Feathers

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    To make a very bad pun, we can say that birds seem to fly in the face of the theory of evolution. That theory claims that every feature of a living organism has come into existence through the interplay between mutation and natural selection. So, we ask, what pattern of mutation and selection gave birds their feathers? They likely inherited their feathers from their theropod dinosaur ancestors, in which feathers first evolved, so we need to look to the dinosaurs to examine the origin of feathers.

Phylum - Chordata

Class - Reptilia

Order - Dinosauria

Suborder - Theropoda

    In your imagination look at the animals that survived the Great Dying at the end of the Permian period of the Paleozoic era, roughly 250 million years ago. The scales that reptiles inherited from their fish ancestors would seem to have been adequate protection from cuts, scrapes, and abrasions, especially in the deserts of Pangaea. We see such protection on snakes and lizards today. But on some reptiles the scales evolved into feathers or hair. What mutations made that change and what promoted them?

    One change that differentiated the dinosaurs and mammals from the other reptiles was the transition to the erect-limb stance from the lizard-like sprawl. Limbs were moved from jutting out the sides of the body to being located under the body. That change, which lifted the animals’ bodies off the ground and up to where breezes could cool it, promoted the evolution of endothermy, which then promoted the evolution of feathers and hair as thermal insulation.

    On the cellular level the mechanism of the change that transformed scales into feathers or hair involved mutations in the placodes that cover the animal’s skin. Mutations certainly arose to alter the way in which the placodes make the keratin that composes body coverings. We must then look for the environmental factors that led natural selection to promote the changes in the placodes that transformed them from producing scales to producing feathers.

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    Like most of life’s productions, a feather is an elaborate manifestation of a basically simple design.

    A feather is based on a hollow shaft, called the rachis, from which slender barbs, each sprouting smaller barbules, grow in a series of parallel structures. Each feather grows out of a cylindrical sheath that protrudes from the skin, the top of the feather coming out first. The sheath erodes when the feather is complete. The process is analogous to the process by which reindeer grow their antlers. The feather itself is made of keratin, the same kind of material found in human fingernails; thus, it grows by a similar process.

    Taken as a whole, feathers constitute the plumage of a bird. The plumage, dermal and subdermal cutaneous muscles, ligaments, and the brain and sense organs form an interconnected structure that must work as a unit, known as a bird. In that unit such details as the angle, thickness, shape and construction of all the feather parts are held to within narrow tolerances. Although not important in flightless creatures, minor deviations in the manifestation of the feathers can render the entire system of flight unworkable. A theory of feather evolution must thus account for the refinement of tolerances in the feathers.

    Most birds shed their feathers at regular intervals, usually once a year, in a process called molting. That process occurs gradually to insure that the bird doesn’t develop bare spots. And the process is so ordered that the bird loses its flight and tail feathers in exact pairs, one from each side, so that the bird maintains its balance. Any bird in which molting did not occur in that way simply did not survive long enough to reproduce itself.

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    We want to know why feathers evolved and fulfilling that desire necessitates that we figure out what benefits feathers confer on their owners. What do feathers accomplish that makes life easier and better for the creature that wears them, even at the most primitive stage of development? Functional integrity provides a guiding principle in the devising of Darwinian hypotheses. As a population mutates, individuals must interact successfully with their environment. Any trait that evolves must, at minimum, not interfere with the proper functioning of the creature.

    The only creatures that we commonly know to have feathers are birds, so we tend to associate feathers with flight, the primary trait that we attribute to birds. Surely, we think, feathers must have evolved to enable birds to fly. But evolution proceeds in small increments, so we don’t expect that a quadrupedal flightless creature would become a bipedal, winged flying creature in one evolutionary leap; rather, we expect that a series of mutations, each promoted by some feature of the environment, transformed an original form into something that can, just barely, fly. Further mutations then refined the ability to fly.

    Certainly a scale would not evolve into a feather in a single mutation. Yet birds require fully developed pennaceous feathers for flight. Thus we must infer that feathers evolved before the ancestors of birds could fly, so we must look for another cause of their evolution.

    Like hair, feathers provide excellent thermal insulation. Because feathers create air pockets between themselves and the creature’s skin, they slow the transfer of heat between the skin and the surrounding air. They also provide a parasol function, intercepting direct solar heat and keeping it away from the creature’s skin. But what circumstances would promote the evolution of such a thermal shielding system?

