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Around 350 BCE,
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ...
and other philosophers of the time attempted to explain the aerodynamics of avian flight. Even after the discovery of the ancestral bird ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'' which lived over 150 million years ago, debates still persist regarding the evolution of flight. There are three leading hypotheses pertaining to avian flight: Pouncing Proavis model,
Cursorial A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often u ...
model, and
Arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose nu ...
model. In March 2018, scientists reported that ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'' was likely capable of
flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
, but in a manner substantially different from that of
modern birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight ...
.


Flight characteristics

For
flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
to occur, four physical forces (
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
and drag,
lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobil ...
and
weight In science and engineering, the weight of an object is the force acting on the object due to gravity. Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar qua ...
) must be favorably combined. In order for birds to balance these forces, certain physical characteristics are required. Asymmetrical wing feathers, found on all flying birds with the exception of
hummingbirds Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae. With about 361 species and 113 genera, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropics aro ...
, help in the production of thrust and lift. Anything that moves through the air produces drag due to
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of ...
. The aerodynamic body of a bird can reduce drag, but when stopping or slowing down a bird will use its tail and feet to increase drag. Weight is the largest obstacle birds must overcome in order to fly. An animal can more easily attain flight by reducing its absolute weight. Birds evolved from other theropod dinosaurs that had already gone through a phase of size reduction during the
Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively ...
, combined with rapid evolutionary changes. Flying birds during their evolution further reduced relative weight through several characteristics such as the loss of teeth, shrinkage of the gonads out of mating season, and fusion of bones. Teeth were replaced by a lightweight bill made of
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up Scale (anatomy), scales, hair, Nail ...
, the food being processed by the bird's
gizzard The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including archosaurs (pterosaurs, crocodiles, alligators, dinosaurs, birds), earthworms, some gastropods, so ...
. Other advanced physical characteristics evolved for flight are a
keel The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. As the laying down of the keel is the initial step in the construction of a ship, in Br ...
for the attachment of flight muscles and an enlarged
cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebe ...
for fine motor coordination. These were gradual changes, though, and not strict conditions for flight: the first birds had teeth, at best a small keel and relatively unfused bones. Pneumatic bone, that is hollow or filled with air sacs, has often been seen as an adaptation reducing weight, but it was already present in non-flying dinosaurs, and birds on average do not have a lighter skeleton than mammals of the same size. The same is true for the
furcula The (Latin for "little fork") or wishbone is a forked bone found in most birds and some species of non-avian dinosaurs, and is formed by the fusion of the two pink clavicles. In birds, its primary function is in the strengthening of the thoracic ...
, a bone which enhances skeletal bracing for the stresses of flight. The mechanics of an avian's wings involve a complex interworking of forces, particularly at the shoulder where most of the wings' motions take place. These functions depend on a precise balance of forces from the muscles, ligaments, and articular cartilages as well as inertial, gravitational, and aerodynamic loads on the wing. Birds have two main muscles in their wing that are responsible for flight: the pectoralis and the supracoracoideus. The pectoralis is the largest muscle in the wing and is the primary depressor and pronator of the wing. The supracoracoideus is the second largest and is the primary elevator and supinator. In addition, there are distal wing muscles that assist the bird in flight. Prior to their existence on birds, feathers were present on the bodies of many dinosaur species. Through natural selection, feathers became more common among the animals as their wings developed over the course of tens of millions of years. The smooth surface of feathers on a bird's body helps to reduce friction while in flight. The tail, also consisting of feathers, helps the bird to maneuver and glide.


