Why do vestigial structures still exist

Introduction A vestigial structure is a biological structure that has lost a major ancestral function and is usually drastically reduced in size. Open in a separate window. Figure 1. Phylogeny of bone vestigialization and loss in mammals. Table 1 List of examined specimens. USNM , A. AMNH T. Identification of vestigial structures Three categories of skeletal structures were examined and considered candidates for identification as vestigial structures: 1 individual bones, 2 parts of individual bones e.

Table 2 Outgroup lists. Tracing phylogenetic patterns We used the cladogram in Fig. Results Mammalian taxa without vestigial skeletal structures There are a few major mammalian clades among whose extant representatives we did not find vestigial structures in the postcranial skeleton.

Sternum Ancestrally, the mammalian sternum consists of several segments called sternebrae, the first of which is called the manubrium Fig. Figure 2. Mammal skeletons with structures in a non-vestigial state. Figure 3. Sterna of whales, with vestigial parts circled with broken line.

Tail Mammalian tails vary widely in length. Figure 4. Pelves of primates, with coccyx vestigial tail circled with broken line. Clavicle Ancestrally, the mammalian clavicle is a robust bone that articulates with the sternum and the acromion process of the scapula, bracing the forelimb against the axial skeleton Fig. Figure 5. Vestigial clavicles, circled with broken line. Forearm Ancestrally, the mammalian forearm contains two bones, the radius and ulna.

Figure 6. Vestigial ulnar shafts in bats, an elephant shrew, and a horse, and nearly-vestigial ulnae in artiodactyls, with distal tip of ulna indicated by arrow. Hand and fingers Ancestrally, the mammalian hand has five digits with two phalanges in the thumb and three phalanges in each other finger, and a metacarpus in which all five metacarpals are of similar diameter Fig.

Figure 7. Vestigial fingers and metacarpals, circled with broken line. Figure 8. Reduced structures that fail to satisfy one or more of the three criteria for vestigiality. Figure 9. Various levels of thumb expression in Hystrix Old World porcupines , with vestigial structures circled with broken line. Pelvic girdle Ancestrally, the mammalian pelvic girdle consists of three bones: the ilium, ischium, and pubis Figs.

Figure Vestigial pelvic girdles, circled with broken line. Femur, tibia and fibula Ancestrally, the mammalian hindlimb includes a single bone in the thigh the femur and two in the shank or crus the tibia and fibula, with the fibula the more lateral of the two. Foot and toes Ancestrally, the mammalian foot has five digits with two phalanges in the first toe and three phalanges in each other toe, and a metatarsus in which all five metatarsals are of similar diameter Fig.

Vestigial toes and metatarsals. Discussion The pattern of vestigialization and loss in the forelimb does not match that of the hindlimb in any mammal taxon Fig. Acknowledgments We would like to thank the following people for their help.

Anatomical abbreviations a acromion process of scapula ac acetabulum c carpals cl clavicle f femur fi fibula h humerus il ilium is ischium m manubrium mc metacarpals mt metatarsals of obturator foramen p pubis ph phalanges r radius s sacrum sc scapula st sternebrae t tibia ta tarsals u ulna I—V first through fifth digits. Funding Statement The authors received no funding for this work. Additional Information and Declarations Competing Interests The authors declare there are no competing interests.

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This objection by Bergman is therefore based on a misunderstanding of the reduction criterion. Bergman correctly states that the evolutionary history of an organ must be known to determine whether it is vestigial. He then argues against the validity of determinations of vestigiality by claiming that evolutionary histories are not known for most such organs and that their identification as vestigial is based on direct comparison with modern and not fossil examples.

That claim shows two errors. First, evolutionary inference does not require direct observation of the history of all structures. Second, the evolutionary histories of vestigial skeletal structures are often well documented by fossil series. For example, in derived tyrannosauroid dinosaurs the third finger is reduced to a metacarpal splint with no phalanges Lambe , whereas early tyrannosauroids had a complete third finger; the fossil record therefore sufficiently documents the evolutionary history of the tyrannosauroid third finger to determine that in derived tyrannosauroids it is vestigial Xu and others This objection by Bergman is based on the incorrect assumption that fossil series are not used to determine vestigiality.

