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Speculation about the evolution of birds began in the eighteenth century when de Maillet claimed that they had arisen from flying fish (Stahl, 174). The earliest known member of Aves is Archaeopteryx, first found in the Late Jurassic rocks of Bavaria, and it is now known that birds are the second most direct (surpassed only by the more obvious reptiles) descendants of the dinosaurs but as yet there has been no universally accepted theory of their exact ancestry. Currently two major hypotheses on the evolution of birds still stand in conflict, with one other slightly more obscure theory still hanging on by a thread.
To start with the least well accepted hypothesis, in 17 A.D. Walker claimed that there were close relations between extant birds and extinct crocodylomorphs (ie Sphenosuchus) based mainly on his detailed studies of the morphology of the braincase and ear region of both. Other proposed evidence included similarities in features of the tarsus, skull and teeth. However, it has since been found that the majority of synamorphies hypothesised between birds and crocodylomorphs are general to archosaurs or other larger taxonomic. Thus levels so they can not be seen as diagnostic of the phylogeny of either taxa and this theory has all but lost its support (Padian & Chiappe, 18).
Of the two major contenders, the theropod hypothesis seems to have the most evidence on its side. First proposed by Huxley, this links birds to a group of small carnivorous bipedal dinosaurs (the theropods) which display striking synamorphies with both Archaeopteryx and extant birds. These include features of the skull, pectoral girdle and hind limb, a longer ischium, a postoventrally directed pubis and many others (Padian & Chiappe, 18). In 16 Heilmann rejected a theropod ancestry for birds based on the lack of clavicles displayed by all theropods which had at that time been found. However, this objection was destroyed a decade later when clavicles were reported in an Early Jurassic theropod dinosaur, Segiasaurus. They have now also been reported in a plethora of other species. The major stumbling block of this theory stems from the apparent absence of theropod dinosaurs from the fossil records before 110 million years ago. As the first Arcaeyopteryx is 150 million years old, Feduccia (16) and others claim that they cannot possibly be the ancestors of the birds. However, Padian and Chiappe (18) counter what they refer to as "The false argument of stratigrapic disjunction" with the example of monotremes which have for a long time been universally accepted as the ancestors of marsupials and placentals but were not until recently known in the fossil records until 80 million years after the first appearance of their descendants. Padian and Chiappe merely take this as evidence for the imperfection of the fossil record.
Mainly because of the momentous gap in the fossil record Feduccia (16) and a number of other scientists still favour thecodonts theory of the ancestry of birds. The thecodonts comprise archosaurian reptiles that are not dinosaurs, crocodiles or pterosaurs, and have no diagnostic characters of there own and are claimed by this group of scientists to be the most likely ancestors of Archaeopteryx. Further evidence to support this claim was found in the form of striking "affinities" between Archaeopteryx and certain earlier thecodonts. However, Padian and Chiappe (18) dismiss this as an over pronounced reliance on features such as small teeth and triangular skulls which they claim can be seen in the juvenals of a large number of tetrapods.
Custom Essays on Discuss how birds evolved from their non-avian ancestors
Whilst the argument over the exact ancestors of the birds has not yet been resolved, it is still possible to speculate on the changes in general which must have taken place for birds to evolve. This is best done by looking at the specific features which are used to diagnose modern birds and which lead to the placing of Archaeopteryx as the earliest know true avian..
On the surface it seems that birds are characterised by 4 main features. Firstly they all have wings which at some point in their ancestry have been capable of sustaining flight. They also all have feathers, elongated skulls and horny beaks. All of these must have evolved due to selection pressure under which their reptilian ancestors lived and whilst they are also displayed separately in a number of non-avian species, (such as the feathered theropods of china (Xu et al. 001)) it is their presence in combination with each other that makes them diagnostic of the order Aves.
In support of the theropod hypothesis of bird development, there are a number of known fossils of distinctly non-avian theropod dinosaurs (ie definitely incapable of flight from the morphology of their wings) which show the presence of either modified scales, ie Sinosauropteryx which has a row of small fringed structures along its vertebral column (Padian & Chiappe, 18), branched integumental structures (protofeathers), ie Sinornithosaurus (Xu et al. 001) or true feathers, ie Protoarchaeopteryx (Quiang et al. 18). Phylogenetic analysis has indicated that these specimens are more primitive than Archaeopteryx and therefore suggests that feathers developed in theropods before a capability to fly gave them their avian distinction. This is fairly astounding as it was always assumed that feathers evolved as a consequence of flight rather than their presence aiding the evolution of the latter.
As feathers did not therefore evolve to allow better flight, there must have been alternative selection pressures which favoured the presence of feathers for another reason. There have been a number of varying theories put forward for the adaptive value of feathers, ranging from active thermoregulation through display to camouflage. If the thermoregulatory function of feathers is to be taken as the most likely then it has been suggested that of the three types of feathers (down, covert and flight) down and covert would have evolved first as they would have afforded the most insulation. In contrast, supporters of the display/camouflage theory believe that flight and covert feathers would have been the first to derive from the reptilian scales (Padian & Chiappe, 18).
