Heckling using Haeckel – What the pharyngula stage says about vertebrate evolution

19th century biologist Ernst Haeckel’s embryo drawings remain one of evolutionary biology’s most controversial subjects, even more than 130 years after their original publication, least not due to the recent attempts by Discovery Institute fellows Casey Luskin and Jonathan Wells to bring the drawings back into the “discussion” about evolutionary biology and intelligent design. Casey and Jonathan’s obsessions with the drawings have lead them to attack certain biology textbook writers and, by extension, evolutionary biologists, playing up the issue as another supposed act of deception by the “Darwinian establishment”, brainwashing high school students and undergraduates with fraudulent evidence for evolutionary theory. However, their claims about both Haeckel’s drawings and their use in some textbooks, as well as the implications of other areas of developmental biology, leave much to be desired, and amount to little more than frantic lunges at anything vaguely controversial in biology and twisting them to prop up their own agenda of the promotion of intelligent design creationism.

Haeckel’s evolutionary ideas and his embryo drawings

During the 19th century, Ernst Haeckel, a German biologist and Charles Darwin enthusiast, proposed an evolutionary hypothesis usually entitled “the biogenic law”, wherein the evolutionary history of a particular organism could be directly traced back through the developmental pathway of its embryonic form. He summarised the idea into the rather neat and impressive-sounding phrase “ontogeny recapitulates phylogeny”, and it can be found all over the Internet and in (some) popular, if misinformed, science writing. Haeckel’s biogenic law, using humans as an example, meant that the human embryo passed through a stage that resembled a fish, complete with “gill slits”, then a stage that resembled a reptile, with four limbs and a tail, and then finally a mammalian human, with human-like features and appendages. In short, the embryo progressed through stages that reflected the adult forms of the organisms in its evolutionary history.

Key to supporting Haeckel’s hypothesis were his comparative drawings of various vertebrate embryos, first published in his 1874 book Anthropogenie: oder, Entwickelungsgeschichte des Menschen (in English – Anthropogenie: or the developmental history of man), which showed remarkable similarities between the embryos at early stages of embryonic development.

An example of Haeckel's embryo drawings, with the top row representing in the pharyngula stage. Note that each column is the developmental pathway of a different organism.

It has been known for a long time in the scientific community that Haeckel exaggerated the similarities and omitted many of the differences between the embryos that he drew, either subconsciously or purposefully, and as such his status as a notable scientist has been thrown into disrepute, despite his other scientific efforts. Haeckel’s biogenic law has also been discredited in the light of modern developmental biology and embryology, which shows that the developmental pathway of an organism does not trace its adult-form evolutionary history.

The pharyngula stage of development

However, Haeckel was not entirely incorrect – while the developmental pathways of organisms fail to reflect their ancestors’ adult forms, they do on some level reflect the embryos of their ancestors and modern, closely-related organisms. The pharyngula stage of vertebrate development is also called the phylotypic stage, and it’s at that stage where vertebrate embryos all resemble each other. This simple fact is shown in Hackel’s drawings, albeit in an exaggerated sense – the embryos are not exactly similar, but share many common features, which are:

• a notochord
• a dorsal nerve cord, which develops into the brain and spinal cord
• pharyngeal arches and pouches (also called “gill slits”, but this term isn’t totally accurate), which develop into gills in fish, but in mammals they develop into the facial bones, parts of the internal and external structures of the ears, the thyroid and thymus glands, etc.
• a postanal tail, which is conserved in a lot of vertebrate species (forming the developed tail), but not in humans

These features suggest that all vertebrates share a common ancestor – a claim that is also backed up by molecular, phylogenetic and fossil evidence.

Casey Luskin and Jonathan Wells’s war on Haeckel

In 2004, Jonathan Wells published a book entitled Icons of Evolution, in which, amongst other things, he attacked biology textbooks that he thought contained Haeckel’s discredited embryo drawings, even if they were critical of Haeckel’s biogenic law, or even, in some circumstances, where they didn’t even use his drawings at all, instead using actual photos of the pharyngula stages of various organisms.

Casey Luskin has recently attacked textbooks showing Haeckel’s embryos in two posts on Evolution News & Views, picking up the fight from where Jonathan left it many years ago. Here’s a sample of his indignation:

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.” 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.”

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.” 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. 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.

Casey is correct that some textbooks still use Haeckel’s drawing uncritically, even if they don’t use them to support the biogenic law but the observation that the pharyngula stage is phylotypic and this supports the common ancestry of vertebrates. This shouldn’t happen, and indicates laziness by the textbook authors – photos of the embryos are far more accurate and eliminate confusion about Haeckel’s biogenic law, which no textbook author wants to be found supporting, even if it is in an indirect manner.

However, Casey does not have a valid point when he complains about the differences between the pharyngula stages of various vertebrates:

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” [sic] stage — are cherry-picked in textbooks to show similarities between vertebrates, even though the embryos are actually more divergent at earlier stages. [Emphasis from original text]

…

Textbooks thus typically cherry pick the [pharyngula] 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.

