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The Evolution of Birds

Birds. They are present everywhere. I am sure that if you look up at the sky, or outside your window, you will most certainly see one of these avian creatures. But have you ever wondered where they came from? Yes, you might know that modern humans evolved from apes or that dogs are the descendant of wolves. But how did birds evolve powered flight? That is what we will find out.

First of all, what is a bird? They fall in the class Aves and their notable characteristics are the presence of feathers on their body, that their young hatch from eggs and that most of them can fly. There are over 9,000 species of birds throughout the world, inhabiting all 7 continents and seas. This begs the question, however, where did they all come from?

Archaeopteryx lithographica. Never heard of it? Maybe you might have heard of Tyrannosaurus Rex or the Velociraptor; its movie-star relatives. This metazoic bird is the earliest known fossil and it provided evidence that supported Darwin’s theory, a few years after the publishing of the Origin of Species. Complete skeletons depict it as being quite different from their modern counterparts: they possessed a long tail, no beak, numerous small teeth and

clawed hands. However, few specimens revealed that it had a brilliant plumage of feathers. Although it was initially used for thermoregulation and mating, mutations likely changed its shape to become more aerodynamic. It is possible that the Archaeopteryx might have been glissant, as it had wings that were large enough for short bursts of flight. Subsequent discoveries of new fossils such as Deinonychus antirrhopus and Microraptor zhaoianus had shocking similarities with the bird. While the Archaeopteryx had weak feather rachides, the Microraptor had long feathers and with an additional set of vaned feathers, it would have glided down in a ‘bi-plane-like’ manner.


A diagram of the upstroke and downstroke of a bird and the musculoskeletal configuration that is necessary.

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These discoveries eventually lead to the hypothesis that birds and dinosaurs are closely related. Further analysis of the wings and feathers revealed that it had limited aerodynamic capability, which could be the basis for the current-day powered flight.

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A fossil of the Archaeopteryx Litographica

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So how the modern birds fly? When flapping their wings, birds use a system of downstrokes and upstrokes to propel themselves. The main muscles that they use are the pectorals, the human chest muscles. In humans, their function is to pull objects toward the front of the body, however, on birds while the pectoralis major has a similar function, the pectoralis minor is used to abduct the wings towards the back of the body. This is due to the tendon supracoracoideus attaching unusually to the sternum and clavicle, which allows for the range of motion. While this musculoskeletal configuration wasn’t present in the archaeopteryx neither is there evidence yet in any other mezazoic fossils, it is crucial for flapping flight. It should be noted that flapping is not the only form of flight-locomotion that modern birds utilise.

There is a lot of debate about how the locomotion of powered flight came to be, it likely originated from the primitive wings of the Mezazoic birds. Their initial use might have been solely for mating purposes; fending of rivals and attracting potential mates but with mutations gradually becoming more aerodynamic and powerful. Currently, there are 3 main ideas. The first is that metazoic birds would glide down from trees to escape from predators. The second is that they would flap their wings in an attempt to make them run faster, with overtime the efficiency increasing until it could provide enough lift. The third is that they would use it to assist with climbing trees. Research with bird chicks has shown that they are able to ascend steep inclines. The chicks do not enjoy being on the ground, so researchers

would put logs at different inclines so that the chicks would be able to get to the ‘high ground’. The chicks were able to climb up an incline as steep as 85º, the use of slow-motion cameras gave the researchers insight into how they achieved this feat: they would flap their wings, with the same up-stroke and down-stroke that birds use to ascend. This led to the 3rd possibility, however, the anatomy of birds such as the Confuciusornis had no adaptations to climb, so a likely outcome is that a co-evolution of these different forms occurred, with the anatomy of each species contributing to the evolution of flight.

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A render of the archaeopteryx litogriphica

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