The process by which certain animals have biologically adapted to engage in three modes of flight: active gliding, passive gliding, and true or powered flight.
Insect flight evolved in the mid to late Carboniferous period, about 325 million years ago. The flight of certain reptiles, the first vertebrates to fly, dates from the Triassic period, between 230 and 195 million years ago. The first primitive birds evolved during the Jurassic period, 195 to 135 million years ago.
Throughout evolutionary history, flight has been a crucial characteristic of many insects, reptiles, birds, and bats, and more than one-half of the animal species now living can fly. Most of these contemporary species are insects, but about nine thousand species of birds and nine hundred bat species are at home in the air. The largest animal capable of flight was an extinct reptile, Quetzalcoatlus northropi, which weighed over 140 pounds (63 kilograms). The smallest flying creature, the chalcid wasp (Encarsia formosa), weighs about 0.0001 ounces (0.025 milligrams). Within the range of these extremes, the variety of flying animals is immense.
Flying animals that succeeded in surviving for long periods of time had to solve basic problems. Their survival in a particular ecological niche depended on achieving an optimum size, bone structure, wingspan, and cardiovascular system. Through trial-and-error methods, some flying animals survived and prospered while many others became extinct. Over the long years of animal evolution, animals have chanced upon a wide spectrum of solutions to the problems of flight. Some fliers developed long wings, others stubby ones. Some flapped their wings vigorously, others used gliding and soaring techniques. Gliding utilizes gravity by either launching toward a target (directed or active gliding) or relying on wind for motion (passive gliding or parachuting). Soaring is sustained gliding, taking advantage of rising columns of air. True or powered flight consists of using muscles to take off, fly, and land. The details of the evolution of animal flight are poorly understood, because most flying animals left no trace in the fossil record. How did primitive insects, reptiles, and birds solve the problems of flight? Did flying vertebrates evolve from running, jumping, or perching animals? Did early fliers have rigid or flexible wings? Some of these questions have been answered, others have been only partially answered, but many others remain to be satisfactorily answered.
To enhance their ability to spend a significant part of their lives in the air, flying animals developed some devices that were similar to those now used by humans in constructing aircraft, but others were unlike the mechanisms found in flying machines. For example, the Wright brothers were helped in devising a wing-warping mechanism to control their Flyer by observing the flight of birds. When a jetliner is landing, the pilot lowers the landing gear and wing flaps to reduce speed in preparation for touchdown, just as birds extend their legs and lower their tails to increase drag when they are landing. On the other hand, aircraft designers have been unable to successfully copy the complex structures of feathers and the intricate ways that birds twist and flap their feathered wings.
Animal flight involves lift, thrust, and control, and such animals as insects, reptiles, birds, and bats have used a variety of methods in achieving flight, some of which are specific to a species, while others are shared across classes. For example, the largest insects have wings similar to those of the smallest hummingbirds, but animals such as flying squirrels lack the ability to flap their “wings” and so are confined to gliding. Scientists have studied the theoretical limits for such variables as size, wingspan, bone mass, and muscular strength in trying to understand flying animals.
To explain how animals developed the devices that allowed them to fly, scientists have proposed various theories. Two classical models, dating from the nineteenth century, are the arboreal and cursorial theories. These theories try to explain how limbs developed into wings, how bones became lightened, and how feathers evolved from scales. Some scientists speculate that elemental wings first sprouted as lengthwise ridges along the sides of vertebrates. These ridges were constructed from available materials such as fur, skin, or scales. According to the arboreal theory, these protowings adapted the animals for life in the trees, especially for leaping from limb to limb. Selective pressures from predators subsequently changed these winglets into wings. The cursorial theory, whose name derives from the Latin cursor, “a runner,” explains the origin of flight differently. To evade enemies, bipedal vertebrates habitually combined running jumps with short glides. Their increasingly winglike forelimbs helped them to generate the lift and thrust to spend time in the air instead of in the jaws of predators.
Both these theories have been criticized, but because of the deficiency of relevant fossils, it has been impossible to eliminate either one. Even when relevant fossils exist, they fail to resolve all problems. For example, Archaeopteryx lithographica, a fossil that had a reptilian skeleton and feathered wings, created a sensation when it was discovered in Germany just two years after Charles Darwin had published On the Origin of Species by Means of Natural Selection (1859).
