Figure 2.1.1. Various organisms displaying nonfunctional, vestigial characters. From top to bottom: A. A blind cave salamander - look closely for the eyes buried underneath the skin. B. Astyanax mexicanus, the Mexican tetra, a blind cave fish. C. Apterocyclus honolulensis, a flightless weevil. The black wing covers cannot open, as they are fused, yet underneath are perfectly formed beetle wings. D. The nonfunctional flower of Taraxacum officinale, the common dandelion. E. A nonfunctional pollen grain from the dandelion.
Some of the more renowned evidences for evolution are the various nonfunctional or rudimentary vestigial characters, both anatomical and molecular, which are found throughout biology. During macroevolutionary history, functions necessarily have been gained and lost. Thus, from common descent and the constraint of gradualism, we predict that many organisms should display vestigial structures, which are structural remnants of lost functions. (Note that the exact evolutionary mechanism is irrelevant here as long as it is a gradual one). Since there is no apparent reason for their existence, nonfunctional characters can be especially puzzling features of organisms. Likewise, rudimentary structures are curious, as they have different and relatively minor functions compared to the same more developed structures in other organisms.
There are many examples of rudimentary and nonfunctional characters carried by organisms, and these can very often be explained in terms of evolutionary histories. For example, snakes are known to be the descendants of four-legged reptiles. Most pythons (which are legless snakes) carry vestigial pelvises hidden beneath their skin (Cohn 2001; Cohn and Tickle 1999). 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, nonfunctional legs underneath their skin, undetectable from the outside (Raynaud and Kan 1992). 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 (Besharse and Brandon 1976; Durand et al. 1993; Jeffery 2001; Kos et al. 2001). The eyes of the Mexican tetra have a lens, a degenerate retina, a degenerate optic nerve, and a sclera, even though the tetra has no use for them, even though the eyes cannot see (Jeffery 2001). The blind salamanders have eyes with retinas and lenses, yet the eyelids grow over the eye, sealing them from outside light (Durand et al. 1993; Kos et al. 2001). Dandelions reproduce without reproduction (a condition known as apomixis), yet they retain flowers and produce pollen (both are sexual organs normally used for sexual fertilization) (Mes et al. 2002). Flowers and pollen are thus useless characters for dandelions. 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 structures 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). There are many examples of flightless beetles (such as the weevils of the genus Lucanidae) which retain perfectly formed wings housed underneath fused wing covers. All of these examples can be explained in terms of the beneficial functions and structures of the organisms' predicted ancestors (Futuyma 1998, pp. 122-123).
No organism can have a vestigial structure that was not previously functional in one of its ancestors. Thus, for each species, the standard phylogenetic tree makes a huge number of predictions about vestigial characters that are allowed and those that are impossible for any given species.
Shared derived characters and molecular sequence data, not vestigial characters, determine the phylogeny and the characteristics of predicted common ancestors. Thus, if common descent is false, vestigial characters very possibly could lack an evolutionary explanation. For example, whales are classified as mammals according to many criteria, such as having mammary glands, a placenta, one bone in the lower jaw, etc. Snakes likewise are classified as reptiles by several other derived features. However, it is theoretically possible that snakes or whales could have been classified as fish (as Linnaeus originally did). If this were the case, the vestigial legs of whales or the vestigial pelvises of snakes would make no sense evolutionarily and would be inconsistent with common descent.
It follows, then, that we should never find vestigial nipples or a vestigial incus bone in any amphibians, birds, or reptiles. No mammals should be found with vestigial feathers (but they can have vestigial tails, as humans do). We should never find any arthropods with vestigial backbones.
Note that this prediction is not invalidated by finding a function for the presumed vestigial structure. Should this happen, the structure merely becomes an example of paralogy, which is considered in prediction 3.1, or, more likely, an example of inefficient design, which is considered in prediction 3.5. Some anti-evolutionist authors have erroneously concluded that it is scientifically and theoretically impossible to demonstrate that a structure has no function (e.g., see Ham et al. 1990; Scadding 1981). Such nihilistic philosophies have no place in practical science. This erroneous conclusion is based upon the false premise that negative evidence cannot be used in science to test a hypothesis. However, negative evidence certainly is admissible if it is acquired with the proper experimental controls. It is obvious that negative evidence is scientifically useful by considering the following analogy with physics - if it is impossible to demonstrate that a certain structure has no function, then by the same logic it is impossible to demonstrate that a given atomic element is not radioactive. However, it is well-established in physics that lead-207 is not radioactive. We know this because radioactivity is detectable from other elements, such as phosphorous-32, yet simultaneously radioactivity is undetectable from lead-207. In this physics example phosphorous-32 is a positive control, which is needed to use the negative evidence gathered from lead-207. Likewise, we can certainly demonstrate that a given structure has no function when we can simultaneously detect a beneficial function from another demonstrably useful structure in the same environment.