The above objection by Bergman is invalid for another reason. In evolutionary studies, a precursor to a rudimentary organ can be deduced by comparison with its non-rudimentary counterparts in close relatives. By the same token, within the creationist paradigm a rudimentary structure in one species must be considered vestigial if the homologous structure is non-rudimentary in other species within the same baramin.

In such a case, even a creationist must concede that a rudimentary structure has evolved from a non-rudimentary homolog. Darwin and others explain that a biological structure may become vestigial if members of the evolutionary lineage in question stop using it. Some creationists claim that this explanation is Lamarckian and therefore false see Glover ; Bergman and Howe The term Lamarckian refers to the now-discredited hypothesis, named after the pre-Darwinian biologist Jean-Baptiste Lamarck, that traits acquired by an organism during its lifetime are heritable.

Lamarckian scenarios are unrealistic, as witness the fact that the children of amputees are born with their limbs intact. The misinterpretation is understandable, because the evolutionary scenario in question is often described with poor wording, as in the first sentence of this paragraph. This scenario could be better worded thus: if members of an evolutionary lineage cease to use a given organ, then the survival of the lineage will not be compromised if one of its members is born with a heritable mutation that results in the reduction of that organ to a rudimentary state; the descendants of that individual will possess a vestigial organ.

This is not a Lamarckian scenario, and the objection that it is one is therefore false. Neither Darwin nor any modern evolutionary biologist makes the Lamarckian claim that atrophy of an organ due to disuse for example, withering of a muscle that an individual does not exercise is heritable.

Clearly, the creationist arguments against the existence of vestigial structures are based on misunderstandings and incorrect assumptions. Even so, these examples of vestigial structures do not necessarily demonstrate that vestigial structures exist within the creationist paradigm, because they relate to taxa that creationist authors have not identified as belonging to a single baramin.

Baraminologists creationist researchers who seek to determine which extant taxa belong to which baramins have not placed pythons in the same baramin as any fully legged animal, and they have not yet studied cassowaries or tyrannosauroids. However, examples of vestigial structures do exist within baramins that have been studied by and are recognized by baraminologists. The fossil horse series offers some examples. Creationists once considered fossil members of Equidae the horse family to have been created separately from modern horses Cousins ; Gish However, recent baraminological studies confirm that there is too much morphological continuity between the various fossil and extant members of Equidae to support that interpretation Garner ; Cavanaugh and others ; Wood The fossil record reveals that in the earliest equids each forelimb had four digits, each hindlimb had three digits, the shaft of the ulna extended the full length of the forearm, and the shaft of the fibula extended the full length of the shank Figure 1.

In each forelimb and hindlimb of later fossil equids all digits but number III were lost, and in modern horses thin splints of bone are all that remain of the metacarpal hand and metatarsal foot bones that supported digits II and IV in each limb.

The metacarpal and metatarsal splints of modern equids are vestigial bones, and the ulnar and fibular splints are vestigial shafts of bones. Because these rudimentary skeletal structures are demonstrably derived from non-rudimentary structures in ancestral members of the same baramin, they must be considered vestigial within the creationist paradigm.

It should be noted that these vestigial skeletal structures perform useful functions in extant horses, and that they are nonetheless vestigial. The metacarpal and metatarsal splints serve as guides for ligaments, and remnants of the ulna and fibula function as muscle attachment sites Smythe Even so, these structures are vestigial because they currently exist in a state of extreme reduction and they are derived from non-rudimentary homologs in ancestral equids.

Other cases exist in which the vestigiality of a structure can be deduced by comparison with close relatives without reference to fossils. For example, the family Columbidae includes flying pigeons and doves with unreduced wings, as well as flightless dodos and solitaires with miniscule wings Figure 2. The family Columbidae is identified by creationists as a single baramin More , and molecular phylogenetic analysis confirms that the dodo and solitaire are phylogenetically nested deeply within the family and are descended from flying columbid ancestors Shapiro and others Therefore, the extremely reduced wings of dodos and solitaires are derived from the flying wings of ancestral columbids and so must be considered vestigial within the creationist paradigm.

