Why Evolution Is True

by Jerry A. Coyne

As one group evolves from another, it often adds its development program on top of the old one. Noting this principle, Ernst Haeckel, a German evolutionist and Darwin’s contemporary, formulated a “biogenetic law” in 1866, famously summarized as “Ontogeny recapitulates phylogeny.” This means that the development of an organism simply replays its evolutionary history. But this notion is true in only a limited sense. Embryonic stages don’t look like the adult forms of their ancestors, as Haeckel claimed, but like the embryonic forms of ancestors. Human fetuses, for example, never resemble adult fish or reptiles, but in certain ways they do resemble embryonic fish and reptiles. Also, the recapitulation is neither strict nor inevitable: not every feature of an ancestor’s embryo appears in its descendants, nor do all stages of development unfold in a strict evolutionary order. Further, some species, like plants, have dispensed with nearly all traces of their ancestry during development. Haeckel’s law has fallen into disrepute not only because it wasn’t strictly true, but also because Haeckel was accused, largely unjustly, of fudging some drawings of early embryos to make them look more similar than they really are.19 Yet we shouldn’t throw out the baby with the bathwater. Embryos still show a form of recapitulation: features that arose earlier in evolution often appear earlier in development. And this makes sense only if species have an evolutionary history.