The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

You can see the appeal of the idea of recapitulation. The life story of every young animal is a telescoped re-enactment of its (adult) ancestry. We all start as a single cell: that represents a protozoan. The next stage in development is a hollow ball of cells, the blastula. Haeckel suggested that this represents an ancestral stage, which he called the blastaea. Next in embryology, the blastula invaginates, like a ball punched in as a dent from one side, to form a cup lined by a double layer of cells, the gastrula. Haeckel imagined a gastrula-stage ancestor, which he called the gastraea. A cnidarian, such as a hydra or a sea anemone, has two layers of cells, like Haeckel’s gastraea. In Haeckel’s recapitulationist view, cnidarians stop climbing up their family tree when they reach the gastrula stage, but we soldier on. Subsequent stages in our embryology resemble a fish with gill slits and a tail. Later we lose our tail. And so on. Each embryo stops climbing up its family tree when it reaches its appropriate evolutionary stage. Appealing as it is, the recapitulation theory has become unfashionable—or rather it is now regarded as a small part of what is sometimes but not always true.