Exaptation List

The following is a list of examples of hypothesized exaptations in biological evolution. Also known as co-option and preadaptation, exaptation occurs when a structure or trait, evolved for some function or for none, is coopted to serve a new function. After the initial exaptation event, the exapted trait or structure will often undergo secondary adaptation as its shape and properties are sculpted by natural selection to amplify its performance at the new task. Exaptation is an important facet of evolutionary biology whereby the myriad side-effects of existing traits act as a second source of variation, alongside mutation, offering up new possibilities, variations, and avenues for selection to exploit. When exaptation occurs, any minimum structural or complexity requirements for the new function, no matter how great, are satisfied from the outset without that complexity having to evolve from scratch for the new function.

Keep in mind that my research area is evolutionary computation and not biological evolution, although I do try to learn as much as I can about it. The exaptation hypotheses listed here vary in terms of the degree to which each is weak or speculative versus well-evidenced and firmly established, and some may not accurately reflect the current most up-to-date consensus of the scientific community. Nevertheless, I provide this list in the hope that it may be useful or interesting for those interested in the phenomenon of exaptation and biological evolution in general.

Alpha-lactalbumin This is a protein produced by lactating mammals. It aides in the production of the milk sugar lactose. Alpha-lactalbumin is hypothesized to be an altered copy of lysozyme, an enzyme that kills bacteria by damaging their cell walls, and is found in many bodily fluids such as tears, mucus, and saliva [Dickerson & Geis, 1969].

Avian wings As birds evolved from therapod dinosaurs (a view now essentially unanimously accepted), their wings would originally have been forelimbs for terrestrial locomotion or grasping before being exapted to their current role in powered flight [Fastovsky & Weishampel, 1996]. Several possible intermediate stages predating their use for powered flight are hypothesized.

Bone The material of bone may have originally functioned as the body's storage mechanism for phosphate and/or other mineral ions, and was later exapted to its role in structural support [Halstead, 1969].

Elephant trunks Once just a nose and upper lip, the elephant's trunk now functions for numerous purposes such as grasping and manipulating objects in the environment, social interactions, and spraying water into the mouth or onto the body for bathing.

Feathers Feathers have been hypothesized to have originally functioned in the catching of insects, and later being exapted for flight. A competing hypothesis is that feathers were adapted primarily for thermoregulation before being exapted for aerodynamics [Ostrom, 1976].

Female hyena genitals The genitals of female hyenas very closely resemble those of the males. This seems to enhance the females' fitness in what's called the meeting ceremony where two hyenas pair to inspect each other's genitals. It is hypothesized that this role for the female genitalia is an exaptation of a non-aptation. The idea is that the resemblance to male genitalia is merely a side-effect of high androgen levels in the females, the genesis of which is tied to the social behavior and morphology of the females; they are larger than the males and dominate them within the social framework of hyena society. [Gould & Vrba, 1982] put forth this hypothesis, though they did not assert that it is correct, only that it had been an overlooked and perfectly plausible possibility for quite a long time due at least in part to what they perceived as the constraints of a conceptual framework in evolutionary biology devoid of a labeled distinction between aptations, adaptations, and exaptations. In particular they felt the conceptual framework was most lacking in that it had no term for co-opted non-aptations. The hyena hypothesis was meant to illustrate their point.

Flat-headedness of the lizard Holaspis guentheri [Arnold, 2002] argues that phylogenetic analyses indicate that the flat head of the lizard Holaspis guentheri first evolved for hunting in the tight spaces under tree bark, and was later exapted to the role it now plays in the gliding behavior of the lizard. The flat-headedness trait now serves both functions.

Gills Though they now serve respiration functions, the gills of aquatic vertebrates are thought to have evolved from pharyngeal slits involved in the filter-feeding behavior of chordate ancestors.

Insect wings There are a number of competing hypotheses regarding the origins of insect wings. As the fossil record for insect protowings is extremely lacking, the issue is far from resolved. [Kingsolver & Koehl, 1994] discusses the strengths and weaknesses of several of these hypotheses, most of which involve some form of exaptation.