In some cases, a single genus contains some species with an unreduced version of a given structure as well as species with a vestigial version. The cave salamanders Eurycea rathbuni and Eurycea tridentifera have strongly reduced, nonfunctional eyes, while other species of the genus Eurycea that do not live in caves have unreduced, functional eyes Petranka In some species of the Australian lizard genus Hemiergis the forelimb and hindlimb both have five full digits, while in other species the outer digits are reduced to metacarpal and metatarsal splints with no finger bones Choquenot and Greer Figure 3B.

Not until forty years later, with the publication of the 12 th edition in , was the author revealed, posthumously, to be Scottish journalist Robert Chambers. Although it sharpened rather than softened resistance to evolutionary ideas among scientists and clergy, Vestiges reached a huge audience and, one might argue, made evolution a mainstream concept.

Along with paleontological evidence from the fossil record and biogeographic evidence from species distributions, Darwin turned, naturally, to comparative anatomy, citing such well-known topics as homologous and analogous convergent features and similarities in skeletons and patterns of embryonic development. Darwin cited another prominent finding of comparative anatomy as an obvious line of evidence for evolution: vestigial features.

Hilaire Muller [ ]. This list was greatly expanded by Robert Wiedersheim [ ] to over a hundred human vestiges. However, vestigial features can be found in any species that has evolved—which is to say, all species. These range from biochemical reactions or physiological properties, such as the mammalian dive reflex present in humans but in much-reduced form relative to other mammals; Elsner and Gooden [ ] , to biogeographic distributions of relict taxa, indicators of formerly larger populations.

Straus et al. Tadpoles undergo a ventilatory transformation from external filamentous gills to pulmonary respiration, with a consequent need to close lungs to avoid ingesting water, which is diverted to gills instead Shubin [ ]. Even behaviors can be considered vestigial: witness the instinct in dogs to turn in a circle multiple times before lying down, presumably, though not assuredly, to tramp down a bed of grass Irvine [ ].

This bird possesses stumpy, stubby wings that are much too small to allow it to fly, unlike the 40 other species of cormorants found worldwide. The carina keel of its sternum is notably reduced, reflecting the much-diminished pectoralis musculature of the breast, which is used in flying birds to produce the powered downstroke of flight. When it emerges from the sea and perches on rocky shores, it, like all cormorants, spreads its wings wide to dry them in the sunlight, since all birds in this family lack waterproof plumage.

Research has shown that this basking behavior is not performed for thermoregulation or digestion but to dry and thus lighten the wings in preparation for flight Sellers [ ]. However, the flightless cormorant need not dry its wings because unlike its relatives and ancestors it cannot fly. Thus P. All of the examples cited in the previous paragraphs—whether anatomical, physiological, or behavioral—have one thing in common: they are vestiges of evolution, artifacts of the progressive change that occurred over generations via natural selection, and which Darwin referred to as descent with modification.

Often, features of organisms structural and otherwise are not modified during the descent from parent to offspring, from ancestor to descendant. The vestigial pelvic and hind limb bones of whales, which have not borne functional legs for 40 million years, are a prime example of vestigial retention although these bones still serve as attachment sites for genital and abdominal musculature, so the cost of their loss might be significant. Darwin claimed these vestiges offer compelling evidence for evolutionary change because even though the function has been lost or altered, the structure, behavior e.

However, evolution is not the only widespread change that is manifested in organisms. Numerous embryonic rudiments may be retained as recognizable remnants in later developmental stages of animals and plants, including adults. Ontogenetic changes i. Nipples in male mammals provide a classic example to be explained in detail. Detailed examples of various developmental relics follow. We must be especially careful not to confuse ontogenetic traces of development like male nipples , whose significance is often overlooked, with phylogenetic vestiges of evolution like our appendix.

This paper presents and distinguishes examples of variously retained features Table 1 , particularly in the human body, as it discusses their importance and utility in relating evolution and development. Given the myriad challenges of evolutionary education and the potential importance of vestiges in explaining evolution, reserving the term vestige solely for holdovers of evolution is recommended to avoid unnecessary confusion. Consider that lungs are not used for pulmonary ventilation by any mammals in utero , but they must be fully developed so that they can function effectively and independently to oxygenate the blood of a newborn infant after oxygen is no longer supplied by the placenta.

