Since the late 19th century, it has been known that some of Ernst Haeckel’s embryo drawings were highly inaccurate (they overstate the degree of similarities between vertebrate embyros and ignore the differences). In the 1990s, some influential science journals (discussed below) hosted articles where leading scientists vocally opposed the use of Ernst Haeckel’s inaccurate — even “fraudulent” — embryo drawings in textbooks. Then, in his 2000 book Icons of Evolution, biologist Jonathan Wells exposed just how many modern textbooks continue to use Haeckel’s drawings. (For articles about some modern textbooks that use Haeckel’s drawings, see here or here.)
One would have thought this outcry would have been enough to send a message to textbook publishers to stop using these inaccurate drawings as evidence for evolution. But it wasn’t.
In 2003, textbooks submitted to the Texas State Board of Education (TSBOE) for adoption again contained Haeckel’s drawings. This story ended well: Jonathan Wells’ work led to the removal of the inaccurate drawings from the proposed textbooks during the 2003 biology textbook adoption process.
Like a zombie that just won’t die, these bogus drawings keep coming back. Now in 2011, Haeckel’s fraudulent embryo drawings are threatening to re-enter the curriculum in Texas.
As elaborated in our report, some publishers (e.g. Adaptive Curriculum and Rice University) have submitted teaching materials for use in Texas that include Haeckel’s inaccurate embryo drawings. Indeed, even many publishers that didn’t use Haeckel’s drawings still overstate the degree of similarity between vertebrate embryos, and ignore the differences.
The Facts About Embryology and Haeckel’s Drawings
Textbooks commonly claim that vertebrate embryos are highly similar in their earliest stages, and that these similarities point to their common ancestry. Some textbooks try to illustrate these claims by using inaccurate embryos drawings based on the work of the 19th century embryologist Ernst Haeckel– drawings which overstate the degree of similarity between embryos. Textbooks may also claim that human embryos have “gill slits.” It is widely acknowledged, even by leading evolutionary scientists, that Haeckel’s embryo drawings were “highly inaccurate, exaggerating the similarities among embryos, while failing to show the differences.” 1 Stephen Jay Gould called them “fraudulent”2 and the leading embryologist Michael Richardson called them “fakes.” 3 Even the journal Science acknowledges 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” because “the impression they give, that the embryos are exactly alike, is wrong.”4
Unfortunately, some current biology textbooks continue to use Haeckel’s inaccurate drawings to advocate for common ancestry. In fact, in 1997 Richardson acknowledged that there are “at least fifty recent biology textbooks which use the drawings uncritically”5 and stated that the 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.”6 This led Gould to exclaim in 2000:
[W]e 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!7
Because of these criticisms, in the past decade more and more biology textbooks have used photographs of embryos instead of inaccurate drawings. However, some current biology textbooks continue to use Haeckel’s inaccurate drawings to advocate for common ancestry,8 and in fact two curricula evaluated in this report use Haeckel’s embryo drawings.9 Nonetheless, whether textbooks use drawings or photographs, most textbooks still overstate the degree of similarity between vertebrate embryos. Contrary to the claims of textbooks, leading embryologists have acknowledged that the earliest stages of vertebrate embryo development are in fact very different. As a 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.10
Vertebrate embryos start developing very differently, and at most temporarily converge at a somewhat similar stage midway through development, and then diverge again. Appearances during this similar or “conserved” stage–called the “tailbud,” “phylotypic,” or “pharyngula” stage–are cherrypicked in textbooks, ignoring earlier stages with much greater differences between embryos. A paper in the journal Anatomy and Embryology explains this “hourglass” pattern 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.”11
This ‘hourglass’ model of development is illustrated below, where it is seen that vertebrate embryos are actually quite different in their earliest stages of development, at the top of the hourglass:12
Copyright Jody F. Sjogren 2000But even the existence of this purportedly similar and conserved “pharyngula” (or “phylotypic” or “tailbud”) stage has been called into question. A paper by leading embryologists, titled “There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development,” found that differences in body size, body plan, growth patterns, and growth timing show “wide variation in morphology among vertebrate embryos,” which “is difficult to reconcile with the idea of a phylogenetically-conserved tailbud stage.”13
