Ernst Haeckel was an influential German scientist who supported Charles Darwins theory of evolution. He published his influential theory of embryology, distilled as ontogeny recapitulates phylogeny in 1866, seven years after Darwins On the Origin of Species. Haeckels theory fell out of favor and hasnt been part of evolutionary theory for many decades, but its still cited today as a cause of mischief by modern Creationists.

Haeckels 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 embryosfish, chicken, human, and so onto illustrate his point.

Ontogeny is the development of an embryo, and phylogeny is an organisms evolutionary history. So by ontogeny recapitulates phylogeny, Haeckel was saying that you can watch through an organisms 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 doesnt work like that.

What embryology actually tells us

Lets put Haeckel aside for now and look for clues to evolution within embryology. Whats 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 visual commonality 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 mammalsyes, even the long-necked giraffethe 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.

The Creationists

Creationists respond that the perfect designer was making variations on a theme. If youve 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 (shrews, mice) and big ones (elephants, whales), cars that are beautiful (peacock, gazelle) and cars for tough environments (camel, yak).

In the first place, 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 dont have a problem here.

And second, if land animals are like cars, why do they begin life looking like submarines? (h/t Troy Britain)

See also: Argument from Design BUSTED!

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 limbsfour legs and two wings. But the Navi 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, wellHollywood.

Creationisms failure

Why do adult animals differ in appearance but look similar as 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 play book 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, which is the target audience.

Even if Creationisms questions were new and insightful (they never are), Creationism doesnt 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 sciences rules is never the goal. Creationists dont participate in the domain of regular biology, which includes conferences, journals, and laboratories. Theyve already lost there, and thats been true for a century. So they peddle their message exclusively to the public, another admission that they arent 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 grows 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.

(h/t commenter MR for links to articles)

Acknowledgements: these excellent articles were helpful in writing this post.

Somebodys gotta stand up to experts. Don McLeroy, Texas board of education,speaking against evolution in public schools

Image credit: Wikimedia

Read more from the original source:
Saving Haeckel: Why Ontogeny Recapitulates Phylogeny Isn't so Wrong - Patheos (blog)