    The original land animals were cold-blooded, like the fish from which they evolved, and we still see that feature manifested in amphibians and reptiles today. Those creatures obtain from their environment the heat necessary to drive the chemical reactions that move their bodies, so we expect that the original land animals existed in the tropics and subtropics of Pangaea. There the air temperature remained more or less constantly close to the average body temperature (about 37 C/98.6 F) of mammals, so the animals required no special thermal insulation.

    In the Triassic period two groups of reptiles evolved the erect-limb stance, one becoming the first mammals and the other becoming the ancestors of the dinosaurs. That stance enabled animals to walk and run more efficiently than they could do with the sprawl-legged stance of lizards, alligators, and tortoises. Both mammals and dinosaurs also became warm-blooded.

    Endothermy offered little or no advantage in the swamps and jungles of tropical Pangaea and, so, likely didn’t evolve there. But in the dryland forests, semideserts, and deserts, especially away from the coasts, the world gets cold at night. As the warm-blooded creatures made their long, slow migrations into those areas, only those individuals manifesting the appropriate heat-conserving mutations would have been favored in reproduction. Mutant mitochondria that produced only heat gave the creature a selective advantage, preventing the creature from going torpid when their environment got cool. At about the same time deformed scales, acting more as windbreaks than as actual insulation, gave those creatures an additional selective advantage by helping them to conserve heat.

    We see a clue to the importance of the association between feathers (or hair) and endothermy in the observation that small birds have a larger number of feathers per unit area of skin than do larger birds. That fact indicates the important role that feathers play in thermal insulation, because smaller birds lose heat more rapidly due to the relatively larger surface area in proportion to their body weight.

    Thus we can infer that feathers originated in the evolution of scales into thermal insulation.

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    A feather begins with a placode, a patch of cells in the skin. Those cells grow downward to form a tube, called a follicle, slanting under the skin and the feather grows in the follicle. Encased in a cornified sheath, the feather grows out of the follicle’s base and emerges as a quill, like those of hedgehogs. When the feather has achieved its full growth, the blood flow in the cornified sheath stops and the sheath, made of horn-like material, disintegrates, thereby allowing the feather’s vane to unfold and spread out.

    The evolution of feathers began after the Permian-Triassic mass extinction event. We know that because alligators have the same gene that guides the building of feathers in birds, so we know that the genetic basis for feathers goes back at least 250 million years, to the time of The Great Dying. Fossil evidence shows us that the first non-avian theropods with simple, single-filament feathers lived about 190 million years ago, and that non-avian theropods with feathers having a complex branching structure like those of present-day birds (pennaceous feathers) existed about 135 million years ago. This fossil evidence raises two important questions. The first asks that, if they were not derived from scales, how did feathers evolve and the second asks, how did simple, single-filament feathers evolve to become much more complex pennaceous feathers? Of course, we have a related question whose answer could lead to the answers to the first two questions: given that non-avian theropods did not fly, what function or functions did these feathers serve?

    Because birds evolved from reptiles and the integument of present-day reptiles (and most extinct reptiles including most dinosaurs) is characterized by scales, early hypotheses concerning the evolution of feathers began with the assumption that feathers developed from scales. In those hypotheses scales elongated, then grew fringed edges and, ultimately, produced hooked and grooved barbules. The problem with that scenario lies in the fact that scales are basically flat folds of the integument whereas feathers are tubular structures. A pennaceous feather becomes ‘flat’ only after it emerges from a cylindrical sheath. In addition, the type and distribution of protein (keratin) in feathers and scales differ. The only feature shared by feathers and scales is that they both begin development as a morphologically distinct placode – an epidermal thickening above a condensation, or congregation, of dermal cells. Feathers, then, are not derived from scales, but, rather, are evolutionary novelties with numerous unique features, including the feather follicle, tubular feather germ (an elevated area of epidermal cells), and a complex branching structure. We infer, then, that the placode evolved and that the product changed as a result.

    Reptiles have placodes, certainly. But in a reptile embryo each placode switches on genes that cause only the skin cells on the back edge of the placode to grow, eventually forming scales. In the late 1990s Richard Prum of Yale University and Alan Brush of the University of Connecticut developed the idea that the transition from scales to feathers might have depended on a simple switch in the wiring of the genetic commands inside placodes, causing their cells to grow vertically through the skin rather than horizontally. In other words, feathers were not merely a variation on a theme: They were using the same genetic instruments to play a whole new kind of music. Once the first filaments had evolved, only minor modifications would have been required to produce increasingly elaborate feathers.

