Modern analysis of vertebrate embryos shows that fish, chick, and cat embryos are remarkably similar at certain stages. This pattern would be expected if all were descended from a common ancestor. Processes occurring later in development then modify the ancestral body structures.
Further evidence of evolution is found in structures called vestigial structures that seem to serve no function but that resemble structures with functional roles in related organisms. For example, the human tailbone, or coccyx, is made up of four fused vertebrae that resemble the bones in an animal's tail. Other examples of vestigial structures are the pelvic bones of modern whales and the human appendix.
The development of animal embryos is also evidence of descent with modification. As shown in Figure 15-9, some stages of vertebrate embryo development are very alike, although the similarities fade as development proceeds. One possible explanation for these similarities is that vertebrates share a common ancestor and have inherited similar stages of development.
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