Therefore the blood vessels that will function outside the womb must be constructed before birth, even though they do not function until after birth, when the fetal vasculature shifts immediately to a new pattern.

That this change occurs at all is a marvelous feat of embryology; that it leaves very few but notable artifacts of the original blood supply network is a wonder of nature. This bypass, essentially an oval-shaped hole in the septum dividing the right and left atria, normally closes when the pressure head in the systemic left half of the mammalian circulation exceeds that of the pulmonary division, pressing a flap of tissue to close the hole.

The tissues soon fuse and all that remains is an oval-shaped depression. As with the other fetal circulatory shunts described here, there is a difference between functional closure, which typically occurs within minutes after birth, and structural closure, which usually takes days or weeks.

It has been speculated that levels of circulating hormones, particularly prostaglandins, may regulate shunt closure and timing Fugelseth et al. Babies born prematurely often have difficulty with the transition to normal postnatal circulation because they retain fetal circulatory patterns due to delayed shunt closure Evans and Iyer [ ]. Even if fetal blood from the right atrium escapes passage through the foramen ovale into the left atrium and travels, as in postnatal mammals, to the right ventricle, it cannot go to the lungs because they are not yet inflated.

The ductus arteriosus AKA ductus Botalli , a bypass from the pulmonary trunk to the aorta i. This remnant, which connects the pulmonary trunk and aorta, can be found in all mammals. The ductus venosus of fetal mammals shunts oxygenated blood returning from the placenta via the umbilical vein away from the liver and instead directly into the inferior vena cava.

This oxygenated blood serves the developing brain and other organs via the systemic circulation. The ductus venosus closes shortly after birth functionally, with full structural closure occurring days later ; its fibrous remnant is known as the ligamentum venosum.

This is often continuous with the round ligament of the liver, also known as the ligamentum teres hepatis, described below.

As noted, the umbilical vein carries oxygenated blood from the placenta. It is open at birth but closes within a week. During the time it is open it can be catheterized, allowing for delivery of drugs or for blood transfusion. Within days after closure the umbilical vein is mostly obliterated except for a fibrous portion on the abdominal wall that remains as the ligamentum teres hepatis. Curiously, this may reopen in adults with extreme hypertension although this is debatable; Lafortune et al.

Umbilical arteries carry deoxygenated fetal blood back to the placenta, and as such are unnecessary after birth. Umbilical arteries give way to a functional section of the internal iliac arteries and superior vesicular artery that delivers blood to the dorsal portion of the urinary bladder, as well as a non-functional, obliterated portion which led to the umbilical cord that becomes the medial umbilical ligament, not to be confused with the median umbilical ligament, a remnant of the urinary urachus, itself an embryonic remnant of the allantois.

The carotid duct or ductus caroticus remains in some postnatal vertebrates lungfishes and some amphibians and reptiles as a remnant of the original embryonic connection between the third and fourth aortic arches.

This occurs because although the sex of a developing embryo is determined at conception in humans, as in other mammals, due to the presence of an X or Y chromosome in the sperm , this sex does not become phenotypically manifested until much later in development. Often, leftovers of earlier stages can be seen in adults.

Homologies of shared urogenital structures occur because sexual development typically occurs along a default female track that, in the absence of any outside influence—namely the testis determining factor TDF produced by the sex-determining region of the Y chromosome SRY —turns the indifferent stage into a female embryo. The mesonephric AKA archinephric or Wolffian and paramesonephric AKA Mullerian ducts of embryogenesis begin developing in both male and female vertebrates, including humans.

In males, the Wolffian duct becomes the efferent or deferent duct including the epididymis, vas deferens, and seminal vesicle that stores and transports sperm and seminal fluids.

In females, the Wolffian duct degenerates and remains solely as a withered rudiment. In females, the Mullerian duct becomes the oviduct, including the Fallopian tubes, uterus, cervix, and upper portion of the vagina, whereas in males the Mullerian duct regresses completely or is seen as a small remnant. Many exceptions can found among vertebrate clades in the fate of both Wolffian and Mullerian ducts, particularly teleost fishes, which have novel testicular and ovarian ducts—the story is much abbreviated here for simplicity—but the basic account of one tubule being used as a functional duct in males and the other in females holds true; retention in other sexes occurs solely as non-functional residues.

