INTRODUCTION The evolution of flight in birds is recognized as the key adaptive breakthrough that contributed to the biological success of the group. The evolution of flight in birds has even contributed to the advancement of the human race; humans were inspired to try and conquer the air themselves after observing these animals. There are two main competing theories for the evolution of flight in birds: the Arboreal Hypothesis and the Cursorial Hypothesis. This essay will argue for the Arboreal Hypothesis, or the tree-to-ground model, and show how the Cursorial Hypothesis, or the ground-up model, is not accurate. ARGUMENTS FOR THE ARBOREAL HYPOTHESIS The Arboreal Hypothesis suggests that ancestors of modern birds were non-dinosaurian tree-dwellers who leapt from branch to branch. There ability to glide and eventually fly developed to slow their fall to the ground when they missed a branch (Figure 1).
This evolutionary process has four stages: climbing, parachuting, gliding, and flapping (Gauthier and Gall, 2001).
The first step is crucial because once a proavian has climbed a tree, there is no problem getting into the air from the arboreal nest (Figure 2).
The best evidence that climbing creatures can learn the power to fly is provided in bats (Paul, 2002).
It can be seen today that bats fly, but can also climb (Figure 3).
The fourth stage of flapping is also important because flapping of the wings allowed proavians to apply increasing amounts of power in order to lengthen their glides (Paul, 2002).
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These different steps allowed flight muscles to become powerful overtime.
There are no awkward “almost flying” stages along the hypothesized evolutionary transition, which makes this hypothesis even more accurate (Shipman, 1998).
The central theme of the Arboreal Hypothesis is the use of gravity as a source of power by proavians to aid in the flight process (Chatterjee, 1997).
These animals took advantage of the cheap energy provided by high places in the form of gravity (Feduccia, 1999).
It is argued that the ancestors of birds must have been arboreal because an assist from gravity was needed to make takeoff possible in the early stages of the evolution of flight (Shipman, 1998).
It can also be argued that it costs less for a proavian to climb a tree and then glide or flap its wings than to run (Chatterjee, 1997) (Figure 4).
ARGUMENTS AGAINST THE CURSORIAL HYPOTHESIS The Cursorial Hypothesis suggests a ground-up model where birds evolved from bipedal ground-dwelling dinosaurs whose front limb scales gradually developed into feathers, giving them an upward thrust when they ran and eventually enabled them to fly.
This hypothesis has several weak points that damage its credibility. The first attack is against the idea that wings would help increase the speed of an animal as it ran along the ground. The use of wings to increase running speed has no living descendants. Outstretched wings would increase drag and as soon as the animal left the ground, its legs could no longer propel it forward causing it to immediately fall back down (Chatterjee, 1997).
Also, taking off from the ground requires more energy. In fact, it requires four times as much power as does ordinary flight (Chatterjee, 1997).
As soon as proavians jumped into the air from the ground they would loose airspeed and altitude and fall back. There was no lift or thrust to keep them aloft, so this hypothesis is bio mechanically impossible (Chatterjee, 1997).
In taking off from the ground, the animal is working against gravity, not with it (Gauthier and Gall, 2001).
It makes more sense for an animal to work with something than against it.
Gravity would just cause additional stress on proavian takeoff. The Cursorial Hypothesis also does not have smooth stages in the evolution of flight. Because it does not have the gliding stage, flight evolved rapidly, which is unlikely (Chatterjee, 1997).
Birds are animals with very special features such as feathers, special lungs, and light body structures. Evolutionists believe that birds evolved from reptiles or maybe even a type of dinosaur. Archaeopteryx is thought to be the first flying bird. Archaeopteryx has feathers like a bird but the skeleton of a small dinosaur. Archaeopteryx lived 150 million years ago and had many features of a ...
In this model, landing was perfected first followed by improvements in flight. In the fossil record of Mesozoic birds, we see an opposite trend: flight evolved first followed by the development of landing (Chatterjee, 1997).
Even fossils disagree with this theory.
It can easily be argued that the evolution of flight from the ground up is “bio physically impossible” (Feduccia, 1999).
REFERENCES CITED Chatterjee, S. 1997. The Rise of Birds.
Johns Hopkins University Press, Baltimore. Feduccia, A. 1999. The Origin and Evolution of Birds. Yale University Press, New Haven. Gauthier, J.
A. , and L. F. Gall (eds. ).
New Perspectives on the Origin and Early Evolution of Birds. Yale University Press, New Haven. Paul, G. S. 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds.
John Hopkins University Press, Baltimore. Shipman, P. 1998. Taking Wing: Archaeopteryx and the Evolution of Bird Flight. Simon & Schuster, New York. web am samoa / bats .