Interpterygoid vacuity as housing for the eyeballs in some lizards The interpterygoid vacuity is a cavity within the head, and a particular group of lizards, called cordylids, use this cavity to house their eyeballs when their heads are vertically compressed, such as when squeezing into tight crevices. As this cavity exists in many creatures, and predates the cordylids, it is not an adaptation for housing eyeballs, but has been exapted (and secondarily further adapted) to that role [Arnold, 1994].

Lens crystallins Lens crystallins are water-soluble proteins used in eye lenses. There are many known cases where strong molecular and genetic evidence shows these proteins to have been exapted for use in the lens from a previous unrelated function. In many cases the original function is still maintained and the protein serves a dual role. Examples include the delta crystallin of chicken lenses, which is also the enzyme argininosuccinate lyase, the epsilon-Crystallin of ducks, which is the enzyme lactate dehydrogenase, the tau crystallin of turtles, which is the enzyme alpha-enolase, the mu-Crystallin in marsuipials, which is the enzyme ornithine cyclodeaminase [Piatigorsky, 1993], the eta-Crystallin in elephant shrews (which makes up more than 25% of the soluble protein in their lenses) is the enzyme cytoplasmic aldehyde dehydrogenase [Piatigorsky & Wistow, 1991], the omega-Crystallin of octopuses is also aldehyde dehydrogenase, and the S-Crystallin of squid is related to the enzyme glutathione S-transferase. [Piatigorsky, 2003] finds even more - the alpha-A-Crystallin in vertebrates is related to a heat shock protein, the zeta-Crystallin in guinea pigs, camels, and llamas is related to the enzyme quinine oxidoreductase, the pi-Crystallin in geckos is related to the enzyme glyceraldehyde-3-phosphate dehydrogenase, and the eta-Crystallin in the elephant shrew is related to the enzyme retinaldehyde dehydrogenase. The family of lens crystallin proteins is rife with examples of exaptation.

Mammalian ear bones The inner ear bones used by mammals for hearing are exaptations, though they've experienced considerable secondary adaptation. One of the best known series of transitional fossils shows these bones to have originally been parts of the reptilian jaw [Kardong, 2002].

Mammary Glands Mammary glands are thought to have evolved from immunoprotective glands that produced secretions that fought bacterial infections on the skin [Vorbach, Capecchi, & Penninger, 2006]. This hypothesis, though still speculative, ties in well with the evidence that the protein a-lactalbumin, found in breast milk, is an exapted version of the bacteria-killing enzyme lysozyme, and (perhaps to a lesser degree) with the fact that some remaining members of early offshoots in the mammalian line, such as platypuses, possess mammary glands but lack teats, instead simply secreting milk directly through the skin.

Pandas' thumbs Originally a sesamoid bone in the wrist, modified to allow the giant panda (Ailuropoda melanoleuca) to hold bamboo, the thumb of the giant panda is not a thumb at all [Gould, 1980]. Interestingly, this same exaptation occurred independently in a distantly related animal named the red panda (Ailurus fulgens), which is more closely related to skunks, raccoons, and weasels than true pandas. Its sesamoid bone has also been exapted and secondarily adapted to serve as an opposable thumb, and, coincidentally it seems, for processing the very same plant: bamboo [Salesa, 2006]. The red panda's thumb, as a further twist, seems to have itself been exapted for manipulating bamboo from an earlier opposable-digit function, as it was found to be present in the red panda's long-extinct and carnivorous (and therefore non-bamboo-eating) relative, Simocyon batelleri. These are both examples of exaptation, and together make a particularly striking example of convergent evolution (evolution hitting upon similar solutions independently).

Penguin feathers The feathers of penguins represent a long chain of exaptations. Originally scales (although there is some dispute over that hypothesis in evolutionary biology), they were exapted, possibly for insulation, to become down and feathers, exapted again for flight, and then in the penguin line, exapted once again for both insulation and swimming in the ocean.

Penguin wings As penguins are birds that have lost the ability to fly, their wings have been exapted and secondarily adapted for swimming.

Post-synaptic neuron proteins A number of proteins that provide a scaffold underlying the receptors on post-synaptic membranes in neurons are now known to have evolved before the so-called Cambrian explosion in which the major animal groups and body-plans appeared in a very short period of geological time, even before they would have been co-opted into use in nervous systems [Sakarya et al., 2007].

