Pterosaurs: dragons of the air
Gliding – a controlled descent using gravity as the driving force – has evolved many times and in many different groups including frogs, lizards, snakes and several different mammals. Even some plants evolved gliding seeds. Gliding is also known among some fossil reptiles. Active flight, where flapping powers an animal through the air, on the other hand, has only evolved four times in nature: once in arthropods and three times in vertebrates. It has only become ‘extinct’ once… in the pterosaurs.
The first animals to evolve flight were insects. The earliest fossil insects with wings are dragonfly-like animals from the Carboniferous, but there are some very fragmentary insect remains from Devonian rocks in Scotland. These fossil remains closely resemble mayflies, but the specimens do not show the wings. It is highly likely that they did have wings, and if this supposition is correct, it means that flight first evolved approximately 410 million years ago. This major evolutionary innovation allowed insects to go on to evolve more than six million different species. Many of these ancient insects reached sizes far larger than modern insects.
During the Permian, a group of small lizard-like reptiles evolved the ability to glide, but they did not go on to become active flyers and most died out in a mass extinction 248 million years ago.
Pterosaurs and dinosaurs
Pterosaurs were the first vertebrates to evolve active flapping flight, but their origins are something of a mystery. The transformation from non-flying, perhaps gliding animal to fully flying pterosaur probably occurred in the forests of the Middle Triassic. Unfortunately, this environment rarely yields fossils, so the search for the oldest pterosaur might be in vain.
The Triassic also saw the evolution of a group of reptiles that were destined for world domination: the dinosaurs. By the end of the Triassic, dinosaurs had diversified and one group, the meat-eating theropods, had become the dominant predators. Although some of these reached gigantic proportions, some were small and lightly constructed. By the Upper Jurassic, some, like Archaeopteryx, had developed the ability to fly using a major evolutionary innovation: the flight feather.
During the Cretaceous, the skies were filled with a wide diversity of both pterosaurs and flying dinosaurs (which most people are happy to call birds), but by the end of the Cretaceous, the pterosaurs and several groups of birds had become extinct. Pterosaurs survived for more than 150 million years, and birds have had a similar innings, but birds are much more diverse now than pterosaurs were at the end of the Cretaceous.
Bats and beyond
Shortly after the Cretaceous, bats appeared – the first active flying mammals. In some respects the origins of flight in bats are as obscure as those of pterosaurs. The first bat fossils come from Paleogene rocks about 50 million years old in Wyoming and Germany. These fossils are not intermediary forms, but proper fully flying bats. About 50 million years after bat ancestors evolved flight, another group of vertebrates achieved the ability to fly. But instead of evolving wings, this group used its brain: it is man. The panels to the right show the story of the evolution of gliding and active powered flight from the first insects to the machines invented by man.