Anatomical atavisms are closely related conceptually to vestigial structures. An atavism is the reappearance of a lost character specific to a remote evolutionary ancestor and not observed in the parents or recent ancestors of the organism displaying the atavistic character. Atavisms have several essential features: (1) presence in adult stages of life, (2) absence in parents or recent ancestors, and (3) extreme rarity in a population (Hall 1984). Of course, without an evolutionary perspective we could not state that an atavism is a structure that was once found in a remote ancestor but has been lost in a recent lineage. Therefore, here we are primarily concerned with potential atavistic structures that are characteristic of taxa to which the organism displaying the structure does not belong. As a hypothetical example, if mutant horses occasionally displayed gills, this would be considered a potential atavism, since gills are diagnostic of taxa (e.g. fish) to which horses do not belong. For developmental reasons, the occasional occurrence of atavisms is expected under common descent if structures or functions are lost between ancestor and descendant lineages (Hall 1984; Hall 1995). As with vestigial structures, no organism can have an atavistic structure that was not previously found in one of its ancestors. Thus, for each species, the standard phylogenetic tree makes a huge number of predictions about atavisms that are allowed and those that are impossible for any given species.
Probably the most well known case of atavism is found in the whales. According to the standard phylogenetic tree, whales are known to be the descendants of terrestrial mammals that had hindlimbs. Thus, we expect the possibility that rare mutant whales might occasionally develop atavistic hindlimbs. In fact, there are many cases where whales have been found with rudimentary atavistic hindlimbs in the wild (for reviews see Berzin 1972, pp. 65-67 and Hall 1984, pp. 90-93). Hindlimbs have been found in baleen whales (Sleptsov 1939), humpback whales (Andrews 1921) and in many specimens of sperm whales (Abel 1908; Berzin 1972, p. 66; Nemoto 1963; Ogawa and Kamiya 1957; Zembskii and Berzin 1961). Most of these examples are of whales with femurs, tibia, and fibulae; however, some even include feet with complete digits.
Many other famous examples of atavisms exist, including (1) rare formation of extra toes (2nd and 4th digits) in horses, similar to what is seen in the archaic horses Mesohippus and Merychippus, (2) atavistic thigh muscles in Passeriform birds and sparrows, (3) hyoid muscles in dogs, (4) wings in earwigs (normally wingless), (5) atavistic fibulae in birds (the fibulae are normally extremely reduced), (6) extra toes in guinea pigs and salamanders, (6) the atavistic dew claw in many dog breeds, and (7) various atavisms in humans (one described in detail below) (Hall 1984).
Figure 2.2.1. X-ray image of an atavistic tail found in a six-year old girl. A radiogram of the sacral region of a six-year old girl with an atavistic tail. The tail was perfectly midline and protruded form the lower back as a soft appendage. The five normal sacral vertebrae are indicated in light blue and numbered; the three coccygeal tail vertebrae are indicated in light yellow. The entire coccyx (usually three or four tiny fused vertebrae) is normally the same size as the fifth sacral vertebrae. In this same study, the surgeons reported two other cases of an atavistic human tail, one with three tail vertebrae, one with five. All were benign, and only one was surgically "corrected" for cosmetic reasons (image reproduced from Bar-Maor et al. 1980, Figure 3.)
Primarily due to intense medical interest, humans are one of the best characterized species and many developmental anomalies are known. There are several human atavisms that reflect our common genetic heritage with other mammals. One of the most striking is the existence of the rare "true human tail" (also variously known as "coccygeal process," "coccygeal projection," "caudal appendage," and "vestigial tail"). More than 100 cases of human tails have been reported in the medical literature. Less than one third of the well-documented cases are what are medically known as "pseudo-tails" (Dao and Netsky 1984; Dubrow et al. 1988). Pseudo-tails are not true tails; they are simply lesions of various types coincidentally found in the caudal region of newborns, often associated with the spinal column, coccyx, and various malformations. In contrast, the true atavistic tail of humans develops from the most distal end of the embryonic tail found in the developing human fetus (see Figure 2.3.1 and the discussion below on the development of the normal human embryonic tail; Belzberg et al. 1991; Dao and Netsky 1984), and it is usually benign in nature (Dubrow et al. 1988; Spiegelmann et al. 1985). The true human tail is characterized by a complex arrangement of adipose and connective tissue, central bundles of longitudinally arranged striated muscle in the core, blood vessels, nerve fibres, nerve ganglion cells, and specialized pressure sensing nerve organs (Vater-Pacini corpuscles). It is covered by normal skin, replete with hair follicles, sweat glands, and sebaceous glands (Dao and Netsky 1984; Dubrow et al. 1988; Spiegelmann et al. 1985). True human tails range in length from about one inch to over 5 inches long (on a newborn baby), and they can move and contract (Baruchin et al. 1983; Dao and Netsky 1984; Lundberg et al. 1962). Although human tails usually lack skeletal structures (some medical articles have claimed that true tails never have vertebrae), several human tails have also been found with cartilage and up to five, well-developed, articulating vertebrae (see Figure 2.2.1; Bar-Maor et al. 1980; Dao and Netsky 1984; Fara 1977; Sugamata et al. 1988). However, caudal vertebrae are not a necessary component of mammalian tails; contrary to what is frequently reported in the medical literature, there is at least one known example of a primate tail which lacks vertebrae, as found in the rudimentary two-inch-long tail of Macaca sylvanus (the "Barbary ape") (Hill 1974, p. 616; Hooten 1947, p. 23). True human tails are rarely inherited, though several familial cases are known (Dao and Netsky 1984; Ikpeze and Onuigbo 1999; Standfast 1992; Touraine 1955). As with other atavistic structures, human tails are most likely the result of either a somatic or germline mutation that reactivates an underlying developmental pathway which has been retained in the human genome (Dao and Netsky 1984; Hall 1984; Hall 1995).
