subphylum. The wide variation in morphology among vertebrate embryos is difficult to reconcile with the idea of a phylogenetically-conserved tailbud stage, and suggests that at least some developmental mechanisms are not highly constrained by the zootype. . . .Contrary to the evolutionary hourglass model, variations in the adult body plan are often foreshadowed by modifications of early development. A good example is the aortic arch system in the rat that, even during the pharyngula stage, begins to presage the adult pattern of arteries. Thus the first arch has already broken down completely by the 25-somite stage in the rat (de Ruiter et al. 1989).

In summary, evolution has produced a number of changes in the embryonic stages of vertebrates including:
1. Differences in body size
2. Differences in body plan (for example, the presence or absence of paired limb buds)
3. Changes in the number of units in repeating series such as the somites and pharyngeal arches
4. Changes in the pattern of growth of different fields (allometry)
5. Changes in the timing of development of different fields (heterochrony)

These modifications of embryonic development are difficult to reconcile with the idea that most or all vertebrate clades pass through an embryonic stage that is highly resistant to evolutionary change. This idea is implicit in Haeckel's drawings, which have been used to substantiate two distinct claims. First, that differences beteen species typically become more apparent at late stages. Second, that vertebrate embryos are virtually identical at earlier stages. This first claim is clearly true. Our survey, however, does not support the second claim, and instead reveals considerable variability -- and evolutionary lability -- of the tailbud stage, the purported phylotypic stage of vertebrates.²⁷⁶

Former NCSE staff member Matzke co-writes that complaints about the use of Haeckel's drawings is a "manufactured scandal."²⁷⁷ Not only are textbooks using inaccurate drawings, but they are using them to illustrate points that are highly disputed by leading embryologists. The earliest stages of vertebrate embryos are quite different and the existence of the cherry-picked conserved stage often portrayed in textbooks as evidence for common ancestry is being called into question.

To say the least, students who are taught that the earliest stages of vertebrate embryos are highly similar, without being told of significant embryological evidence that challenges that view and the very existence of the conserved developmental stage portrayed in many textbooks, are not being adequately informed about the evidence regarding evolution.

[Note: This excerpt is adapted from Casey Luskin, "The Constitutionality and Pedagogical Benefits of Teaching Evolution Scientifically," University of St. Thomas Journal of Law & Public Policy, Vol. VI (1): 204-277 (Fall, 2009). For the full article, click here]

References Cited:
[261.] 10 Questions, and Answers, about Evolution, NEW YORK TIMES, August 23, 2008,

[262.] Even Matzke and Gross recognize that "Haeckel did exaggerate similarities in very early embryos of different species, and his figures, or derivatives of them, have appeared in a few textbooks." Nicholas J. Matzke & Paul R. Gross, Analyzing Critical Analysis: The Fallback Antievolutionist Strategy, in NOT IN OUR CLASSROOMS: WHY INTELLIGENT DESIGN IS WRONG FOR OUR SCHOOLS 40 ( Eugenie C. Scott & Glenn Branch, eds. 2006).

[263.] See Casey Luskin, What Do Modern Textbooks Really Say About Haeckel's Embryos? (Mar. 27, 2007) (citing several examples).

[264.] Stephen Jay Gould, Abscheulich!(Atrocious!), NATURAL HISTORY, Mar. 2000, at 42, 44--45.

[265.] Id. at 45.

[266.] Elizabeth Pennisi, Haeckel's Embryos: Fraud Rediscovered, 277 SCIENCE 1435, 1435 (1997).

[267.] Id.

[268.] Michael K. Richardson et al., There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development, 196 ANATOMY AND EMBRYOLOGY, 91, 92--104 (1997) (internal citations omitted).

[269.] KENNETH R.MILLER & JOSEPH LEVINE, BIOLOGY 385 (2008).

[270.] See COLLEEN BELK & VIRGINIA BORDEN, BIOLOGY: SCIENCE FOR LIFE 240 (2d. ed. 2007).

[271.] SYLVIA S.MADER, BIOLOGY 278 (10th ed. 2010).

[272.] See DISCOVERY INST., supra note 228, at 23.

[273.] Andres Collazo, Developmental Variation, Homology, and the Pharyngula Stage, 49 SYSTEMATIC BIOLOGY 3, 9 (2000) (internal citations omitted).

[274.] Richardson et al., supra note 268, at 92 (internal citations omitted).

[275.] The Embryonic Hourglass as published in JONATHAN WELLS, ICONS OF EVOLUTION: WHY MUCH OF WHAT WE TEACH ABOUT EVOLUTION IS WRONG 100 (2002). Diagram Copyright 2000 by Jody Sjogren.

[276.] Richardson, supra note 268, at 91, 105.

[277.] Matzke & Gross, supra note 262, at 41.