Developmental Similarities: Karl von Baer
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 chickensas well as mammals like us, amphibians, and reptilesthey 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 serieswe "recapitulate" iton 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.
What about Baer's claim that vertebrates couldn't be aligned with invertebrate animals? Embryologists working in the mid-1800s showed that the division was not unbridgeable. Some invertebrates known as sea squirts, for example, develop the same kind of stiff rod that vertebrates form in their back as embryos, known as a notochord. In vertebrates the notochord turns into the disks between the vertebrae. If this were in fact a sign of common ancestry, you'd expect sea squirts to be close relatives of vertebrates. And indeed, studies on the DNA of sea squirts show that they are in fact the closest invertebrate relatives of vertebrates yet known.
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Karl von Baer image courtesy of the Karl Ernst von Baer home page.; Adult tunicates image courtesy of Crissy Huffard, UCMP; Tunicate larva image courtesy of Richard Grosberg, UC Davis
Understanding Evolution © 2015 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California