An embryo passes through complete adult stages of its evolutionary ancestors during development.

Nineteenth-century embryologists noticed striking similarities among developing vertebrates. Ernst Haeckel turned those observations into the famous phrase “ontogeny recapitulates phylogeny”: the development of an individual was supposed to replay the evolutionary history of its species.

In the strongest version of the idea, an embryo passed through stages corresponding to the adult forms of its ancestors. A human embryo, for example, could be described as temporarily passing through a fish-like stage.

The real similarities are more subtle.

Related animals inherit many of the same developmental genes and begin by building some of the same basic structures. Vertebrate embryos, for example, form pharyngeal arches in the head and neck region. In fish, some of these structures contribute to gills; in mammals, related embryonic structures develop into parts of the jaw, ear, throat, and neck.

That shared starting point reflects common ancestry. But a human embryo is never a small adult fish. Embryos branch into species-specific development, and evolution can change the timing, shape, and eventual use of inherited developmental structures.

Modern evolutionary developmental biology therefore preserves the connection between development and ancestry while rejecting Haeckel’s literal replay of adult ancestral stages.