    The first feathers were likely hollow cylinders (Stage I) with undifferentiated collars that developed from an evolutionary novel follicle collar. They would have resembled the spines that we see on a hedgehog. The long, hollow filaments on theropods posed a puzzle. If they were early feathers, how had they evolved from flat scales? They have the form of bristles erupting from tiny patches of skin cells called placodes. A ring of fast-growing cells on the top of the placode builds a cylindrical wall that becomes a bristle. Originally the ring of cells was merely an arc, which extruded a flat scale. We see the first feathers in Sinosauropteryx, a chicken-sized dinosaur that lived 260 - 201 million years ago. Its feathers took the form of hollow, thread-like bristles.

    The advantage of a tubular feather germ is that growth of a structure (in this case, a feather) can occur without an increase in the size of the skin itself (in contrast to, for example, scales). An important step in the evolution of the first feathers changed the characteristics of the placode. Both scales and feathers begin development from placodes, but feather development, in contrast to scale development, requires generation of suprabasal cell populations (dermal condensations) to form the follicle. The development of placodes where dermal condensations occur, an evolutionary novelty, required changes in gene expression and timing. However, such changes are known to be an important mechanism in the origin of morphological innovations in many other organisms.

    Based on Prum’s 1999 model of feather evolution, the next step after the origin of the feather follicle was the differentiation of the follicle collar into barb ridges to generate barbs (Stage II). The resulting feather consisted of a tuft of barbs extending from the calamus. Such a feather is hypothesized to have evolved before the origin of the rachis (Stage IIIA) because the rachis is initially formed by the fusion of barb ridges. In addition, barbs are hypothesized to evolve before barbules because barbules develop within layers of pre-existing barb ridges. Feathers comparable in structure to hypothesized Stage II feathers have been reported from fossils of non-avian theropods, such as Sinornithosaurus millenii.

    The majority of dinosaurs known to have had feathers or protofeathers are saurischians in Suborder Theropoda. In most of those the feathers would not have functioned for flight. Our theory tells us that feathers originally evolved on dinosaurs as a result of their thermal insulation properties. Small dinosaurs that then grew longer feathers may have found them helpful in gliding, which gave them an evolutionary advantage and lead to the evolution of proto-birds like Archaeopteryx and Microraptor zhaoianus. Since the 1990s, dozens of feathered dinosaurs have been discovered in the clade Maniraptora, which includes the clade Avialae and the recent common ancestors of birds, Oviraptorosauria and Deinonychosauria. Branched feathers with ranchis, barbs, and barbules were discovered in many members including Sinornithosaurus millenii, a dromaeosaurid found in the Yixian formation (124.6 million years ago).

    Thus, an abundant fossil record of both birds and feathers exists that enables paleontologists to draw some firm conclusions about the evolution of birds and feathers. The scales of dinosaurs and reptiles, feathers, leaves, and even the gossamer wings of insects stand clearly outlined in detail in the fossil record, thereby enabling paleontologists to study these life forms in detail and place feathers in their proper evolutionary context.

    The discovery, in 2011, of feathers preserved in amber, within samples dating to 80 million years ago, suggests the coexistence of theropods and birds, with both theropod and avian feather types commingled in the samples. Led by paleobiologist Alex Wolfe, researchers at the University of Alberta examined amber from 80 million years ago and found eleven examples of feathers, spanning the evolutionary range from simple strands to fully developed flight and diving feathers.

    Although much speculation and major disagreements exist on how feathers ‘could have’ evolved, all existing hypotheses are ‘just-so stories’ until fossil evidence transforms them into theories. And, in regard to feathers, we have that evidence.

Appendix: Endothermy

    Once feathers had evolved into thermal insulation the reptile could warm itself by shivering instead of basking in the sun. It could thus warm itself while in hiding (there’s a life-or-death advantage for a small creature).

    It began with a creature shivering. The twitching of the muscles compelled the mitochondria to carry out the chemical reactions that generate heat. A mutation made the mitochondria run those reactions continuously and gave the creature a competitive advantage.

