Out of several thousand species of birds, almost all of them can fly. They all have the ability to fly because they evolved from a common ancestor that could fly. Bats can all fly because they evolved from a common ancestor that could fly. But why can both birds and bats fly? Did they evolve from a common ancestor that could fly? While they did evolve from a common ancestor, this ancestor could not fly. How, then, are both birds and bats able to fly?
In biology, there is a concept called “convergent evolution.” Some types of organisms have similar traits because they evolved from a common ancestor that had those traits. With only a few exceptions, all mammals, amphibians and reptiles (including birds) have four limbs — two arms/wings and two legs. This is because these three lineages all evolved from a common ancestor that had four limbs. Similar traits that are due to common ancestry are called “homologous traits.”
Other types of organisms have similar traits but did not evolve from a common ancestor that had those traits. Fish and whales are a classic example of convergent evolution. They both have a tail fin that propels them through the water, forward fins that help them steer, a fusiform body that makes them hydrodynamic, and a dorsal fin that keeps them stable.
But there are more differences than similarities. Here are a few:
- The tail fin of a fish is oriented vertically, whereas the tail fin of a whale is oriented horizontally
- Fish lay eggs, whereas whales give live birth
- Baby fish are fed by a yolk sack in their egg, whereas baby whales are fed from mammary milk
- Fish use gills to extract oxygen from the water, whereas whales breathe atmospheric air
- Fish are cold-blooded, whereas whales maintain a high body temperature.
- Fish have scales covering their skin, but whales do not.
- Whales have typical mammalian wrist and finger bones inside their pectoral fins, but fish do not.
- Whales have hair, but fish do not.
Whales do, of course, share a common ancestor with fish, but this common ancestor is not the reason that whales have their aquatic adaptations. The ancestors of whales first evolved into a terrestrial life, then evolved back into the water, much later in life.
When two or more different types of organisms evolve a similar trait independently, these traits are called “analogous traits” and the process of evolving these analogous traits is called “convergent evolution.”
Off the top of my head, I can think of nine independent evolutionary origins of flight — that is, nine separate events of convergent evolution. There are probably more that I don’t know about. Let’s start with the three best fliers that are currently alive: Birds, bats and insects.
Birds and bats are both tetrapods, so they are stuck with four limbs. They both use primarily their front limbs for flight, but they do it differently. Bird hand and wrist bones are fused together to make a short, stumpy end bone. Feathers produce the area required to produce lift.
When birds are in flight, they keep their legs and feet tucked out of the way so they do not interfere with flying.
Bats have a membrane of skin that stretches between their arms and legs that help produce lift. The legs and feet of bats are very important for flight.
Bats have elongated fingers that make up most of the wings. They use skin that is stretched between their fingers to create the area required to produce lift.
The arm bones in the bats and birds are homologous to one another, but their wings are the result of convergent evolution.
Insects have six legs and two pairs of wings. Insect wings are inflexible, except for where the connect to the body; a little bit like the oars on a boat. There are no bones or muscles inside the wings. Birds and bats have aerodynamic bodies that allow them to pass through the air efficiently. Some insects, like the dragonflies, have aerodynamic bodies, but bees and beetles do not.
The pterosaurs were not technically dinosaurs, but they were close relatives. Modern birds, which are dinosaurs, are not direct descendants of the pterosaurs, but birds are more closely related to the pterosaurs than they are to bats. Despite the closer genetic relatedness, the pterosaurs flight ability resembles bats more than birds in a variety of ways. First, they did not appear to have had feathers. Instead, they probably used a membrane of skin to form their wings much the way bats do.
Bats use fingers 2-4 (index through pinkie) for flight, and finger 1 (the thumb) for limited gripping. Pterosaurs only had four fingers, and only finger 4 was used for flight, whereas fingers 1-3 were used for gripping.
Other, lesser fliers:
These are animals that fly sort of like a paper airplane. They cannot propel themselves once they are in the air — they have to jump to get their initial momentum. But once they are in the air, they can control their direction and create an air foil to slow their falling. Humans can do this with the aid of a wingsuit:
Flying squirrels: A little bit like bats, flying squirrels have a membrane of skin that stretches between their front and rear legs. This allows them to glide over longer distances than they would otherwise be able to jump.
Flying lizards: Although the word “dinosaur” literally means “terrible lizard,” lizards and dinosaurs are completely different types of reptile. Flying lizards in the genus “Draco” are not very closely related to the flying dinosaurs. The flying lizards are very unusual because they do not use any of their four limbs for flying. Instead, they are able to spread out their ribs to form fairly immobile wings which allow them to glide for short distances.
Flying fish: Flying fish are much better at flying than you would expect. They use their tail to get out of the water and get speed. Once they are in the air they can glide for fairly long distances. If they want to increase their speed, they can put their tail back into the water and give themselves another push. This makes them the only glider that I know of that can add energy to their glide without landing.
Flying frogs: Like bats, flying frogs create “wings” by stretching skin between long fingers. Unlike bats, the “wings” of the flying frogs are limited to their feet, and do not include any skin on the arms or legs.
Flying snakes: To people who are afraid of snakes, nothing sounds more horrifying than snakes that can fly. But don’t worry — the flying snakes are the worst flyers of the group. They are able to flatten out their bodies to create a very minimal air foil. Their “flight” looks a lot like jumping or falling, but research has shown that they are able to steer themselves in the air. It may not seem like I should have included these in a list of things that evolved to fly, but remember that everything that evolved to fly had to go through many stages of flying ability. In the first stages, the animals would have just been jumping. In later stages, they would have a rudimentary ability to glide and navigate. For this reason, I firmly consider these snakes to be an example of incipient flight.
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