Hypotheses


Pouncing Proavis model

A theory of a pouncing proavis was first proposed by Garner, Taylor, and Thomas in 1999:
We propose that birds evolved from predators that specialized in ambush from elevated sites, using their raptorial hindlimbs in a leaping attack. Drag-based, and later lift-based, mechanisms evolved under selection for improved control of body position and locomotion during the aerial part of the attack. Selection for enhanced lift-based control led to improved lift coefficients, incidentally turning a pounce into a swoop as lift production increased. Selection for greater swooping range would finally lead to the origin of true flight.
The authors believed that this theory had four main virtues: *It predicts the observed sequence of character acquisition in avian evolution. *It predicts an ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
''-like animal, with a skeleton more or less identical to terrestrial theropods, with few adaptations to flapping, but very advanced aerodynamic asymmetrical feathers. *It explains that primitive pouncers (perhaps like ''
Microraptor ''Microraptor'' ( Greek, μικρός, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China. ...
'') could coexist with more advanced fliers (like ''
Confuciusornis ''Confuciusornis'' is a genus of basal crow-sized avialan from the Early Cretaceous Period of the Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago. Like modern birds, ''Confuciusornis'' had a toothless beak, b ...
'' or '' Sapeornis'') since they did not compete for flying niches. *It explains that the evolution of elongated
rachis In biology, a rachis (from the grc, ῥάχις [], "backbone, spine") is a main axis or "shaft". In zoology and microbiology In vertebrates, ''rachis'' can refer to the series of articulated vertebrae, which encase the spinal cord. In this c ...
-bearing feathers began with simple forms that produced a benefit by increasing drag. Later, more refined feather shapes could begin to also provide lift.


Cursorial model

A cursorial, or "running" model was originally proposed by
Samuel Wendell Williston Samuel Wendell Williston (July 10, 1852 – August 30, 1918) was an American educator, entomologist, and paleontologist who was the first to propose that birds developed flight cursorially (by running), rather than arboreally (by leaping from tr ...
in 1879. This theory states that "flight evolved in running bipeds through a series of short jumps". As the length of the jumps extended, the wings were used not only for thrust but also for stability, and eventually eliminated the gliding intermediate. This theory was modified in the 1970s by John Ostrom to describe the use of wings as an insect-foraging mechanism which then evolved into a wing stroke. Research was conducted by comparing the amount of energy expended by each hunting method with the amount of food gathered. The potential hunting volume doubles by running and jumping. To gather the same volume of food, ''Archaeopteryx'' would expend less energy by running and jumping than by running alone. Therefore, the cost/benefit ratio would be more favorable for this model. Due to ''Archaeopteryx's'' long and erect leg, supporters of this model say the species was a terrestrial bird. This characteristic allows for more strength and stability in the hindlimbs. Thrust produced by the wings coupled with propulsion in the legs generates the minimum velocity required to achieve flight. This wing motion is thought to have evolved from asymmetrical propulsion flapping motion. Thus, through these mechanisms, ''Archaeopteryx'' was able to achieve flight from the ground up. Although the evidence in favor of this model is scientifically plausible, the evidence against it is substantial. For instance, a cursorial flight model would be energetically less favorable when compared to the alternative hypotheses. In order to achieve
liftoff Liftoff, lift-off, or lift off may refer to: Technology * Lift-off (microtechnology), a fabrication technique * Flame lift-off, a separation of flame from burner device * Takeoff, the first moment of flight of an aerospace vehicle * Reduction ...
, ''Archaeopteryx'' would have to run faster than modern birds by a factor of three, due to its weight. Furthermore, the mass of ''Archaeopteryx'' versus the distance needed for minimum velocity to obtain liftoff speed is proportional, therefore, as mass increases, the energy required for takeoff increases. Other research has shown that the physics involved in cursorial flight would not make this a likely answer to the origin of avian flight. Once flight speed is obtained and ''Archaeopteryx'' is in the air, drag would cause the velocity to instantaneously decrease; balance could not be maintained due to this immediate reduction in velocity. Hence, ''Archaeopteryx'' would have a very short and ineffective flight. In contrast to Ostrom's theory regarding flight as a hunting mechanism, physics again does not support this model. In order to effectively trap insects with the wings, ''Archaeopteryx'' would require a mechanism such as holes in the wings to reduce air resistance. Without this mechanism, the cost/benefit ratio would not be feasible. The decrease in efficiency when looking at the cursorial model is caused by the flapping stroke needed to achieve flight. This stroke motion needs both wings to move in a symmetrical motion, or together. This is opposed to an asymmetrical motion like that in humans' arms while running. The symmetrical motion would be costly in the cursorial model because it would be difficult while running on the ground, compared to the arboreal model where it is natural for an animal to move both arms together when falling. There is also a large fitness reduction between the two extremes of asymmetrical and symmetrical flapping motion so the theropods would have evolved to one of the extremes. However, new research on the mechanics of bipedal running has suggested that oscillations produced by the running motion could induce symmetrical flapping of the wings at the natural frequency of the oscillation.