Likewise, a 2003 paper in Proceedings of the Royal Society, B contends that differences between vertebrate embryos “argu[e] against the existence of a phylotypic stage in vertebrates.”14
Finally, some textbooks also claim that human embryos have “gill slits,” allegedly reflecting our fish ancestry. But biologist Jonathan Wells explains in The American Biology Teacher that human embryos do not have gill slits:
[H]uman embryos do not really have gills or gill slits: like all vertebrate embryos at one stage in their development, they possess a series of ‘pharyngeal pouches,’ or tiny ridges in the neck region. In fish embryos these actually go on to form gills, but in other vertebrates they develop into unrelated structures such as the inner ear and parathyroid gland. The embryos of reptiles, birds and mammals never possess gills.15
Textbooks thus commonly provide an inaccurate depiction of vertebrate development, overstating the degree of similarity between vertebrate embryos while ignoring the differences, especially in the earliest stages. The result is misleading claims based upon cherry-picked data, or simply flat-out inaccurate claims, that are used in textbooks to bolster common ancestry of vertebrates.
What Do Proposed Supplemental Materials in Texas Say?
As noted, various publishers have proposed instructional materials to the TSBOE that present inaccurate, misleading, or one-sided information about embryology, often using Haeckel’s fraudulent embryo drawings. By doing so, these publishers have failed to follow state science curriculum standards, which call for materials that help students “analyze, evaluate, and critique scientific explanations by using empirical evidence, logical reasoning, and experimental and observational testing, including examining all sides of scientific evidence of those scientific explanations, so as to encourage critical thinking by the student.”
Here is a run-down of the inaccurate information presented:
In a section titled “Anatomical and Developmental Homologies as Evidence for Evolution,” this publisher claims that the embryos of humans, birds, reptiles, fish, and amphibians are “very similar and share many characteristics,” thereby providing “evidence that they evolved from a common ancestor.” There is no discussion of any differences between the early stages of embryonic development. To bolster the curriculum’s incomplete, misleading, and inaccurate claims, it uses Haeckel’s embryo drawings:
It also wrongly claims that human embryos have gill slits, stating that in fish “embryonic gill slits develop into true gills while in humans they develop into the ears and throat.”
In a section titled “Evidence of Common Ancestry,” materials submitted by Rice University inaccurately state: “embryological studies also show similar developmental stages among different species indicating the evolution from a common ancestor.” The materials then use Haeckel’s discredited embryos to further overstate the degree of similarity between early vertebrate embryos:
Students are then asked to arrange Haeckel’s inaccurate drawings in the ‘right’ order. The answer key is given at the end as follows:
A test question also forces students to inaccurately assent that these drawings provide evidence for evolution:
Some materials submitted did not use Haeckel’s exact drawings, but nonetheless overstated the degree of similarity between vertebrate embryos. For example, Apex Learning’s proposed materials claim that “Organisms with similar embryonic development are often considered to have similar ancestry.” But it makes no mention differences in early embryos in vertebrates.
Similarly, Technical Lab Systems submitted materials which claim that “Species which have a common ancestor typically share the early stages of embryo development and differ in later stages.” It further claims “in their early stages of development, chickens, turtles and humans look similar providing evidence that they shared a common ancestor.” Again, there is no discussion of differences between the embryos, nor is there any critique or evaluation of evolutionary claims, or presentation of all sides of the data.
In a section titled “Similarities in Embryology” the Holt McDougal materials state: “The early stages of different vertebrate embryos are strikingly similar to each other. These similarities may provide another indication that vertebrates share a common ancestry. However, it is important to note that there are major differences in the embryonic development of different types of vertebrates, and the similarities fade as development proceeds.” It also presents Haeckel-like drawings that overstate the degree of similarities between embryos. The qualifications are meager and there is no mention of the differences between embryos.