Sections of the Wolffian archinephric duct normally remain in human females. The Wolffian duct develops in males into the vas deferens. In mammals testes normally descend permanently or temporarily into a scrotal sac for better spermatogenesis, the cooler temperatures of the scrotum allowing for improved production of viable sperm.

This weak spot in the abdominal wall is the cause of inguinal hernias in men. The vas deferens efferent duct takes a long, looping path over the ureter, a developmental holdover of the original descent of the testes into the scrotal pouch. The odd looping pathway the ureter takes is reminiscent of the recurrent laryngeal nerve, a branch of the vagus nerve cranial nerve X , whose unusual circuitous detour on the left side, as it loops under the aorta, was noted in giraffes by anatomist Richard Owen [ ].

These are the first and most important difference in the indifferent gonad. In females, the primary medullary sex cords degenerate, where they may remain as ontogenetic vestiges, and instead secondary sex cords invade the cortex of the gonad and become ovarian follicles. In all mammals, nipples develop along the milk line of the mammary ridge. Cats, dogs, and other familiar pet or barnyard mammals have abdominal mammaries that develop much more posteriorly inferiorly than in humans.

Ungulates have inguinal mammary glands; rodents and pigs develop them along the entire trunk, including both the thorax and abdomen. In humans the milk lines or mammary ridges develop around the sixth week, and although two nipples usually form, extra supernumerary nipples occur not infrequently in males and females. But why should male mammals have nipples at all? Interestingly, Darwin [ ] perpetuated the myth that male nipples are true evolutionary vestiges that stem from lactation in male as well as female ancestral mammals.

This mistaken notion was based on rumors that circulated with the first, largely erroneous European scientific descriptions of the duck-billed platypus. We now know male monotremes do not produce milk, so male nipples are not in any way an evolutionary holdover. They are, however, a developmental holdover from the early, indifferent stage of embryonic development, when the milk line arises but before the embryo has developed along the default female track or divergent male pathway.

After this switch has occurred, nipples are already present and are not resorbed. Evidently, there is little or no cost to retaining nipples and associated mammary tissue in males, aside, perhaps, from a slight risk of breast cancer. If there was a true cost to retention of male nipples or selection pressure against their presence, they would likely not be retained. Like the urogenital ducts described above, male nipples are default structures formed earlier in development than the differentiation between males and females.

Just as footprints reveal traces of a person or animal that is no longer present, numerous features of organisms reveal historical information about phylogeny and ontogeny.

In many cases there are features of organisms that may unlike vestiges retain an original function but nonetheless reveal glimpses of history via comparative cladistic analysis of a species and its lineage Table 1. Again these can be divided into traces from the past that indicate prior states of evolution or development. On the evolutionary side, these include shared, derived characters synapomorphies like the stereoscopic binocular vision and fingernails of primates, which yield information about the ancestry and evolutionary relationships of this mammalian order when compared to other mammals.

Bilaterally symmetric embryos of echinoderms, which as adults display pentamerous radial symmetry, likewise are relics that disclose secrets of echinoderm origins. Even the possession of a head region at one end rather than in the center of cephalized animals is a form of historical remain: still obviously functional yet a reminder of the past, when heads first arose.

In this sense they are indeed faint footprints, but not true vestiges in the proper evolutionary sense. Other functional remains indicate developmental history. Examples include cranial and other bony sutures that reveal formerly distinct ossification centers. During formation of endochondral cartilage replacement long bones, a growth zone forms between the bony shaft diaphysis and end plate epiphysis.

These elements eventually fuse when growth ends, leaving a tell-tale epiphyseal line where elements were formerly separated. This too is an artifact of ontogeny. Strictly speaking this is a scar: although the umbilical cord has a function, the umbilicus itself never had one, making this a nonfunctional embryonic holdover, unlike the formerly functional circulatory and urogenital remains described earlier.

For example, the vestigial coccyx, a remnant of a now-missing tail, shows how developmental remnants reflect evolutionary history, revealing the complex association that leads to such structures being expressed or not , which is itself an informative aspect of organismal history.