Wing-assisted incline running

The WAIR hypothesis, a version of the "cursorial model" of the evolution of avian flight, in which birds' wings originated from forelimb modifications that provided downforce, enabling the proto-birds to run up extremely steep slopes such as the trunks of trees, was prompted by observation of young chukar chicks, and proposes that wings developed their
aerodynamic Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators. Note that in this scenario birds need ''downforce'' to give their feet increased grip. Summarized in It has been argued that early birds, including ''Archaeopteryx'', lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes; since the downforce on which WAIR depends is generated by upstrokes, it seems that early birds were incapable of WAIR. However, a study that found lift generated from wings to be the primary factor for successfully accelerating a body toward a substrate during WAIR indicated the onset of flight ability was constrained by neuromuscular control or power output rather than by external wing morphology itself and that partially developed wings not yet capable of flight could indeed provide useful lift during WAIR. Additionally, examination of the work and power requirements for extant bird pectoralis contractile behavior during WAIR at different angles of substrate incline demonstrated incremental increases in these requirements, both as WAIR angles increased and in the transition from WAIR to flapping flight. This provides a model for an evolutionary transition from terrestrial to aerial locomotion as transitional forms incrementally adapted to meet the work and power requirements to scale steeper and steeper inclines using WAIR and the incremental increases from WAIR to flight. Birds use wing-assisted inclined running from the day they hatch to increase locomotion. This can also be said for birds or feathered theropods whose wing muscles cannot generate enough force to fly, and shows how this behavior could have evolved to help these theropods then eventually led to flight. The progression from wing-assisted incline running to flight can be seen in the growth of birds, from when they are hatchlings to fully grown. They begin with wing-assisted incline running and slowly alter their wing strokes for flight as they grow and are able to make enough force. These transitional stages that lead to flight are both physical and behavioral. The transitions over a hatchling's life can be correlated with the evolution of flight on a macro scale. If protobirds are compared to hatchlings their physical traits such as wing size and behavior may have been similar. Flapping flight is limited by the size and muscle force of a wing. Even while using the correct model of arboreal or cursorial, protobirds' wings were not able to sustain flight, but they did most likely gain the behaviors needed for the arboreal or cursorial model like today's birds do when hatched. There are similar steps between the two. Wing-assisted incline running can also produce a useful lift in babies but is very small compared to that of juveniles and adult birds. This lift was found responsible for body acceleration when going up an incline and leads to flight as the bird grows.


Arboreal model

This model was originally proposed in 1880 by
Othniel C. Marsh Othniel Charles Marsh (October 29, 1831 – March 18, 1899) was an American professor of Paleontology in Yale College and President of the National Academy of Sciences. He was one of the preeminent scientists in the field of paleontology. Among h ...
. The theory states ''Archaeopteryx'' was a reptilian bird that soared from tree to tree. After the leap, ''Archaeopteryx'' would then use its wings as a balancing mechanism. According to this model, ''Archaeopteryx'' developed a gliding method to conserve energy. Even though an arboreal ''Archaeopteryx'' exerts energy climbing the tree, it is able to achieve higher velocities and cover greater distances during the gliding phase, which conserves more energy in the long run than a cursorial bipedal runner. Conserving energy during the gliding phase makes this a more energy-efficient model. Therefore, the benefits gained by gliding outweigh the energy used in climbing the tree. A modern behavior model to compare against would be that of the
flying squirrel Flying squirrels (scientifically known as Pteromyini or Petauristini) are a tribe of 50 species of squirrels in the family Sciuridae. Despite their name, they are not in fact capable of full flight in the same way as birds or bats, but they a ...
. In addition to energy conservation, arboreality is generally associated positively with survival, at least in mammals. The evolutionary path between arboreality and flight has been proposed through a number of hypotheses. Dudley and Yanoviak proposed that animals that live in trees generally end up high enough that a fall, purposeful or otherwise, would generate enough speed for aerodynamic forces to have an effect on the body. Many animals, even those which do not fly, demonstrate the ability to right themselves and face the ground ventrally, then exhibiting behaviors that act against aerodynamic forces to slow their rate of descent in a process known as parachuting. Arboreal animals that were forced by predators or simply fell from trees that exhibited these kinds of behaviors would have been in a better position to eventually evolve capabilities that were more akin to flight as we know them today. Researchers in support of this model have suggested that ''Archaeopteryx'' possessed skeletal features similar to those of modern birds. The first such feature to be noted was the supposed similarity between the foot of ''Archaeopteryx'' and that of modern perching birds. The
hallux Toes are the digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being ''digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being ''plan ...
, or modified of the first digit of the foot, was long thought to have pointed posterior to the remaining digits, as in perching birds. Therefore, researchers once concluded that ''Archaeopteryx'' used the hallux as a balancing mechanism on tree limbs. However, study of the Thermopolis specimen of ''Archeopteryx'', which has the most complete foot of any known, showed that the hallux was not in fact reversed, limiting the creature's ability to perch on branches and implying a terrestrial or trunk-climbing lifestyle. Another skeletal feature that is similar in ''Archaeopteryx'' and modern birds is the curvature of the claws. ''Archaeopteryx'' possessed the same claw curvature of the foot to that of perching birds. However, the claw curvature of the hand in ''Archaeopteryx'' was similar to that in basal birds. Based upon the comparisons of modern birds to ''Archaeopteryx'', perching characteristics were present, signifying an arboreal habitat. The ability for takeoff and flight was originally thought to require a supracoracoideus pulley system (SC). This system consists of a tendon joining the
humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a r ...
and
coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...
bones, allowing rotation of the humerus during the upstroke. However, this system is lacking in ''Archaeopteryx''. Based on experiments performed by M. Sy in 1936, it was proven that the SC pulley system was not required for flight from an elevated position but was necessary for cursorial takeoff.