It should be noted here that the existence of true human tails is unfortunately quite shocking for many religiously motivated anti-evolutionists, such as Duane Gish, who has written an often-quoted article entitled "Evolution and the human tail" (Gish 1983; see also Menton 1994; ReMine 1982). Solely based on the particulars of a single case study (Ledley 1982), these authors have erroneously concluded that atavistic human tails are "nothing more than anomalous malformations not traceable to any imaginary ancestral state" (Gish 1983). However, their arguments are clearly directed against pseudo-tails, not true tails, since true human tails are complex structures which have muscle, blood vessels, occasional vertebrae and cartilage (Bar-Maor et al. 1980; Dao and Netsky 1984; Fara 1977; Sugamata et al. 1988), they can move and contract, and they are occasionally inherited (Dao and Netsky 1984; Ikpeze and Onuigbo 1999; Standfast 1992; Touraine 1955). Furthermore, Gish, Menton, and ReMine all argue that human vestigial tails are not true tails if they lack vertebrae - an erroneous claim since M. sylvanus is a primate whose fleshy tail also lacks vertebrae (Hill 1974, p. 616; Hooten 1947, p. 23).
These are essentially the same as for vestigial structures above.
Vestigial characters should also be found at the molecular level. Humans do not have the capability to synthesize ascorbic acid (otherwise known as Vitamin C), and the unfortunate consequence can be the nutritional deficiency called scurvy. However, the predicted ancestors of humans had this function (as do most other animals except primates and guinea pigs). Therefore, we predict that humans, other primates, and guinea pigs should carry evidence of this lost function as a molecular vestigial character (nota bene: this very prediction was explicitly made by Nishikimi and others and was the impetus for the research detailed below) (Nishikimi et al. 1992; Nishikimi et al. 1994).
Recently, the L-gulano-g-lactone oxidase gene, the gene required for Vitamin C synthesis, was found in humans and guinea pigs (Nishikimi et al. 1992; Nishikimi et al. 1994). It exists as a pseudogene, present but incapable of functioning (see prediction 4.4 for more about pseudogenes). In fact, since this was originally written the vitamin C pseudogene has been found in other primates, exactly as predicted by evolutionary theory. We now have the DNA sequences for this broken gene in chimpanzees, orangutans, and macaques (Ohta and Nishikimi 1999). And, as predicted, the nonfunctional human and chimpanzee pseudogenes are the most similar, followed by the human and orangutan genes, followed by the human and macaque genes, precisely as predicted by evolutionary theory. Furthermore, all of these genes have accumulated mutations at the exact rate predicted (the background rate of mutation for neutral DNA regions like pseudogenes) (Ohta and Nishikimi 1999).
There are several other examples of vestigial human genes, including multiple odorant receptor genes (Rouquier et al. 2000), the RT6 protein gene (Haag et al. 1994), the galactosyl transferase gene (Galili and Swanson 1991), and the tyrosinase-related gene (TYRL) (Oetting et al. 1993).
Our odorant receptor (OR) genes once coded for proteins involved in now lost olfactory functions. Our predicted ancestors, like other mammals, had a more acute sense of smell than we do now; humans have >99 odorant receptor genes, of which ~70% are pseudogenes. Many other mammals, such as mice and marmosets, have many of the same OR genes as us, but all of theirs actually work. An extreme case is the dolphin, which is the descendant of land mammals. It no longer has any need to smell volatile odorants, yet it contains many OR genes, of which none are functional – they are all pseudogenes (Freitag et al. 1998).
The RT6 protein is expressed on the surface of T lymphocytes in other mammals, but not on ours. The galactosyl transferase gene is involved in making a certain carbohydrate found on the cell membranes of other mammals. Tyrosinase is the major enzyme responsible for melanin pigment in all animals. TYRL is a pseudogene of tyrosinase.
It is satisfying to note that we share these vestigial genes with other primates, and that the mutations that made these genes nonfunctional are also shared with several other primates (see predictions 4.3-4.5 for more about shared nonfunctional characters).
It would be very puzzling if we had not found the L-gulano-g-lactone oxidase
pseudogene or the other vestigial genes mentioned. In addition, we can predict
that we will never find vestigial chloroplast genes in any metazoans (i.e.
animals) (Li 1997, pp. 284-286, 348-354).