Evolution of flight

The evolution of flight has provided an enormous selective advantage to those organisms that have mastered it. True flight has developed independently on four separate occasions throughout geologic history, beginning with insects and followed by pterosaurs, birds, and bats. By studying how these animals have mastered flight, humans have learned to fly via machines and have also learned about the evolutionary process.

Development of Flight

Insects have dominated the air for about 300 million years and remain the most successful of all animals in terms of abundance and diversity. Following insects into the air were the flying reptiles, or the pterosaurs, which is now extinct. They were rapidly followed by birds. About fifty million years ago, the last group of flying animals joined the birds and insects: bats.

Although these animal groups are capable of true powered flight, there are many additional species said to “fly”—flying squirrels (the marsupial flying phalangers of Australia) and the dermopterans, or “flying lemurs.” There are also more than twenty species of flying lizards, frogs, and snakes. These animals are actually gliders rather than true flyers. They can leap from a branch, extend a skin membrane outward to increase their surface area, and coast to a lower branch or the ground like a paper airplane. In the case of flying squirrels and dermopterans, the gliding membrane consists of a skin fold extending between the fore and hind limbs, sometimes also enclosing the tail. The living dermopteran of the genus Cynocephalus has been recorded gliding a horizontal distance of 136 meters with a fall of only eleven meters. Gliding lizards can extend a membrane from their sides that is not connected to the limbs. Gliding frogs use their webbed feet as a membrane. The Indian parachute snake expands its ribs and draws in its belly after leaping from a branch, parachuting downward to a soft landing.

Flying fish use their abilities to escape submerged predators by launching themselves into the air. The fish have highly enlarged pectoral fins, which fan out into gliding surfaces in the air while the fish vigorously sways its tail just below the water. In this way, it can reach launch speeds of up to seventy-five kilometers per hour and glide freely in the air for a few hundred meters. One family of freshwater flying fish in South America actively flaps with its wing-like pectoral fins, making a buzzing sound, although these fish cannot remain airborne for more than a few meters.

For an animal to develop true powered flight, it must be able to lift itself upward, propel itself forward, and overcome the friction of the air (drag) as it moves. Birds, pterosaurs, and bats have streamlined their wings in such a way that the upper surface of the wing is more curved than the lower surface. This shape produces an acceleration of air above the wing, which generates upward suction, or lift. Flying animals can lift themselves only when moving forward so that air flows over the wing, which induces friction, or drag. Thus, the wings must be flapped vigorously to maintain forward motion and to replace the energy lost to friction, and the body must be as streamlined as possible.

The structures that the flying animals have evolved are designed to maximize lift and minimize drag. Insects have developed true wings, but the flying vertebrates (animals with backbones, such as birds, pterosaurs, and bats) have modified their forelimbs for this purpose. The forelimb bones must be exceptionally strong, lightweight, and powered by large, well-anchored muscles, which can make up to one-fourth the body weight in some birds. In addition, all flying vertebrates must consume a large amount of energy to overcome drag. Therefore, they must have highly efficient hearts, lungs, and metabolisms, which means that they must be warm-blooded, or able to generate their own internal energy. Flying vertebrates also need greater brainpower (larger skulls) to coordinate their flying and regulate their metabolism.

Insects and Pterosaurs

The first and most successful animals to develop flight were insects. Because insects lack bones or teeth, they are not readily preserved in sediments, so their fossil record is very poor. The first insects, which were wingless, probably crawled up from the sea onto land around 380 to 400 million years ago, following the colonization of the land by primitive plant life. The first winged insect appears in the fossil record about 300 million years ago, by which time flight was already highly developed in insects. Thus, scientists can only speculate about these first flying animals by studying the fossils available and by observing those living insects whose characteristics seem “primitive.” It is likely that insect wings were first developed for other purposes, such as gills, gill plates, camouflage, or armor plating to protect from predators. These structures probably became useful as gliding surfaces, allowing the insects to launch into the wind and drift to a new location or escape predators. Insects that could fan their protowings could prolong the length of their glides and, eventually, develop true flight. The advantages of true flight to an insect were (and are) enormous in migration, finding food and mates, and escaping predators.

The first vertebrate animal to share the skies with insects, the pterosaurs, appeared suddenly in the fossil record about 200 million years ago during one of those creative spurts in the evolutionary process that also produced dinosaurs, crocodiles, turtles, primitive mammals, and many other animals. Pterosaurs, about eighty-five known species, flew on wings made of a skin membrane connected to the side (or, in some, the hind leg) and stretched out over the greatly elongated fourth digit. (The thumb is digit one, index finger number two, and so on.) The membrane was reinforced with stiff fibers of connective tissue. When not flying, the pterosaurs could neatly tuck their wings next to their scaly bodies and scramble about on all fours, using their three-clawed front digits for walking, climbing trees, or grasping. Most had long, toothy snouts and large heads on long necks; some had long tails, and several had a head crest. Pterosaurs are thought to have mostly fed on fish, like shorebirds, and probably strongly resembled modern pelicans (without feathers). Modern investigation has shown that pterosaurs were agile, active flyers, and possibly endothermic (warm-blooded).