Finally, Cengage Learning submitted materials that don’t use Haeckel’s drawings, but nonetheless overstate the degree of similarity between early vertebrate embryos and even hints at a long-abandoned ‘ontogeny recapitulates phylogeny’ view of development:
Support for the theory of evolution comes from a number of sources. One of these sources is the science of embryology, the study of early forms of an organism. Darwin reasoned that organisms that have passed through a period of evolution will retain some reminders of that history within their bodies. As its [sic] turns out, virtually all living creatures possess vestigial features. A vestigial feature is a structure that once served some function in an ancestor and remains in an organism at some stage of its development. But the structure no longer serves any function in that organism. As an example, the embryos of all vertebrates (animals with backbones) look remarkably alike at an early stage.
This inaccurately promotes a concept similar to recapitulation theory, and also ignores differences between early stages of embryos.
Just as Haeckel twisted and overstated the embryological evidence, Darwin lobbyists typically respond to our criticisms by twisting and overstating them. Lest Darwin lobbyists try to confuse the issue here: we’re not saying (and never have said) that recapitulation theory or Haeckel’s drawings are necessarily the bedrock of modern evolutionary biology. Nonetheless, Haeckel’s inaccurate drawings and inaccurate claims about vertebrate embryos keep appearing in textbooks, and this inaccurate material is being used for the purpose of overstating the evidence for evolution. The evidence should be presented accurately and these drawings and other inaccurate claims should be removed.
[1.] Michael K. Richardson et al., “There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development,” Anatomy and Embryology, Vol. 196:91 (1997) (internal citations omitted).
[2.] See for example Stephen Jay Gould, “Abscheulich!(Atrocious!),” Natural History, (Mar. 2000).
[3.] Michael K. Richardson quoted in Elizabeth Pennisi, “Haeckel’s Embryos: Fraud Rediscovered,” Science, Vol. 277:1435 (1997).
[4.] Elizabeth Pennisi, “Haeckel’s Embryos: Fraud Rediscovered,” Science, Vol. 277:1435 (1997).
[5.] Michael K. Richardson quoted in Elizabeth Pennisi, “Haeckel’s Embryos: Fraud Rediscovered,” Science, Vol. 277:1435 (1997).
[6.] Michael K. Richardson et al., “There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development,” Anatomy and Embryology, Vol. 196:91 (1997) (internal citations omitted).
[7.] Stephen Jay Gould, “Abscheulich!(Atrocious!),” Natural History, (Mar. 2000).
[8.] For discussions, see: 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); Casey Luskin, “What do Modern Textbooks Really Say about Haeckel’s Embryos?”
[9.] For example, Adaptive Curriculum uses a colorized version of Haeckel’s original drawings, and Rice University uses the original drawings themselves.
[10.] Andres Collazo, “Developmental Variation, Homology, and the Pharyngula Stage,” Systematic Biology, Vol. 49:3 (2000) (internal citations omitted).
[11.] Michael K. Richardson et al., “There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development,” Anatomy and Embryology, Vol. 196:91 (1997).
[12.] The Embryonic Hourglass as published in Jonathan Wells, Icons of Evolution: Why Much of What We Teach about
Evolution is Wrong, p. 100 (2000). Diagram Copyright 2000 by Jody Sjogren.
[13.] Michael K. Richardson et al., “There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development,” Anatomy and Embryology, Vol. 196:91 (1997).
[14.] Olaf R. P. Bininda-Emonds, Jonathan E. Jeffery, and Michael K. Richardson, “Inverting the hourglass: quantitative evidence against the phylotypic stage in vertebrate development,” Proceedings of the Royal Society of London, B, Vol. 270:341-346 (2003).
[15.] Jonathan Wells, “Haeckel’s Embryos & Evolution: Setting the Record Straight,” The American Biology Teacher, Vol. 61(5):345-349 (May, 1999) (internal citations removed).