A structure or organ is vestigial if it has diminished in size or usefulness in the course of evolution. Vestigial structures are markers of evolutionary descent. For example, boa constrictors, which are descended from four-legged reptiles, grow tiny hind legs. Duckbill platypuses, which are descended from extinct platypus species that had teeth as adults, grow and re-absorb teeth before birth. In human beings, the vermiform appendix (a hollow, worm-shaped organ about the size of a pencil, attached to the beginning of the large intestine) marks descent from mammals that had a much larger sac in this position and used it to digest their high-cellulose diet (as many species, including other primates, still do).

From the late nineteenth century until the 1960s, biologists thought that the human body contained scores of useless vestigial structures, including the coccyx, ear muscles, pineal gland, thymus, vermiform appendix, wisdom teeth, and...