Science

Recapitulation Is the Debunked Theory That Embryos Repeat Evolution

They say science is a process, not a destination. If that's the case, we've hit on some very dumb things in that process. Sometimes, those things are downright bizarre: why did people ever think barnacles gave birth to geese? But sometimes, it's understandable: from where you're standing, it really looks like the sun revolves around the Earth, doesn't it? That second category is where recapitulation theory falls. A developing embryo kind of looks like it repeats its species' stages of evolution — even though it definitely does not.

History in the Womb

The most famous pioneer of recapitulation theory was German zoologist Ernst Haeckel, who famously said "Ontogeny recapitulates phylogeny." In layman's terms, that means the development of an embryo (ontogeny) repeats the evolutionary changes its species took over the millennia to appear in its modern form (phylogeny). Haeckel's biogenetic law, as he called it, was a form of recapitulation theory that said each developmental stage of an embryo represents the adult form of one evolutionary ancestor.

Haeckel's law broke down into three elements. First, each development stage of "higher" animals corresponded to the adult stages of "lower" animals. For example, human embryos develop "gill slits" around the fourth week of development, which eventually develop into lungs. In Haeckel's view, those corresponded to the adult form of humans' fish-like ancestors.

The second element involved the order in which these features arose. Haeckel believed that new, unique features only showed up near the end of development. Third, the law said that because "higher" species had more of these new features, their development had to be faster than "lower" species to keep the gestation period from going on forever.

Haeckel was a big fan of Charles Darwin, but Darwin didn't return the favor. Darwin didn't believe in "higher" or "lower" species, or that species only got more complex as they evolved. In fact, Haeckel's theory may have started as a misunderstanding of Darwin, who pointed out that vertebrate embryos look similar to each other early in their development, and used that as evidence of evolution. The important term there is early in their development — Darwin argued that they looked more and more different from each other the  they got.

In fact, Haeckel's theory was kind of a mish-mash of Darwinian evolution and its competitor, Lamarckian evolution. French scientist Jean-Baptiste Lamarck said that organisms could pass on characteristics acquired within their lifetimes, which squared well with Haeckel's belief that embryos developed more and more complex structures.

We All Make Mistakes

There are a lot of problems with Haeckel's theory, mostly because saying "hey, this thing kinda looks like that thing!" doesn't really hold up scientifically. For one, he fudged his drawings. He drew fish embryos alongside human embryos in a way that glossed over their differences and highlighted their similarities. In 1997, researchers led by Michael Richardson of St. George's Hospital Medical School in London used photographs of vertebrate embryos to show that there isn't actually any development stage where all vertebrate embryos are identical, proving Haeckel's drawings wrong.

That's not to say there are no similarities among developing embryos. Darwin called those similarities "by far the strongest single class of facts in favor" of evolution by natural selection. It's just that you can't look at a developing embryo and know what forms it evolved from. Still, as recently as the 1990s, textbooks were citing Haeckel's theories in chapters on evolution. Falsehoods are stubborn things, but we get rid of them eventually.

Pregnancy & Development

Written by Ashley Hamer October 30, 2017