Synthesis

Some researchers have suggested that treating arboreal and cursorial hypotheses as mutually exclusive explanations of the origin of bird flight is incorrect. Researchers in support of synthesizing cite studies that show incipient wings have adaptive advantages for a variety of functions, including arboreal parachuting, WAIR, and horizontal flap-leaping. Other research also shows that ancestral avialans were not necessarily exclusively arboreal or cursorial, but rather lived on a spectrum of habitats. The capability for powered flight evolved due to a multitude of selective advantages of incipient wings in navigating a more complex environment than previously thought.


See also

*
Origin of birds The scientific question of within which larger group of animals birds evolved has traditionally been called the "origin of birds". The present scientific consensus is that birds are a group of maniraptoran theropod dinosaurs that originated ...
*
Bird flight Bird flight is the primary mode of locomotion used by most bird species in which birds take off and fly. Flight assists birds with feeding, breeding, avoiding predators, and migrating. Bird flight is one of the most complex forms of locomo ...
*
Flying and gliding animals A number of animals are capable of aerial locomotion, either by powered flight or by gliding. This trait has appeared by evolution many times, without any single common ancestor. Flight has evolved at least four times in separate animals: i ...
*
Insect flight Insects are the only group of invertebrates that have evolved wings and flight. Insects first flew in the Carboniferous, some 350 to 400 million years ago, making them the first animals to evolve flight. Wings may have evolved from appenda ...
* Tetrapteryx, a four-winged stage proposed by William Beebe; hindlimb feathers on
Microraptor ''Microraptor'' ( Greek, μικρός, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China. ...
and
Anchiornis ''Anchiornis'' is a genus of small, four-winged paravian dinosaurs, with only one known species, the type species ''Anchiornis huxleyi'', named for its similarity to modern birds. The Latin name ''Anchiornis'' derives from a Greek word meaning " ...
have been interpreted as evidence of four-winged gliding.


Footnotes


References

* *Chatterjee, S. 1997. The Rise of Birds. The Johns Hopkins University Press. Baltimore. p. 150-151, 153, 158. * *Elzanowoski, A. 2000. "The Flying Dinosaurs." Ed. Paul, G. The Scientific American Book of Dinosaurs. p. 178. *Feduccia, A. 1999. The Origin and Evolution of Birds. Yale University Press. London. p. 95, 97, 101, 103–104, 136. * *Gill, F. 2007. Ornithology. W.H. Freeman and Company. New York. p. 25, 29, 40–41. * * * *Paul, G. 2002. Dinosaurs of the Air. The Johns Hopkins University Press. London. p. 134-135. *Videler, J. 2005. Avian Flight. Oxford University Press. Oxford. P. 2, 91–98. *


External links


Flight of the ''Archaeopteryx''
(journal article)

{{fins, limbs and wings Avian flight
Flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
Bird flight