This 150-million-year-old protobird was acclaimed as the “missing link” between reptiles and birds. As more Archaeopteryx fossils were found, a debate ensued over whether it was a birdlike reptile or a reptilelike bird. It resembled birds because it had feathers and a wishbone, but it resembled dinosaurian reptiles because it had teeth in its jaws, three clawed fingers on its wings, and a lizardlike tail. Some argued that Archaeopteryx was cold-blooded and flightless; others that it was warm-blooded and a flier.
Theories of the origin of animal flight have encountered what evolutionist Stephen Jay Gould has called the “5-percent-of-a-wing problem.” Aerodynamic analysis revealed that flying animals needed to have wings that were long, light, flexible, and strong, but critics of Darwin’s theory pointed out that protowings, which were short, dense, rigid, and weak, would have been a hindrance rather than a help. However, Darwinists responded that this all-or-nothing argument that wings are worthless until well developed ignores the possibility that winglets could confer such benefits as aiding animals in escaping from their enemies. Furthermore, rudimentary feathers could have served as heat insulators. Some scientists have called these winglets and protofeathers “preadaptations,” and some writers have called these primordial creatures “hopeful monsters.”
Flying creatures developed independently several times during the course of evolution, but the fossil record reveals that the first to fly were insects. Because of their small size, insects need little energy to launch themselves into the air, but due to an inadequate fossil record, scientists have been unable to develop incontestable interpretations of the evolution of insect flight. Fossils of flying insects in amber have not yet been discovered from the Paleozoic era (600 to 230 million years ago), and the sparse evidence scientists possess of the number, variety, structures, and ecological niches of flying insects provides only meager clues for constructing explanations of how insect flight originated. Paleontological evidence does exist for giant dragonflies (Protodonata) during the Carboniferous and Permian periods (between 300 and 225 million years ago). Some scientists think that these insects were aerial predators. Meganeura, the largest known flying insect, dates from this period, and it had a wingspan of 2.4 feet (73 centimeters) and weighed just under 8 ounces (200 grams). Evolution certainly played a role in the creation of these giant insects, and the mechanism may have been an escalating competition between predator and prey.
Some paleontologists have used later fossils and aerodynamic studies of contemporary insects to theorize about early flying-insect evolution. Many scientists believe that insect wings evolved only once and that all fliers derive from a wingless ancestor, but how these wings evolved has been passionately debated. Some scientists hold that winged insects originated on land, where jumping due to a startle reflex in response to predatory attacks may have been the selective force that promoted crude flying. Other scientists argue that insect flight began in an aquatic environment, where winglets allowed insects to walk on water since flapping their winglets kept them from breaking through the surface tension of the water.
Explanations of the history of insect flight are complicated by the great structural and functional diversity of insects. Flying insects now range from large butterflies who oscillate their wings about five times per second to tiny midges who beat their wings a thousand times per second. As impressive as contemporary insect diversity is, even more remarkable are the varieties found in Mesozoic fossils (245 to 65 million years ago). Paleontologists have estimated that about twenty-eight insect orders existed prior to the great Permian extinctions that occurred about 245 million years ago. These insects had many kinds of wings, and they differed in their ability to use them. Enhanced maneuverability in the air also facilitated sexual selection. For example, in some species, males hovered, explored, and then engaged in high-speed chases to capture females. In short, flight has been a key element in the survival and proliferation of an increasing variety of insect species.
The evolution of flying vertebrates presents paleontologists with different problems from those of flying-insect evolution. For example, adult vertebrates generally lack exoskeletons and weigh much more than insects. Because of these differences, scientists have had to modify their theories of insect evolution to explain how vertebrate fliers such as pterosaurs, birds, and bats evolved. A commonly held theory has pterosaurs (flying reptiles) and birds evolving from thecodonts, the direct ancestors of the dinosaurs. Most thecodonts were small reptiles that walked on their two hind legs, as do many birds.
