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 others. Most of these structures are now known to have at least minor functions, leading to controversy over whether the human body contains any vestigial structures at all. However, the discovery of a function for a structure does not necessarily mean that it is not vestigial. A vestigial structure may be completely without function, like fetal platypus teeth, or it may be changed and diminished in function. The human appendix appears to be a vestigial structure with changed and diminished function; it is attached to the digestive system just where an anatomically similar, essential digestive organ is attached in many other mammals, but performs no digestive or other essential function. (People who have had their appendixes removed suffer no known ill effects.) Some biologists assert that the appendix assists the immune system; if so, the appendix's tiny opening on the large intestine still appears to be a truly useless (or worse than useless) vestige of this organ's digestive origin. Hardened feces can block this opening and cause the appendix to swell and rupture, a potentially fatal disorder that afflicts about one person in every thousand annually.
Another vestigial structure with diminished usefulness is the wisdom teeth. When these succeed in coming in properly, they are useful for chewing. However, they are not essential for chewing, and often come in sideways, fail to grow at all, or remain embedded in the jawbone. They are vestiges of a more massive chewing apparatus in our primate ancestors.
Vestigial structures may also be molecular, as in the case of vestigial genes that exist in most species. For example, although humans cannot manufacture their own vitamin C, most other mammals can because they possess a gene enabling them to produce an enzyme (L-gulono-gamma-lactone oxidase) which in turn makes it possible for them to produce vitamin C. Humans possess a defective copy of this gene that does not produce the required enzyme (or any other product). This gene was presumably disabled by mutation at a time in primate evolution when its loss was not a significant disadvantage, and now remains as a vestigial genetic sequence.
Men's nipples, sometimes cited as vestigial structures, are not truly vestigial because they are not remnants of functional male nipples in ancestral species. They occur because nipple precursors are grown early in the development of the human embryo, before sexual differentiation. Later in life these structures become more fully developed in women, while remaining undeveloped in men. The navel, too, is a by-product of embryonic development rather than an evolutionary vestige.
Truly useless vestigial structures are few or absent in humans, but well-known in other species: blind cave-dwelling species of crayfish grow eye-stalks but no eyes, embryonic baleen whales grow teeth which they re-absorb before birth, and so on. However, evolution tends to eliminate useless organs, for every structure requires energy to grow, sustain, and transport. Individuals that expend their limited resources on useless organs are therefore less likely to leave offspring. This explains the tendency of useless anatomic structures to diminish in size during evolution. Vestigial genes, in contrast, can linger indefinitely because they can passed on to one's descendants at essentially zero cost. Vestigial genes inherited from shared ancestors have been preserved for many millions of years in species as far removed from each other today as humans, cows, and mice.
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