Why are there only these three types of vertebrates that have conquered the sky?

Review expert: Ran Hao, a well-known popular science writer

Soaring in the blue sky has been a long-standing dream of mankind. As early as 500 BC, people tied themselves to kites and tried to overlook the earth from the sky.

In 1783, humans achieved manned flight in a lightweight aircraft. In 1902, humans achieved manned flight in a heavy aircraft that could provide its own energy. It took humans more than 2,000 years from the earliest idea of ​​flying to the realization of this dream.

Painting of the Montgolfier brothers' experiment with manned hot air balloons in 1783 Source: Wikipedia

In fact, in the quadrupedal animal kingdom, there were already species that saw the scenery on the ground from the air before humans. The first pterosaurs that left the ground and looked down at the world from the sky appeared on the earth more than 200 million years ago. As species continued to evolve, birds and bats also began to fly in the sky, and compared to humans, they did not need any tools.

So, how do pterosaurs, birds and bats fly?

The Secret of Flying to the Blue Sky

Whether it is pterosaurs, birds or bats, they all have wings, and the structure of wings is the basis for their ability to fly.

According to Bernoulli's principle, in a low-speed fluid, the pressure is high where the flow rate is low, and low where the flow rate is high. When pterosaurs, birds, and bats take off, the air (i.e., fluid) is cut into two parts by the front of the wings as soon as it touches the wings. Because the wings have a certain curvature, the distances traveled by the upper and lower parts of the air are not the same when passing through the wings.

Source: en.wikiversity.org

The distance air passes over the wings is longer, and the distance air passes under the wings is shorter, but the time spent passing through is equal. Therefore, the air flows faster above the wings and slower under the wings. This causes the pressure above the wings to be lower than the pressure under the wings, thus generating an upward force. It is precisely relying on the lift generated that pterosaurs, birds and bats are able to leave the ground and fly into the sky.

The basic principles of flight of pterosaurs, birds, and bats have been explained above, but in actual flight, there are some differences in the flying methods of the three.

Source｜Interactive Science

The wings of pterosaurs (usually called wing membranes) are supported by only one extended finger. This structure determines that pterosaurs cannot fly at high speeds like birds.

An imaginary picture of Quetzalcoatlus moving on the ground Source: Wikipedia Some pterosaurs were huge and relied heavily on air currents when flying, which made it easier for them to leave the ground. Through simulation experiments, scientists have speculated that pterosaurs did not fly very fast and their flying style was relatively simple.

Birds have many ways of flying, such as flapping, gliding, hovering, etc., which are related to factors such as the shape and size of the bird's wings and muscle strength. Generally speaking, there are four types of bird wings: elliptical, high-speed, high-aspect ratio, and high-lift with wing slots.

The oval wings are short and round, which are suitable for quick take-off and turning. Birds with such wings can move freely in the forest. Woodpeckers and magpies belong to this type.

Magpie source: Wikipedia

The high-speed type has short and pointed wings , which can help birds reach higher flight speeds and hunt in the air. Hummingbirds and swallows are representatives of this type.

Hummingbird Source: Wikipedia

The wings of the large aspect ratio type are long, narrow and flat, and can make better use of the difference in wind speed to carry the bird gliding in the sky and flying over the endless ocean. The wings of albatrosses and other seabirds are mostly of this type.

Albatross source丨unsplash

High-lift wings with slits can greatly reduce air resistance and use thermal air currents to allow birds to fly slowly over land. They are often seen in birds of prey such as vultures and owls.

Vulture source: pixabay
The wings of bats are similar to those of pterosaurs, but there are four fingers supporting the wings of bats, which makes the wings more stable, so bats have better control over flight than pterosaurs.

However, due to underdeveloped hind limbs and degraded jumping ability, it is difficult for bats to take off from the ground like birds. Therefore, the way bats take off is quite special - they take off in an upside-down position.

Bat source: Wikipedia

Are all those who can fly relatives?

Among vertebrates, only pterosaurs, birds, and bats have truly broken through the ground space and flown into the vast sky. According to biological classification, pterosaurs belong to reptiles, bats belong to mammals, and birds evolved from dinosaurs and are on par with these two categories. These three are distantly related, and all evolved wings independently. So why do they have similar structures and can all fly?

Biologists call this phenomenon convergent evolution , which may be caused by their ancestors living in the same or similar environment for a long time.

In order to adapt to the environment, organisms will evolve specific structures and functions, and similar environments make them evolve in similar directions, so they have similar wings and the ability to fly. Similar cases include the Mesozoic marine reptile ichthyosaurs and modern dolphins, and the similar-shaped digging forelimbs (front feet) of moles and mole crickets.

Ichthyosaur fossils (above) and dolphins (below) Source: Wikipedia Although humans have not evolved wings and cannot achieve autonomous flight, fortunately, humans can use their wisdom to study the body structure of flying animals, explore the secrets of their flight, and use the principles contained therein to design and manufacture various mechanical devices to realize the long-cherished dream of flying.

Animals initially took off for the purpose of survival , either to avoid predators or to catch prey; while humans took off for development, change, and breakthroughs. Regardless of their original intentions, in the process of ascending to the sky, whether it was pterosaurs, birds, bats, or humans, they have all made a significant mark in the history of the Earth.