Flight is not only an extraordinary ability, but a huge evolutionary advantage. Occasionally, when I see a bird soaring high up in the sky I imagine what it would be like to fly. To see the world from a whole new perspective and feel so free. Powered flight has evolved at least four separate times in history: first in insects, next in pterosaurs, then in birds, and lastly in bats. Powered flight uses muscles to generate lift and thrust; this differs from parachuting and gliding which has evolved in every major group of vertebrates (for example, flying squirrels and flying lizards).
The development of flight is an example of convergent evolution – this is the process whereby unrelated organisms independently evolve similar traits as a result of having to adapt to similar ecological niches. In this post, I want to discuss the evolution and biology of flight in each major group that has acquired this fascinating ability. I also want to delve into what makes flight such a successful strategy, as both predator and prey, and the future of flying.
Four hundred million years ago, insects became the first animals to ever take to the skies. To this day, they are still the only group of invertebrates capable of powered flight. There are two methods of flight in insects. The first method is called direct flight – this was the first method to evolve and can still be seen in dragonflies and mayflies today. Direct flight involves flight muscles that are attached directly to the wings and they contract or relax to move the wings up and down. The alternative method is called indirect flight which is far more common, being present in many insects such as flies, bees and butterflies. This technique relies on flight muscles attached to the thorax, making it oscillate and indirectly causing the wings to beat. The origin and purpose of insect flight is still disputed. However, one thing is certain and that’s that it has proved to be a highly effective competitive advantage. Today, there are more species of insects than all other animal groups combined!
The next group of animals to take flight was the pterosaurs. The pterosaurs were flying reptiles and the first vertebrates to evolve powered flight. They lived through a good chunk of the Mesozoic Era (which includes the Triassic, Jurassic and Cretaceous periods) but disappeared along with the dinosaurs 66 million years ago in the Cretaceous-Paleogene extinction event. Unlike insects, the wings of pterosaurs were formed by a membrane of skin stretching from the ankles to an extended fourth finger. Their ability to fly would have allowed them to fill a previously unoccupied niche. By hunting from the skies, they would have been able to pin-point and dive to catch their prey with great accuracy. Their diet would have consisted of insects and small vertebrates such as fish, although this would have varied massively depending on the species. Towards the end of the Mesozoic, pterosaurs became giants of the skies. Some of the last species were the largest flying animals ever to exist. For instance, Quetzalcoatlus had a wingspan of over 10 metres! The pterosaurs reign lasted for millions of years and this is a testament to their flying prowess. Who knows what our world would look like if pterosaurs were still around.
No vertebrate group has as many flying species as birds. Their unbelievable diversity – from the tiny hummingbird to the mighty albatross – and their powerful flight technique have enabled them to conquer the skies of our modern world. The unique characteristic of birds that sets them apart from other flying animals is of course feathers. Feathers are highly complex structures that serve a range of purposes in birds, including thermoregulation, communication and waterproofing. Birds’ flight feathers provide aerodynamic surfaces that allow them to negotiate the air with ease. Birds were the third group of animals to develop powered flight after evolving from small, feathered theropod dinosaurs. However, the way in which they took to the skies is still questioned. One idea is the ‘from the trees down’ hypothesis in which birds’ ancestors first glided down from trees and gradually evolved true flight over time. A second suggestion is the ‘from the ground up’ hypothesis whereby a fast-running predatory ancestor used feathers for a speed boost and to help catch prey, and these feathers eventually developed into wings.
Birds have powerful chest muscles that contract to flap their wings. Their method of flight can be simplified to a series of upstrokes and downstrokes. This action gives the thrust needed to move forwards. The downstroke provides the propulsion whilst lift is generated by the wings’ convex shape. On the upstroke, wings fold closer to the body to reduce their surface area and thus minimise air resistance. Bird flight is one of the most complex forms of locomotion in the animal kingdom. It took millions of years for birds to perfect this skill but now they are true masters of the sky. If you are prey, flying helps you to swiftly escape a predator, and if you are a predator, flying helps you to accurately target your prey and exert a powerful and unexpected strike to your victim from above.
The most recent group of animals to obtain powered flight is the bats. Evolving around 60 million years ago, bats are the only animals capable of true flight, although some mammals like colugos and sugar gliders have developed to ability to glide. Bats beat their wings to generate the thrust needed to move them through the air. Whereas birds have stiff flight feathers providing the aerodynamic surface, bats have a skin membrane, stretched between elongated finger bones and their feet. Wings of living skin are more adjustable and sensitive to air around them, giving bats unmatched manoeuvrability. This is crucial for flying through dense habitats in their search for insects, fruit or nectar. Bats’ aerial agility means that they can change direction up to ten times faster than a bird. With over 1,000 species and making up around 20% of all known mammals, their competence in the air may be the reason why bats are some of the most successful mammals on earth.
So there we are! That was a brief overview of when, how and why flight has evolved on four separate occasions in earth’s history. After each group took to the skies, they seemed to flourish and bring a whole new dynamic to the world’s ecosystems. Aside from the tragic fate of the pterosaurs, all of the other animal groups are still alive and thriving today. The future of flight is unknown. Who knows what the world will look like in a few million years (I do hope we haven’t destroyed it by then), maybe present-day gliding animals like flying lizards or flying fish will be next to evolve powered flight. The opportunities are exciting and endless, but in the meantime we need to protect and conserve the life on our planet currently under threat.
Thank you for reading!
Sources:
The Natural History Museum (2019) The Science of Animals. 1st edn. London. Dorling Kindersley.
https://www.scienceworld.ca/stories/evolution-flight/
https://en.wikipedia.org/wiki/Flying_and_gliding_animals
https://en.wikipedia.org/wiki/Bird_flight#Evolution_of_bird_flight
https://en.wikipedia.org/wiki/Insect_flight
https://askabiologist.asu.edu/how-do-birds-fly
https://ucmp.berkeley.edu/vertebrates/flight/evolve.html
The Wonders of Wildlife 