Yes, it’s true that there are large differences between the earliest stages of development between vertebrate species – for example, the amount of yolk and extra-embryonic tissues vary wildly from fish to birds to mammals to reptiles – but this is largely irrelevant, as we shall soon see. Casey is also slightly misquoting Michael Richardson’s 1997 paper, There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development, as Richardson does not conclude that there is no phylotypic stage in all vertebrates, merely that this stage is less conserved than was once thought. The phylotypic stage still exists, whether intelligent design proponents like it or not.

Differences in developmental pathways are to be expected from evolutionary theory

Casey seems to think that not informing students about the differences between the earliest developmental stages (and the pharyngula stages) in vertebrates amounts to deceiving them:

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.

He’s implying, of course, that this evidence goes against what would be expected from evolutionary theory, but this is not the case. Here’s the last couple of sentences from Richardson’s paper:

We suggest that not all developmental mechanisms are highly constrained by conserved developmental mechanisms such as the zootype. Embryonic stages may be key targets for macroevolutionary change.

What Casey doesn’t understand is that there’s nothing in evolutionary theory that would prevent macroevolutionary change taking place at any stage of embryonic development. Modern evolutionary biology does not claim that the developmental sequence has to be extremely conserved, only that it could be, and a moderate amount of conservation is what we see – the pharyngula stage is basically phylotypic when you consider that the four common features of the stage are present in every vertebrate embryo, and the changes that are found between vertebrates are not that extreme.

Evolutionary changes can be effected by changes to any part of the developmental sequence, and it could be that the pharyngula stage is the most conserved because the embryos are all in a relatively similar environment, while the earlier stages are more divergent because of differing fertilisation techniques, while the later stages are more divergent still because of novel and unique gene pathways being activated to produce the highly different final adult forms of each vertebrate group. Perhaps someone could research that? It sounds like an intriguing hypothesis.

It’s easy to get caught up in the idea of progressive evolution and the idea of an evolutionary ladder, but the truth is that evolutionary changes can occur at any point in development, rendering this vaguely-Haeckelian idea out of touch with reality. The fact that a phylotypic stage even exists is quite remarkable when you think of how many potential ways development could have changed amongst vertebrates, but one does, and it is powerful evidence that vertebrates share a common ancestor at some point. Without common ancestry, this developmental quirk makes absolutely no sense – why would a fish embryo and a primate embryo share a developmental stage where they have a similar body shape, a notochord, a dorsal nerve cord, a postanal tail and pharyngeal pouches and arches, after developing from two quite different earlier stages?

Concluding thoughts

Developmental biology can give us clues about the level of phylogenetic similarity between organisms, but like the morphological differences between adult forms, there is a limit – some features are so different that there is no homology present, for example, the homology between echinoderm and mammalian arms is non-existent, but that doesn’t mean that echinoderms and mammals lack a common ancestor. Put another way, the differences between early developmental stages in vertebrates say nothing substantial about their evolutionary relationships, it’s the similarities that tell us that.

The pharyngula stage is great evidence for the existence of a common ancestor for all vertebrates, but it doesn’t tell us much about that common ancestor. The molecular, phylogenetic and fossil evidence is far better when it comes to determining those ancestral forms.

The developmental evidence is not as obvious as the genetic evidence is – basic developmental pathways are far more easily changed by evolutionary forces than the underlying genetic pathways that produce those changes (ie. birds lacking teeth, but retaining the genes for developing teeth), as well as the overall genetic tools used by organisms in development (ie. Hox genes). In other words, it’s easier to obscure phylogeny through developmental change when you look at gross morphology, but not so easy to obscure it on the genetic level (of course, this only applies when you’re comparing two relatively separate taxonomic groups – subgroups will retain the developmental pathways of the larger group, for example, whales will still develop like mammals despite lacking many of the adult features like hair).

Introductory biology textbooks could theoretically talk about all this and give students “the complete picture”, but it’s not really necessary, as this information is really only relevant when you study developmental biology or evolutionary biology. This information is also not contrary to evolutionary theory, regardless of what Casey and other intelligent design proponents say, so there is no “withholding” of contradictory evidence.

A level of detail that is appropriate to an introductory course would be: talking about the history of Haeckel’s ideas, showing his drawings (including information about the inaccuracies), then showing photos of the pharyngula stages of various vertebrates and briefly touching on the significance of their similarities. Going into detail about the differences between the pharyngula stages and the earlier developmental stages misses the point about what you’d be trying to teach in this section of the textbook.

It’s clear from Casey’s obfuscation and rhetoric that he’s far more interested in confusing the issue of developmental biology with regards to evolution for students than he is in trying to clear the air. After all, the more confused someone is about the real science, the more likely they’ll be to accept the junk science of intelligent design that is constantly promoted by the Discovery Institute.

19th century biologist Ernst Haeckel’s embryo drawings remain one of evolutionary biology’s most controversial subjects, even more than 130 years after their original publication, least not due to the recent...