    Many endotherms have a larger number of mitochondria per cell than ectotherms do. That surplus of mitochondria enables the animals to generate heat by increasing the rate at which they metabolize fats and sugars. As a consequence those animals require a much greater quantity of food than do ectothermic animals in order to sustain their higher metabolism. We infer that endothermy enables an animal to acquire more food with less effort than would its ectothermic counterpart; otherwise, natural selection would not have promoted the necessary mutations.

    Note that when resting the human body generates about 67% of its heat inside the organs located in the thorax and abdomen and in the brain, which generates about 16% of the total heat produced by the body.

    But once animals started generating their own heat, even weakly (lukewarm-blooded rather than warm-blooded), heat loss became a major threat, especially to smaller creatures, which have a larger ratio of surface area to volume. Natural selection would then promote the mutations that helped to minimize heat loss. Thus, most small warm-blooded animals have insulation in the form of fur or feathers. Aquatic, warm-blooded animals, such as seals and whales, generally have deep layers of fat under the skin for insulation, since fur or feathers would spoil their streamlining, but those creatures evolved after hair and feathers did. Penguins have both feathers and fat, since their need for streamlining limits the degree of insulation which feathers alone can give them. Birds, especially waders, have blood vessels in their lower legs which act as heat exchangers: the veins run adjacent to the arteries and, thus, extract heat from the arteries and carry it back into the trunk. Other warm-blooded animals reduce blood flow to the skin by vasoconstriction in response to cold to reduce heat loss. As a result, they blanch (become paler).

    Another factor in the evolution of warm-blooded animals comes up when the animals do not need extra body heat. In many endothermic animals, a controlled state of hypothermia, called hibernation or torpor, conserves energy by lowering the body temperature when the animal sleeps. Many birds and small mammals (e.g. tenrecs) reduce their body temperature during daily inactivity, such as at night for diurnal animals or during the day for nocturnal animals, thus reducing the energy cost of maintaining body temperature. Human metabolism also slows down slightly during sleep.

    On the other hand, in equatorial climates and during temperate summers, overheating (hyperthermia) threatens an animal as severely as does cold. In hot conditions, many warm-blooded animals increase heat loss by panting, which cools the animal by increasing water evaporation in the breath, and/or flushing, increasing the blood flow to the skin so the heat will radiate and convect into the environment. Hairless and short-haired mammals also perspire, using the evaporation of the water in sweat to remove heat. Elephants keep cool by using their huge ears like radiators in automobiles: their ears are thin and the blood vessels are close to the skin, so flapping their ears to increase the airflow over them causes the blood to cool, which reduces the animal’s core body temperature when the blood moves through the rest of the circulatory system.

    The primary advantage of endothermy lies in the fact that, over a range of temperatures between freezing to death and dying of heat stroke, an animal's overall metabolic rate increases by a factor of about two for every 10 °C (18 °F) rise in body temperature. Endothermy does not provide an animal with greater speed in movement than we find in cold-blooded animals: when the ectothermic animal is near or at its optimum temperature it can move as fast as do warm-blooded animals of the same size and build, though often for not as long as endotherms. Endothermy simply maintains a constant core temperature in the animal’s body for optimum enzyme activity.

    Endothermic/homeothermic animals can achieve and maintain optimal activity at more times during the diurnal cycle, especially in places of sharp temperature variations between day and night. The animal can also remain active during more of the year in places of great seasonal differences of temperature. This activity is accompanied by the need to expend more energy to maintain the constant internal temperature and thus imposes a greater food requirement upon the animal. The benefits coming from endothermy must outweigh that disadvantage in order for the evolutionary process to select for warm-bloodedness.

    Endothermy may also provide a protection against fungal infection, such as Candidiasis in humans. While tens of thousands of fungal species infect insects, only a few hundred target mammals, and often infect only those mammals with a compromised immune system. A recent study suggests that fungi are fundamentally ill-equipped to thrive at mammalian temperatures. The high temperatures produced by endothermy might have provided an evolutionary advantage and thus promoted the mutations that gave animals the ability to generate body heat.

Appendix: Avian and Dinosaurian Respiration

    It seems reasonable to assert that lungs evolved from the swim bladders of lobe-finned fish, the precursors of amphibians, such as Tiktaalik. In modern amphibians the lungs are still little more than simple bladders, sufficient for their purpose because frogs and salamanders are small and they also carry out oxygen/carbon dioxide exchange through their moist skins. Proper lungs only evolved when land animals grew large or moved away from water and evolved dry, impermeable skins.

