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Unformatted text preview: in different organisms are inherited from a common ancestor. Vertebrate forelimbs contain the same sets of bones organized in similar ways, despite their dissimilar functions. Vestigial structures are remains of a structure that was functional in some ancestor but is no longer functional in the organism in question. Most birds have well-developed wings, although some birds have reduced wings and do not fly. Humans have a tail bone (the coccyx) but no tail. The presence of vestigial structures is explained by the common descent hypothesis. Embryological development reveals a unity of plan. During development, all vertebrates have a notochord and paired pharyngeal pouches. In fishes and amphibian larvae, the pouches become gills. In humans, first pair of pouches becomes a cavity of middle ear and auditory tube; second pair becomes tonsils, while third and fourth pairs become thymus and parathyroid glands. This makes sense only if fish are ancestral to other vertebrate groups....
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