Ernst Haeckel published his influential theory of embryology, distilled as “ontogeny recapitulates phylogeny” in 1866, seven years after Darwin’s On the Origin of Species. Haeckel’s theory fell out of favor and hasn’t been part of evolutionary theory for decades, but it’s still cited today as a cause of mischief by modern Creationists.

Haeckel’s theory

The similarities between embryos of different animal species were noted decades before Darwin: while adults of different species are easy to tell apart, their embryos are not. Haeckel took this further and is most known for his 1874 drawing (above) of the development of various animal embryos—fish, chicken, human, and so on—to illustrate his point.

Ontogeny is the development of an embryo, and phylogeny is an organism’s evolutionary history. So by “ontogeny recapitulates phylogeny,” Haeckel was saying that you can watch through an organism’s development as an embryo a replay of its development through hundreds of million years of evolution. For example, a human embryo first looks like a fish (notice the gill-like structure), then like a reptile (four limbs and a tail), and finally like a mammal, which is the evolutionary path that humans took.

But it doesn’t work like that.

What embryology actually tells us

Let’s put Haeckel aside for now and look for clues to evolution within embryology. If Creationists could get beyond “Haeckel was wrong,” which no one denies, there are important insights here. What’s fascinating is how embryonic structures that developed in animals that preceded humans, like fish and reptiles, have been repurposed by evolution for humans.

Pharyngeal arches or folds (often improperly called “gill slits”) are the double-chin-like folds under the head in the early embryo stage. This striking feature is found in all vertebrate embryos.

The arches that develop into gills in fish become various cartilages, glands, muscles, and other tissue in the human neck and face.

These arches explain the strange path of the recurrent laryngeal nerve. Pharyngeal arches four, five, and six (arch one is closest to the head) fuse early in the development of mammal embryos. The recurrent laryngeal nerve comes from the fourth arch, and after the fusion, it is near an artery from the sixth arch. This creates a straightforward layout in fish, but in mammals the neck takes the brain and larynx (connected by this nerve) away from the heart. The problem is that the nerve is hooked around that artery. That means that in all mammals—yes, even the long-necked giraffe—the nerve goes from the brain, down around this artery, and back up to the larynx. No perfect designer would create this, but it is nicely explained by evolution.

Another example of repurposing (technically, exaptation) is the mammalian ear. Structures that develop into a multi-bone jaw in reptiles have been repurposed to become ear bones in mammals. In fact, it was embryology, not fossils, that provided the first clues of this evolution.

Turn back humans’ evolutionary clock and we see the tail grow back (as in other mammals), the ear bones become jaws (as in reptiles), and the throat becomes gills (as in fish).

The Creationists

Creationists respond that the perfect designer was making variations on a theme. If you’ve got a great design, why design everything from scratch? Why not simply tweak it for various environments? This designer is like a car company that makes small cars (shrew, mouse) and big ones (elephant, whale), cars that are beautiful (peacock, gazelle), and cars for tough environments (camel, yak), and so on.

The supernatural assumption adds nothing when we have a natural theory that explains evolution just fine. “God” is a solution looking for a problem, and we don’t have a problem here.

Another obvious similarity across early embryos is the tail. Human embryonic tails are absorbed later in development. The hind limbs of cetaceans like whales also appear in embryos and are likewise absorbed.

If you saw the movie Avatar, did you catch the evolution mistake it makes? The land animals had six limbs and breathed through a second mouth on their shoulders. The winged creatures also had six limbs—four legs and two wings. But the Na’vi people had four limbs and no shoulder mouths. If they had a common ancestor with the other animals of their world, like people on earth, you would see these fundamental characteristics shared.

Ah, well—Hollywood.

Creationism’s failure

Why do adult animals differ in appearance but look similar to embryos? Why should the same basic embryonic components become gills in fish but faces in mammals? Why do human embryos have a tail that is later reabsorbed? The common beginning as early embryos and later divergence to satisfy different body plans points to common ancestry, not design. Evolution explains all this nicely, while Creationism has no explanation.

The Creationist playbook is to attack evolution, usually by asking questions that are important but already answered. Biologists have a ready answer, but these questions stump the average person, and deceiving a lay audience is the goal, not changing biologists’ minds.

Even if Creationism’s questions were new and insightful (they never are), Creationism doesn’t become the dominant scientific paradigm by showing flaws in evolution; it could only do that by explaining the evidence better. But since Creationists are only pretending to be scientific, playing by science’s rules is never the goal. Creationists don’t participate in the domain of regular biology, which includes conferences, journals, and laboratories. They’ve already lost there, and that’s been true for a century. So they peddle their message exclusively to the public, a glaring admission that they aren’t doing science.

Yes, Haeckel was wrong, and his error, like any popular wrong turn, delayed progress. But evolution was never built with this as part of its foundation. Turn back humans’ evolutionary clock and we see the tail grow back (as in other mammals), the ear bones become jaws (as in reptiles), and the throat becomes gills (as in fish). Haeckel got a lot wrong, but he was right that embryology holds clues to where we came from.

Somebody’s gotta stand up to experts.
— Don McLeroy,
on the Texas board of education but not a biologist,
speaking against evolution in public schools