Prediction 2.1: Anatomical vestiges
Some of the most renowned Evidence for evolution are the various nonfunctional or rudimentary vestigial characters, both anatomical and molecular, that are found throughout biology. A vestige is defined, independently of evolutionary theory, as a reduced and rudimentary structure compared to the same complex structure in other organisms. Vestigial characters, if functional, perform relatively simple, minor, or inessential functions using structures that were clearly designed for other complex purposes. Though many vestigial organs have no function, complete non-functionality is not a requirement for vestigiality (Crapo 1985; Culver et al. 1995; Darwin 1872, pp. 601-609; Dodson 1960, p. 44; Griffiths 1992; Hall 2003; McCabe 1912, p. 264; Merrell 1962, p. 101; Moody 1962, p. 40; Muller 2002; Naylor 1982; Strickberger 2000; Weismann 1886, pp. 9-10; Wiedersheim 1893, p. 2, p. 200, p. 205).
[Figure2.1.1 (ostrich with wings extended)] [Figure2.1.1 (blind cave salamander)] [Figure2.1.1 (blind cave fish, the Mexican tetra)]
For example, wings are very complex anatomical structures specifically adapted for powered flight, yet ostriches have flightless wings. The vestigial wings of ostriches may be used for relatively simple functions, such as balance during running and courtship displays—a situation akin to hammering tacks with a computer keyboard. The specific complexity of the ostrich wing indicates a function which it does not perform, and it performs functions incommensurate with its complexity. Ostrich wings are not vestigial because they are useless structures per se, nor are they vestigial simply because they have different functions compared to wings in other birds. Rather, what defines ostrich wings as vestigial is that they are rudimentary wings which are useless as wings.
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Geoffroy was at a loss for why exactly nature "always leaves vestiges of an organ", yet he could not deny his empirical observations. Ten years later, Jean-Baptiste Lamarck (1744-1829) identified several vestigial structures in his Zoological Philosophy (Lamarck 1809, pp. 115-116):
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...these "Hypocritical" structures profess something that they do Not do—they clearly appear designed for a certain function which they do Not perform.
However, Common Descent provides a scientific explanation for these peculiar structures. Existing species have different structures and perform different functions. If all living organisms descended from a common ancestor, then both functions and structures necessarily have been gained and lost in each lineage during macroevolutionary history. Therefore, from Common Descent and the constraint of gradualism, we predict that many organisms should retain vestigial structures as structural remnants of lost functions. Note that the exact evolutionary mechanism which created a vestigial structure is irrelevant as long as the mechanism is a gradual one.
Confirmation:
[Pictures/Illustrations]
There are Many examples of rudimentary and nonfunctional vestigial characters carried by organisms, and these can very often be explained in terms of evolutionary histories.
For example, from independent phylogenetic evidence, snakes are known to be the descendants of four-legged reptiles.
Most Pythons (which are legless snakes) carry Vestigial Pelvises hidden beneath their skin.. The Vestigial pelvis in pythons is Not attached to vertebrae (as is the normal case in most vertebrates),and it simply floats in the abdominal cavity. Some lizards carry rudimentary, vestigial legs underneath their skin, undetectable from the outside...
Many cave dwelling animals, such as the fish Astyanax mexicanus (the Mexican tetra) and the salamander species Typhlotriton spelaeus and Proteus anguinus, are blind yet have rudimentary, Vestigial eyes.... The eyes of the Mexican tetra have a lens, a degenerate retina, a degenerate optic nerve, and a sclera, even though the tetra cannot see... The blind salamanders have eyes with retinas and lenses, yet the eyelids grow over the eye, sealing them from outside light....
Dandelions reproduce without fertilization (a condition known as apomixis), yet they retain flowers and produce pollen (both are sexual organs normally used for sexual fertilization).... Flowers and pollen are thus useless characters for dandelions in terms of sexual reproduction.
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The ancestors of Humans are known to have been herbivorous, and molar teeth are required for chewing and grinding plant material. Over 90% of all adult humans develop third molars (otherwise known as wisdom teeth).
Usually these teeth never erupt from the gums, and in one Third of all individuals they are Malformed and Impacted (Hattab et al. 1995; Schersten et al. 1989). These Useless teeth can cause significant pain, increased risk for injury, and may result in illness and even death (Litonjua 1996; Obiechina et al. 2001; Rakprasitkul 2001; Tevepaugh and Dodson 1995).
Another Vestige of our herbivorous ancestry is the vermiform appendix. While this intestinal structure may retain a function of some sort, perhaps in the development of the immune system, it is a rudimentary version of the much larger caecum that is essential for digestion of plants in other mammals. For a detailed discussion of the vestigiality of the human vermiform appendix, see The Vestigiality of the human vermiform appendix: A modern reappraisal.
Yet another human Vestigial structure is the coccyx, the four fused caudal vertebrae found at the base of the spine, exactly where most mammals and many other primates have external Tails protruding from the back. Humans and other apes are some of the only vertebrates that lack an external tail as an adult.
The coccyx is a developmental Remnant of the embryonic tail that forms in humans and then is degraded and eaten by our immune system (for more detail see the sections on the embryonic human tail and the atavistic human tail).
Our internal tail is Unnecessary for sitting, walking, and elimination (all of which are functions attributed to the coccyx by many anti-evolutionists). The caudal vertebrae of the coccyx can cause extreme and unnecessary chronic pain in some unfortunate people, a condition called coccydynia. The entire coccyx can be surgically removed without any ill effects (besides surgical complications), with the only complaint, in a small fraction of patients, being that the removal of the coccyx sadly did not remove their pain (Grossovan and Dam 1995; Perkins et al. 2003; Postacchini Massobrio 1983; Ramsey et al. 2003; Shaposhnikov 1997; Wray 1991). Our small, rudimentary, fused caudal vertebrae might have some minor and inessential functions, but these vertebrae are useless for balance and grasping, their usual functions in other mammals.
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