Flight has evolved at least four separate times in different animals in history, rendering it a highly coveted trait supporting the survival of many animals today: quick journeys, rapid escapes from predators, and the ability to reach usually unreachable places are all enabled by sky-travel.
There are two types of flight: powered and unpowered. Unpowered flight begins from a raised location, converting gravitational potential energy into kinetic energy and using aerodynamic forces to control the trajectory of descent. Since energy is lost to drag and not replaced, unpowered flight is restricted by its limited range, rendering it most useful for short distance travel in dense rainforests. Sugar gliders, for instance, use their gliding ability to hop between trees clustered in Northern and Eastern Australian woodlands.
Powered flight first evolved within insects, about 350 million years ago. The problem with insect flight though, is that there is no consensus on its evolutionary origins between biologists. Over time, however, two main theories became mainstream. One theory, the tergal origin hypothesis, is that wings emerged from their tergum, or backs, as membranes first used for gliding. Another is the pleural origin hypothesis, which posits that their wings originated as limbs from their pleura, or torsos, and moved to their back over time as insect flight progressed from parachuting, to gliding and flying. A third school of thought seeks to merge the two hypotheses into a ‘dual origin hypothesis' yet none of these suggestions have been fully accepted by the scientific community. The origins of this debate are rooted in the lack of fossil records to definitively prove a theory: as insects tend to be smaller and lighter, they fossilize infrequently and poorly in comparison to terrestrial animals, leaving few traces of evidence for evolutionary biologists to work with.
Early flying insects, such as dragonflies, were terrifying displays and often had wingspans of more than two and a half feet – mainly due to abundance of oxygen in the late Paleozoic period, allowing insect spiracles (air holes that take in oxygen to allow them to conduct gaseous exchange) to take in more oxygen and facilitate greater growth. Again, the little fossil evidence of transitional forms leaves scientists to ponder how they have evolved into insects we see today.
After insects, pterosaurs arose, about 228 million years ago. Being relatives of the dinosaurs, they’re not quite active anymore and all their evidence is locked in fossil records: their flight mechanism is hinged on their forelimbs – their long, tapering wings evolved from the same body part as our arms. As pterosaurs' arm and hand bones evolved for flying, they lengthened, and the bones of one finger—the equivalent of our ring finger—became extraordinarily long. They were quite successful as well, cruising the skies for over 160 million years until they became extinct.
Early pterosaurs were quite small, with less than a metre of wingspan and were insectivorous; see suborder Rhamphorhynchoidea or genus Eudimorphodon for pterosaurs such as these. The more impressive pterosaurs were of the genus Pterodactylus, the behemoths of the sky.
Once more, two main theories emerged to explain the origins of pterosaurs’ flight: firstly, the arboreal leaping theory, which states that pterosaurs developed their wings first to glide from tree to tree and then to fly over time; secondly, the ground up theory, suggesting that pterosaur ancestors were terrestrial and ran and jumped to catch prey, developing wings over time to become more effective than that. They were the largest flying creatures to ever exist, with some of them (particularly Quetzalcoatlus) reaching a wingspan of over 40 feet - over two thirds the length of a Boeing 747.
Birds’ fossil records are much vaster and more extensive. Their common ancestor is found 125 million years ago, and it is called the Archaeopteryx, or Urvogel (its German name). Archaeopteryx refers to a genus of bird-like dinosaurs, which became the birds we have nowadays, and was discovered only two years after Darwin’s publication of The Origin of Species. Its unusual appearance sparked a debate about its mixture of avian and reptilian features, such as it having feathers for flight (a trait of birds) and teeth, which aren’t found on birds. Birds have the most species of any class of terrestrial vertebrates, hinting at their success.
The most prominent theory behind the origins of avian flight is the trees down theory, as wings were seen as more effective through gliding rather than through running and flapping consecutively. This is supported through the discovery of the micro raptor, whose hind leg feathers would have been a hindrance while running. Birds are also very well adapted for flight, sporting hollow bones, like pterosaurs, and a unidirectional respiratory system, meaning that when they breathe in, they fill two air sacs around their lungs with air, and when they exhale, those air sacs release their air into the lungs. Air from the lungs goes into the anterior air sack, one of the two air sacs, and some of it leaves the system while some of it goes back for a second round of gaseous exchange. This system allows birds to gain much more oxygen much quicker, and power their flight much more effectively than how humans breathe (total by phasic air flow).
Birds also typically have impressive eyesight, especially birds of prey such as the Harpy Eagle, which have brilliant eyesight through having more photoreceptors, packed closely together to produce a more precise image. Birds have a precise degree of control over flight direction through rotation of primary feathers attached to the phalanxes and metacarpi of their wings, giving them an ability to manoeuvre in the sky and render it difficult for their prey to escape. Through these adaptations birds have become a significantly successful family with many extant species filling all kinds of ecological niches. However, recently they have gained a competitor, bats.
Bats recently evolved about 60 million years ago and fly with a completely different mechanism. They have a thin membrane of skin stretched between their “fingers”, and attached to their ankles, which now span across the wing acting as support for the skin wing (called a patagium), no longer resembling whatever appendage it originally converged from. Bats are more flexible than birds, sporting 25 joints in their patagium, allowing a greater degree of motor control and, instead of sharp eyesight, they rely on echolocation to find prey. Ancestral bats were gliders, moving from tree to tree seeking fruits or insects to eat and evolving flight gradually to improve at this; now, over 1400 species of bats exist today, found on every continent that exists apart from Antarctica. Bats do have a hindrance however, and that is the fact that they use total by phasic air flow, so their breathing is much less efficient than that of birds and so cannot grow as large as birds can, or fly as high as birds can, where there is less oxygen.
Yet, the evolution of flight remains a highly contested topic. It is a fascinating example of convergent evolution, often regarded as an indication of the success of an adaptation, and has been seen in countless species from 350 million years ago up until today.
References
Dudley R., Byrnes G., Yanoviak S., Borrell B., Brown R., McGuire J. “Gliding and the Functional Origings of Flight: Biomechanical Novelty or Necessity?” (2012) Annual Review of Ecology, Evolution and Systematics 38 (1) 179-201 https://doi.org/10.1146/annurev.ecolsys.37.091305.110014
Jackson S., Schouten P., “Gliding Mammals of the World” (2012) doi:10.1071/9780643104051
Linz D.M., Tomoyasu Y. “Dual evolutionary origin of insect wings supported by an investigation of the abdominal wing serial homologs in Tribolium” (2017) https://www.pnas.org/doi/pdf/10.1073/pnas.1711128115#:~:text=The%20pleural%20origin%20hypothesis%20proposes,structures%20of%20insects%20(8).
Tomoyasu Y., Ohde T., Clark-Hatchel C. “What serial homologs can tell us about the origin of insect wings” (2017) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5357031/
DOI: 13 facts about bats https://www.doi.gov/blog/13-facts-about-bats
Britannica: Pteranodon https://www.britannica.com/animal/Pteranodon
Britannica: Pterodactyl https://www.britannica.com/animal/pterodactyl
Britannica: Pterosaur https://www.britannica.com/animal/pterosaur
Wikipedia: Flying and gliding animals https://en.wikipedia.org/wiki/Flying_and_gliding_animals
RSPB: How do Birds survive https://www.rspb.org.uk/birds-and-wildlife/natures-home-magazine/birds-and-wildlife-articles/how-do-birds-survive/adapted-for-flight/how-birds-bodies-help-them-fly/