Flying reptiles have been found fully developed in the Lower Jurassic (195 to 135 million years ago), but they had a much longer history, even though paleontologists have failed to find any intermediate forms between thecodonts and pterosaurs. The most distinctive trait of a flying saurian was its membranous wings buttressed by greatly elongated fourth fingers. Their three other fingers bore claws that may have allowed them to cling to rocks or tree limbs, from which they either hung head down (like bats) or perched head up (like birds). Some paleontologists believe that pterosaurs descended from a small arboreal reptile that spent its life in trees where, like modern flying squirrels, it used flaps of skin attached to its limbs to facilitate its glides and brake its falls. Others believe that pterosaurs evolved from bipedal reptiles that ran along the ground, perhaps spreading their upper limbs for balance. Through gradual growth, these forelimbs evolved into wings.
All these ideas are highly speculative, and most paleontologists agree that the question of pterosaur origins remains open. However, sufficient pterosaur fossils have been found for scientists to conclude that these “dragons of the air” were one of evolution’s early success stories. Unhampered by enemies in the air, pterosaurs diversified into over 120 known species. The sparrow-sized Pterodactylus elegans, the smallest pterosaur, had a ten-inch wingspan, whereas the largest, Quetzalcoatlus, known from a fossil in the Big Bend region of Texas, had a wingspan of nearly 40 feet (12.2 meters). The small pterosaurs fed chiefly on insects, while the large pteranodons preyed primarily on fish. Though often described as flying reptiles, the pterosaurs were unlike modern reptiles in several respects. They were most likely warm-blooded and had a hairlike surface and highly developed brains. Pterosaurs lived and thrived for 150 million years, but all of them became extinct in a very short period of time 65 million years ago, at the end of the Cretaceous. Just as the origin of pterosaurs is disputed, so too is their extinction. Some paleontologists claim that pterosaurs, with their fragile bodies, had become so specialized that they were unable to adapt to changes in the Cretaceous climate. Others blame a large asteroid that slammed into Earth, making the demise of the pterosaurs part of the calamity that wiped out the dinosaurs. However, critics of the catastrophic theory point out that pterosaur species had been dwindling for millions of years prior to the cataclysm, suggesting that other factors may have contributed to their fate.
Paleontologists have found no transitional forms between pterosaurs and the first birds, and most believe that birds and pterosaurs evolved separately from thecodonts rather than that birds evolved directly from pterosaurs. All modern theories of bird evolution have been influenced by a crow-sized creature that perished in a shallow lagoon 150 million years ago and whose bones and feather imprints were preserved in lithographic limestone found in 1861. Scientists naturally focused on this Archaeopteryx lithographica in their studies on the descent of birds. Strangely, during the nineteenth century, Archaeopteryx was not accepted as the missing link between reptiles and birds by most scientists nor by the public. Influenced by religious views of the fixity of species, the public viewed reptiles and birds as unchanging and unchangeable forms. In contrast, scientists had multiple interpretations of Archaeopteryx. For Richard Owen, the scientist who coined the name “dinosaur,” Archaeopteryx was the earliest bird, which was a transmuted form of a long-tailed pterosaur. On the other hand, Thomas Henry Huxley, traditionally described as “Darwin’s bulldog” because he eloquently defended natural selection, saw this fossil as proof that birds had evolved from dinosaurs.
The first and most influential book on avian evolution, The Origin of Birds, was written by a Dane, Gerhard Heilmann, in 1926. He argued for a thecondontian ancestry of birds, and his theory was supported by most textbooks and scholarly works on avian origins for the next fifty years. However, toward the end of the twentieth century, this classic theory came under attack. Some researchers interpreted thecodonts as a heterogeneous assemblage rather than a well-defined group. Others emphasized that birds were descendants of dinosaurs who were warm-blooded land-dwellers, and the first fliers, with their feathered wings, originated “from the ground up.” This cursorial theory of avian flight denied dinosaurs any life in the trees, and thus directly contradicted the arboreal theory championed by Heilmann, who had reconstructed in detail the evolution of birds from tree-dwelling to flying animals.
