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Lines of evidence: The science of evolution :

Homologies: developmental biology

Studying the embryological development of living things provides clues to the evolution of present-day organisms. During some stages of development, organisms exhibit ancestral features in whole or incomplete form.

Snakes have legged ancestors.
Some species of living snakes have hind limb-buds as early embryos but rapidly lose the buds and develop into legless adults. The study of developmental stages of snakes, combined with fossil evidence of snakes with hind limbs, supports the hypothesis that snakes evolved from a limbed ancestor.

fossil snake hindlimb Fossil snake hindlimb bones labelled
Above left, the Cretaceous snake Pachyrhachis problematicus clearly had small hindlimbs. The drawing at right shows a reconstruction of the pelvis and hindlimb of Pachyrhachis.

Baleen whales have toothed ancestors.
Toothed whales have full sets of teeth throughout their lives. Baleen whales, however, only possess teeth in the early fetal stage and lose them before birth. The possession of teeth in fetal baleen whales provides evidence of common ancestry with toothed whales and other mammals. In addition, fossil evidence indicates that the late Oligocene whale Aetiocetus (below), from Oregon, which is considered to be the earliest example of baleen whales, also bore a full set of teeth.

Aetiocetus skull

Again, these observations make most sense in an evolutionary framework where snakes have legged ancestors and whales have toothed ancestors.
Pachyrhachis photo © Michael Caldwell; reconstruction of Pachyrhachis bones after an illustration by Michael Caldwell, 1997; Aetiocetus image courtesy of Tom Deméré, San Diego Natural History Museum

Lines of Evidence
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Learn more about cetaceans at UCMP's Introduction to Cetaceans page.