Repetitive DNA Repetitive segments of DNA in genomes may be non-aptations [Orgel & Crick, 1980], and [Doolittle & Sapienza, 1980] that can act and have acted as a source of raw materials for molecular exaptations.

Shell space for egg-brooding in some snails Some species of snails brood their eggs within a space available in their shells. This space, called the umbilicus, seems to be a mere byproduct of the way shells develop as snails grow. Evidence strongly suggests that the snails that use the space for egg brooding evolved after those that possess it but don't use it, making this a spandrel that was exapted for brooding [Gould, 2002], [Andrews, Gangestad, & Matthews, 2000], and [Lindberg & Dobberten, 1981].

Shoulder hump decoration in Megaloceros giganteus Megaloceros giganteus is also known as the Irish Elk (though it is technically a deer). It was the largest species of deer to have ever existed, and went extinct about 8000 years ago [Stuart et al., 2004]. It had the largest antlers of any deer, weighting up to 35kg on a 2kg skull. To support the mass of this structure, the spines of the vertebrae near the head were enlarged to provide a greater surface area for attachment of large ligaments and muscles. The resulting hump above the animal's shoulder was then exapted for mating displays by taking on colours and stripes as decoration. A twist in the story behind this example is that this type of colouration does not fossilize, and evidence for its existence comes from Cro-Magnon cave paintings of these animals. This example is perhaps more speculative and less firmly established than some of the others, but no less interesting [Gould, 2002 p. 1260].

Suborbital foramen as housing for the eyeballs in some lizards The suborbital foramen is a cavity which evolved for other purposes but has been exapted in certain lizards (scincids and lacertids) to house the eyes when the head is squeezed into thin crevices [Arnold, 1994]. Like the exaptation of the interpterygoid vacuity in the cordylid lizards, above, this cavity was exapted to the same function in these lizards. It's an alternative exaptive solution to the same general problem.

Sutures in mammalian skulls In mammals, skull sutures serve to aid in parturition (the birthing process), and may be indispensable for that purpose. However, as they exist in the skulls of birds and reptiles alike, and as birds and reptiles have no need of them since their young merely escape from eggs, they most likely exist as a consequence or side-effect of the way skull bones grow alongside one another. In their useful role in mammalian birth, they can be considered exaptations - specifically, exapted spandrels [Gould & Vrba, 1982]. This point was famously made even by [Darwin, 1859] himself, who realized that their utility and origins were functionally unrelated.

Swim-bladders The swim bladders present in many fish seem to have originally been lungs [Liem, 1988]. Interestingly, [Darwin, 1859] also hypothesized this particular exaptation, but he had it backwards since swim bladders were more common in fish than lungs, and because air-breathing tetrapods evolved later than fish.

Tetrapod limbs As tetrapods evolved from fish, so it seems likely that their limbs would have been exaptations to a function involved with terrestrial locomotion, from an original swimming function.

Vertebrate jaws It is thought that the jaws of early vertebrates evolved from skeletal rods called pharyngeal arches. The arches originally served to keep pharyngeal gill slits open in an ancient ancestor.

Wing mantling in the Black Heron/Egret, Egretta ardesiaca The Black Egret mantles its wings over water (spreads them out to create a large shadow over the water's surface). This allows it to more easily see its prey below the surface [McLachlan & Liversidge 1978]. The wings differ little from closely-related species whose members don't hunt this way, and the behavior seems to have a genetic component.

[Andrews, Gangestad, & Matthews, 2000] Andrews P.W., Gangestad S.W., and Matthews D., 2002. Adaptationism - How to Carry Out an Exaptationist Program. Behavioral and Brain Sciences, Volume 25, Number 4, Cambridge University Press. p. 489-553.

[Arnold, 1994] Arnold E.N., 1994. Investigating the origins of performance advantage: adaptation, exaptation and lineage effects. In Phylogenetics and Ecology, London: The Linnean Society of London.

[Arnold, 2002] Arnold E.N., 2002. Holaspis, a lizard that glided by accident: mosaics of cooption and adaptation in a tropical forest lacertid (Reptilia, Lacertidae). Bulletin of the Natural History Museum, Zoology Series, Volume 68, Issue 2, November 2002. p. 155-163.

[Darwin, 1859] Darwin C., 1859. On the Origin of Species. Murray J.: London.