Besides questions of whether birds evolved “from the ground up” or “from the trees down,” problems arose over the origin of feathers, the most beautiful and well-known adaptation in evolutionary history. The central difficulty with the explanation of feathers’ evolution from reptilian scales is that a feathered airfoil had to meet stringent aerodynamic criteria to function as a manipulable wing for controlled flight. Some have proposed that feathers arose as netlike devices to catch insects, but nets must be pervious to air whereas airfoils need to be impervious to air. Furthermore, feathers are present in bird tails, where they could scarcely serve fly-catching functions. Other scientists speculate that feathers initially developed for temperature regulation, a preadaptation later used for flight. The difficulty with this thermoregulatory theory is that the microarchitecture of feathers, with their numerous filaments (barbs) and interlocking fringes (barbules), is so well adapted to flight that some scientists proposed that avian feathers evolved directly for flight. However birds originated, they rapidly diversified and colonized a variety of environments. The cataclysm that annihilated the pterosaurs at the end of the Cretaceous created ecological voids that birds increasingly occupied in an extraordinarily explosive evolutionary diversification.
The final flying vertebrate to evolve was the bat. The only mammals to have developed true flight, bats have existed since the start of the Cenozoic era, 65 million years ago. As with pterosaurs and birds, transitional forms of bats have not been found in the fossil record. The wings of the earliest fossilized bat, from about 50 million years ago, were as completely developed as those in modern species. This absence of incipient-winged ancestors has not prevented scientists from speculating about bat evolution. Some paleontologists interpret bat evolution as a succession of small mammals whose fingers gradually lengthened as wings and the specialized muscles necessary to power them developed, but this theory encountered the criticism that these protowings, which restricted normal hand movements, would be disadvantageous. It was argued that a creature would not sacrifice usable hands for half-developed wings. Other paleontologists use bats as an example of quantum evolution, believing that wings and other biomachinery needed to support flight developed in an evolutionary spurt. In this theory, bats arose from arboreal insectivores who developed a membrane that stretched between their fore- and hind-appendages. These protowings were initially used for gliding from tree to tree. Gliding evolved into flying, because wings allowed bats to occupy the niche of flying insect eaters. However, at that time the niche of daylight insect feeding was occupied by birds, forcing primitive bats into becoming nocturnal insect eaters. Like birds, bats quickly diversified and are now represented by over 850 species. They have been particularly successful in tropical regions, where more bat species exist than all other mammals combined.
Paleontologists have been able to sketch an outline for the evolution of animal flight, but the details of this picture have yet to be worked out in any completely satisfactory way. New fossil discoveries continue to expand and deepen scientific knowledge of flying-animal evolution, and computers have come to the aid of paleontologists, who are able to model prehistoric flying creatures by making use of detailed aerodynamic and physiological data. Thus scientists have new evidence to determine whether certain vanished creatures really flew. These studies have enhanced the appreciation of scientists and their students for the great diversity of flying insects, pterosaurs, birds, and bats that have flourished through evolutionary time. Once these animals conquered the air, new worlds were opened up to them, from deserts to mountains, from arctic tundra to tropical forests. The impressive distances covered by migratory birds and butterflies show the great energies these flying animals are willing to expend to traverse vast distances in their search for suitable environments in which to feed and reproduce. It took 300 million years to create the variety of flying creatures that live in the modern world, but it took Homo sapiens a much shorter time to achieve heavier-than-air powered flight. However, humans, who profited from their observations of birds in solving the problems of flight, have yet to solve the myriad of puzzles posed by the evolution of animal flight.
Dudley, Robert. The Biomechanics of Insect Flight: Form, Function, Evolution. Princeton, N.J.: Princeton University Press, 2000. The first detailed study of how insects actually fly and how they evolved into fliers, this well-researched book also contains an analysis of the roles that natural and sexual selection played in insect evolution. Feduccia, Alan. The Origin and Evolution of Birds. New Haven, Conn.: Yale University Press, 1996. A heavily illustrated volume treating the origin and early evolution of birds and avian flight and the later evolution of a great diversity of highly developed birds, including raptorial and flightless birds. Templin, R. J. “The Spectrum of Animal Flight: Insects to Pterosaurs.” Progress in Aerospace Sciences 36 (2000): 393-436. A comprehensive scientific paper summarizing data on the flight characteristics of many kinds of winged animals, it uses flight simulation to explore the characteristics of hypothetical proto-fliers, favoring the “trees-down” rather than the “ground-up” theory of vertebrate flight origins.
Forces of flight