    As animals developed a need for a greater rate of oxygen/carbon dioxide exchange, natural selection promoted mutations that increased the surface area of the lungs, primarily by convoluting them. At first little change was necessary. Modern reptiles have lungs that are little different from those of amphibians. The evolution of endothermy necessitated a more radical change in lungs, which change we see reflected in the lungs of the mammals and the birds (the heirs of the dinosaurs).

    In the mammals the lungs evolved by splitting and branching. The obvious mutation to start the process would make the balloon-like lungs of an amphibian grow extra sacs. In the mutant the sacs were smaller than normal, but the net surface area of the lung was greater than normal, thereby giving the creature more power and a selective advantage that promoted the mutation and others like it. The improved exchange of carbon dioxide for oxygen enabled the creature to survive the mutations that cause endothermy. The benefits of endothermy then promoted any other mutations that elaborated the lungs, driving a positive-feedback process. Eventually fully warm-blooded creatures had lungs in which myriads of clusters of alveoli, the descendants of the balloon-like lungs where the gas exchange takes place, sit at the ends of bush-like arrays of bronchi and bronchioles.

    As we see in the crocodilians, the evolution of the respiratory apparatus in the dinosaurs and, thence, in the birds followed a different pattern. Consider the twin air sacs that form the basic amphibian lung. They’re inherently poor at gas exchange because they have little surface area. Eventually a mutation created wrinkles or dimples on the surfaces of the sacs, thereby increasing the area available for gas exchange. The resulting increased rate of gas exchange gave the creature an advantage over other members of its species that lacked the mutation; thus, natural selection preserved and promoted the mutation in the population of that species.

    Where the air sacs pressed against each other some of the wrinkles and dimples came together as they grew and the dividing cells could merge and connect the two sacs together. A simple mutation made those connections open up into holes that allowed air to pass between the sacs. That was a neutral mutation in that it gave the creature neither advantage nor disadvantage, so natural selection neither promoted nor demoted it. But the mutation enabled others which lengthened the openings into tubes, which acted as flues that carried air between the air sacs. Those flues added area to the lungs and augmented gas exchange enough to make the mutations promotable. It wasn’t the most efficient gas exchange possible, because the air in the flues tended to stagnate, with no net flow in one direction or the other.

    At the places where the air sacs joined the creature’s trachea a simple birth defect began the evolution of a superbly efficient lung. Part of one opening’s rim grew outward as a flap that partially restricted airflow in one direction and allowed the air to flow freely in the opposite direction. That feature made air flow a little more unidirectionally through the system, thereby bringing fresher into the flues either during inhalation or exhalation. The resulting increased diffusion of oxygen into the creature’s blood gave the creature an advantage over others of its kind and promoted additional mutations that improved the effect. In their life-sculpting pas de deux, mutation and selection created a system in which both air sacs had flap valves that compelled air to flow in only one direction through the lungs. Currently we see this kind of respiratory system in crocodiles and alligators.

    Unidirectional airflow also promotes mutations that caused the flues in the lungs to lengthen and to grow outpocketings, called atria, that also increased the surface area available for gas exchange. The flues thus became myriads of parabronchi extending between the air sacs, which became smaller as they evolved into the dorsobronchi and ventrobronchi.

    At the same time, more or less, additional air sacs evolved, both anterior and posterior to the evolving lung. With their own flap valves, those additional sacs augmented the movement of air in the uniflow system. With the addition of those air sacs, dinosaurs evolved the respiratory system that is now unique to birds.

    Dinosaurs and birds don’t have a diaphragm; rather, their body cavity acts as a bellows. In mammals muscular contractions pull the diaphragm down, thereby making inhalation the active phase of respiration: in birds muscular contractions squeeze the bellows, thereby making exhalation the active phase of respiration. When a bird exhales, the stale air in its anterior air sacs gets expelled into the trachea and out of the bird and the fresh air in the posterior air sacs gets pushed into the lungs, where it pushes the stale air into the trachea. When the bird then inhales, stale air from the lungs flows into the anterior air sacs and fresh air coming through the trachea flows into the lungs and the posterior air sacs. Both phases of respiration blow fresh air through the lungs, so dinosaurs and birds have an efficient means of oxygenating their blood.

    That latter fact enables natural selection to promote mutations that give animals with flow-through lungs features that require a rapid expenditure of energy. Because of their lungs, dinosaurs became warm-blooded and birds acquired the ability to fly.

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