The head, neck, ankle, and shoulder anatomy indicate that pterosaurs evolved from a primitive archosaurian reptile such as Lagosuchus. Still, because they appear so suddenly in the fossil record, there is no evidence to show how the forelimbs evolved into wings. It is not the pterosaurs but the first birds that shed some light on this dilemma. The first bird, the dinosaur-bird Archaeopteryx, appeared in the fossil record about 150 million years ago, and further research revealed it must have originated before the most recent Jurassic period. Birds shared the skies with pterosaurs for about eighty million more years, after which the pterosaurs (non-avian dinosaurs) became extinct.

Archaeopteryx

Unlike insects, pterosaurs, and bats, which appear suddenly and already as highly developed flyers in the fossil record, Archaeopteryx provides the ideal transitional animal, in both time and structures, between reptiles and modern birds. Like birds, Archaeopteryx had feathers on streamlined wings. The feathers, which are modified reptilian scales, vastly increased the lift and surface area of the wing while adding little weight. The wing consisted of two and three elongated digits, tightly connected by tissue in Archaeopteryx and fused solidly together in modern birds. Also, like modern birds and unlike most reptiles, Archaeopteryx was warm-blooded and had fused collarbones (the wishbone). Unlike modern birds, Archaeopteryx lacked the large-keeled breastbone and the ability to fold its wings tightly. Also more like reptiles, Archaeopteryx had a long, toothy snout, a long, bony, feathered tail, and claws on its digits. If its feathers had not been preserved in such exquisite detail in the fossil record, Archaeopteryx would most likely have been classified as a small theropod. Theropods are a branch of dinosaurs that ran on their strong, birdlike hind limbs, were carnivorous, and had deep, compact bodies with long necks. They were not related to pterosaurs. If a theropod were warm-blooded and small enough (Archaeopteryx was pigeon-sized), its scales could have evolved into feathers to help keep it warm.

From its other anatomical details, Archaeopteryx was a competent flyer despite its lack of a keeled breastbone to anchor flight muscles. However, it probably lacked the maneuverability of modern bird flight. Traditionally, it was thought that Archaeopteryx developed the ability to fly via a gliding stage (the trees-down hypothesis), but additional research has indicated that this scenario is unlikely. Instead, perhaps Archaeopteryx ran along the ground, pursuing insects, and used its wings as aerodynamic stabilizers to balance itself after leaping into the air to catch its prey in its mouth (the cursorial hypothesis). Another possibility is that Archaeopteryx scrambled among the trees and used its wings to balance itself as it leaped between branches (the arboreal hypothesis).

Birds and Bats

Modern birds have adapted flight and feathers to various lifestyles and environments, from the tiny tropical hummingbird to the penguin and ostrich. The huge success of insectivorous birds has undoubtedly placed strong selective pressure on many insects to become nocturnal, as almost all birds are inactive at night. This adaptation allowed the evolution of the fourth and final group of flying animals, the bats.

Bats are the only flying mammals. Like other mammals, they are warm-blooded, have fur, bear live young, and nurse their young with milk. Bat wings are a skin membrane stretched like an umbrella over the elongated digits two through five. Digit one is short, free of the membrane, and bears a claw for grasping. The large wing membrane, also attached to the animal's side, encloses the weak hind leg and, in many species, even the long tail. When not flying, bats can scramble over the ground on all fours, but generally, they prefer to hang upside down from their hind limbs. Most are tropical, nocturnal insectivores and use echolocation (sonar), not sight, to find their prey.

Fossil bats are quite sparse, but the oldest fossil is around fifty million years old. Icaronycteris appears almost indistinguishable from modern bats, so scientists cannot explain how the bats learned to fly. The teeth of Icaronycteris bear some resemblances to those of fossil shrews and moles, which were also nocturnal insectivores (some of which also use a rudimentary form of echolocation), so the bats likely split from this family sometime before fifty million years ago.

Principal Terms

Archaeopteryx: a now-extinct transitional animal between dinosaurs and birds but generally regarded as the first bird because it flew on feathered wings

dermopteran: a gliding squirrel-sized mammal probably most closely related to primates; the so-called flying lemur of Southeast Asia

drag: the component of force that slows the speed of a moving wing as a result of friction and turbulence of the air

insectivore: any animal that eats insects, such as shrews, bats, and many birds

lift: the component of air force that acts on a wing in an upward direction

pterosaur (formerly pterodactyl): a now-extinct flying animal closely related to the dinosaurs; it had scales and wings made of a membrane attached to its “fingers”

theropod: a branch of carnivorous dinosaurs, thought to be ancestral to birds, that had large heads on long necks and a deep, compact body and that ran on two strong, birdlike hind limbs

vertebrate: any animal with a backbone, such as birds, dinosaurs, and fish; insects are invertebrates

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