Developmental Similarities: Karl von Baer (1 of 2)

How does life begin? At the dawn of the nineteenth century, naturalists were staring through microscopes in hopes of finding the answer. In the process, they discovered some peculiar things about embryos. A chicken may look very different from a fish, but their embryos share some striking similarities. They both develop from a single cell into tube-shaped bodies, for example. They share many traits early on, such as a set of arching blood vessels in their necks. In fish, the vessels retain this arrangement, so that they can take in oxygen from their gills. But in chickens—as well as mammals like us, amphibians, and reptiles—they are reworked into a very different anatomy suited to getting oxygen through lungs.

In Germany, where much of this study was done, some researchers claimed that these similarities were signs that life formed a series from simple forms to lofty ones (the loftiest being, of course, ourselves). As embryos we pass through this series—we “recapitulate” it—on our way to becoming human. We started out life as a worm, became a fish (complete with gill arches), a reptile, and so on. Some naturalists even claimed that recapitulation was evidence that life had changed through time, as higher and higher forms emerged on Earth.

von Baer: Recapitulation Is Kaput
In 1828, the Estonian-born embryologist Karl von Baer launched a withering attack on recapitulation. A careful look at embryos revealed that it was impossible to arrange them in any meaningful series. From the earliest stages, vertebrates all share an anatomy that invertebrates such as insects or worms never acquire. And even within vertebrates, there are facts that clash with recapitulation. A human does not develop a wing or a hoof before forming a hand—humans, birds, and horses all begin with limb buds, which then diverge into different adult limbs.

• Karl von Baer image courtesy of the Karl Ernst von Baer home page.