[Doolittle & Sapienza, 1980] Doolittle W.F., and Sapienza C., 1980. Selfish genes, the phenotype paradigm, and genome evolution. Nature, Volume 284. p. 601-603.

[Dickerson & Geis, 1969] Dickerson R.E., and Geis I., 1969. The Structure and Action of Proteins. Harper and Row, New York.

[Fastovsky & Weishampel, 1996] Fastovsky D.E., and Weishampel D.B., 1996. The Evolution and Extinction of the Dinosaurs. Cambridge University Press.

[Gould, 1980] Gould S.J., 1980. The Panda's Thumb. W.W. Norton & Company.

[Gould & Vrba, 1982] Gould S.J., and Vrba E., 1982. Exaptation: A missing term in the science of form. Paleobiology, Volume 8, Number 1. p. 4-15.

[Gould, 2002] Gould S. J., 2002. The Structure of Evolutionary Theory. The Belknap Press of Harvard University Press, Cambridge, Massachusetts and London, England.

[Halstead, 1969] Halstead L.B., 1969. The Pattern of Vertebrate Evolution. Oliver and Boyd, Edinburgh.

[Kardong, 2002] Kardong K.V., 2002. Vertebrates: Comparative Anatomy, Function, Evolution. Third edition, McGraw Hill, New York.

[Kingsolver & Koehl, 1994] Kingsolver J.G., and Koehl M.A.R., 1994. Selective Factors in the Evolution of Insect Wings. Annual Review of Entomology, Volume 39. p. 425-451.

[Liem, 1988] Liem K.F., 1988. Form and function of lungs: the evolution of airbreathing mechanisms. American Zoologist, Volume 28, Number 2. p. 739-759.

[Lindberg & Dobberten, 1981] Lindberg D.R., and Dobberteen R.A., 1981. Umbilical brood protection and sexual dimorphism in the boreal trochid gastropod Margarites vorticiferus. Dall. International Journal of Invertebrate Reproduction, Volume 3. p. 347-355.

[McLachlan & Liversidge 1978] McLachlan G.R., and Liversidge R., 1978. Roberts' Birds of South Africa, 4th Edition (first published in 1940), John Voelcker Bird Book Fund, Cape Town.

[Orgel & Crick, 1980] Orgel L.E., and Crick F.H.C., 1980. Selfish DNA: the ultimate parasite. Nature, Volume 284. p. 604-607.

[Ostrom, 1976] Ostrom J.H., 1976. Archaeopteryx and the origin of birds. Biological Journal of the Linnean Society, Volume 8. p. 91-182.

[Piatigorsky & Wistow, 1991] Piatigorsky J., and Wistow G.J., 1991. The recruitment of crystallins: new functions precede gene duplication. Science, Volume 252, Number 5009, May 1991. p. 1078-1079.

[Piatigorsky, 1993] Piatigorsky J., 1993. Puzzle of crystalline diversity in eye lenses. Developmental Dynamics, Volume 196, Issue 4. p. 267-272.

[Piatigorsky, 2003] Piatigorsky J., 2003. Gene Sharing, Lens Crystallins and Speculations on an Eye/Ear Evolutionary Relationship. Integrative and Comparative Biology, Volume 43, Number 4, The Society for Integrative and Comparative Biology. p. 492-499.

[Sakarya et al., 2007] Sakarya O., Armstrong K.A., Adamska M., Adamski M., Wang I.F. et al., 2007. A Post-Synaptic Scaffold at the Origin of the Animal Kingdom. PLoS ONE 2(6): e506 doi:10.1371/journal.pone.0000506

[Salesa, 2006] Salesa M.J., Anton M., Peigne S., and Morales J., 2006. Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas. Proceedings of the National Academy of Sciences of the United States of America, Volume 103, Number 2. p. 379-382.

[Stuart et al., 2004] Stuart A.J., Kosintsev P.A., Higham T.F.G., and Lister A.M., 2004. Pleistocene to Holocene extinction dynamics in giant deer and woolly mammoth. Nature, Volume 431, Number 7009. p. 684-689.

[Vorbach, Capecchi, & Penninger, 2006] Vorbach C., Capecchi M.R., Penninger J.M., 2006. Evolution of the mammary gland from the innate immune system? BioEssays, Volume 28. p. 606-616.