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Developmental Similarities: Karl von Baer (2 of 2)

Compelling Evidence for Evolution
Baer was no fan of evolution, and so it was much to his chagrin that Darwin used his work to provide some of the most compelling evidence in Origin of Species. A species inherits its developmental program from its ancestors, and so two closely related species would be expected to have similar—but not necessarily identical—embryos. Over time, as lineages evolve further away from each other, natural selection modifies their embryos in various ways, but some vestiges of their common ancestry survive. That’s why we still bear a limited resemblance to fish in our early embryonic stages. Darwin did not argue that life was arrayed in a linear series from lower to higher; instead, he saw life branching like a tree as new species emerged.That branching was reflected in the similar paths of development that ultimately produced hooves, claws, and hands.

Some researchers believed life developed in a linear fashion, from simple to more complex forms (left). Darwin compared the emergence of new species to the branching of a tree (right).

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.

Left: Since Baer’s time, tunicates (several varieties of adults, top) have been found to be a bridge between invertebrates and vertebrates. Tunicate larvae (below) have a notochord much like that found in vertebrate embryos.

• Adult tunicates image courtesy of Crissy Huffard, UCMP.
• Tunicate larva image courtesy of Richard Grosberg, UC Davis.

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