Once the evolution of flight has been uncoupled from the development of feathers in birds (by the discovery of them non-flying, reptilian ancestors) then we are free to hypothesise on the adaptive significance of flight without having to worry about the apparent lack of some of the necessary structures at the beginning of flight history. The two proposed models of flight evolution in existence have enjoyed varying levels of favour within the scientific world over the years the arboreal model suggests a "trees down" gliding origin of flight whilst the cursorial model relies on a run-and-jump "ground up" beginning.
Having spent many years at the top of the pile, the arboreal theory is beginning to be toppled. This model suggests that powered flight originated through a growing use of wings whilst gliding from a higher level to a lower one. These high places have most frequently been assumed to be trees thus giving the model it's name. Although it has been argued that the claws of its pes and manus have a curvature typical of birds adapted to perching on branches (Feduccia, 1), this argument was countered by Ostrom (176) who suggests that Archaeopteryx has claws commensurate with ground dwelling birds. There is also evidence to suggest that trees of substantial size would have been absent from the habitats in which the fossilised Archaeopteryx has been found (ie de Buisonj, 185). This, accompanied by the other evidence, such as the lack of typical gliding morphology associated with the skeleton of Archaeopteryx, has lead to an increase in the number of proponents for the cursorial model which does not rely on the presence of trees or climbing ability in the ancestors (Padian & Chiappe, 18).
However, the cursorial model relies on the inference from fossils that the ancestors of the birds were strongly-bipedal, swift running and capable of attaining suitable speeds to allow for take off. Unfortunately, the maximum running speed of Archaeopteryx has been estimated at ms-1 and it's minimum flying speed at 6ms-1 and this apparent disparity in figures has caused an instability in the "ground up" theory. This, coupled with the high energy demands necessary to attain lift against gravity and the unsolved problem of the origin of the flight stroke have been used by proponents of the "tree down" theory to discredit the cursorial model. However, recent calculations including the wings of Archaeopteryx as primary thrust generators used to increase speed (Burgers & Chiappe, 1) have apparently provided a solution to the gap between the running and flight velocities and have given the cursorial model much greater credibility. Also, the discovery that the sister group of birds, Deinonychus, already had the side-ways flexing wrist essential for the development of the wing beat and the production of thrust has helped calm fears over problems associated with the evolution of the flight stroke (Padian & Chiappe, 18). It has also been suggested that this movement would have served Deinonychus as a prey-seizing stroke and therefore could well have been used by the ancestors of both in the pursuit of prey and have developed into a thrust generating mechanism from there.
Whether it developed via the cursorial or arboreal model, or even by a combination of the two, flight is an exceptionally energetically expensive pastime and the ancestors of modern birds must have been under phenomenal selection pressures to make it a favourable option. Theories on causes of flight origin tend to cast the ancestors of birds in either a predatorial or a prey role. As the animals were carnivores it has been suggested (as mentioned before) that the wing beat evolved as a means to increase their speed when in the pursuit of terrestrial prey. However, it has also been hypothesised that forelimb flapping may well have served to increase the moment of suspension when jumping to catch insects which were also a food source. Alternatively, these creatures were small compared to the top predators of the time and the extra thrust generated by a wing beat, and even the ability to propel themselves through the air, may well have increased their chances of avoiding predation.
The energetic demands of flight may well have been instrumental in the evolution of the distinct avian respiratory system. This system ensures via a system of air sacs that with every muscle movement involved in breathing, fresh a supply of oxygen is passed across the respiratory surface, - unlike in other vertebrates where fresh air is only present on every other muscle movement. This would therefore allow a greater efficiency in the diffusion of oxygen into the blood meaning that more was available to be used in respiration within muscles such as those controlling the wings during flight.
Other skeletal characteristics that must have evolved in the development of the birds include major alterations to the digits to produce wings and the skull. In the past it seems to have been argued at some time or another that the wing of a bird comprises of almost every combination of three digits conceivable. However, irrelevant of digit numbering (apparent discrepancies in which have been used to attack the theropod ancestor theory (Hecht, 14) but in turn has been rebuffed by the claim that a change in digit identity would only necessitate a single simple mutation and is therefore not strong enough evidence to discredit the theropod theory (Galis, 001)) it is obvious that a reduction in digit number and morphology has occurred to produce the wings exhibited by Archaeopteryx. From the non-avian ancestors, it has been suggested that a complete loss of digits IV and V and a significant reduction in digits I and III would have lead to the production of the wing seen in Archaeopteryx (Padian & Chiappe, 18).
The development of the beak and loss of teeth would have opened up new food sources to the proto birds whilst also greatly changing the aerodynamics of the creature. By losing the teeth and replacing it with a gizzard containing stones (as in the case many species of extant birds) the weight distribution would change from more at the front to a greater mass in the region of the sternum. This may well have acted as a type of ballast by stabilizing the bird during flight and thus giving it an adaptive favourability through which natural selection could act.
As can be seen from the evidence laid out in this essay, the evolution of birds is a dynamic subject with many contrasting and conflicting theories. However, irrelevant of the exact starting point, the evolution of birds was characterised by the development of the essentially avian features of feathers, wings, flight and beaks in combination. Since the time of Archyopteryx the order Aves has undergone many further modifications at the hands of evolution to produce the birds of modern day. For instance, drastic reduction of the number of tail vertebrae and the development of a pygostyle have afford birds greater control of the flight feathers in their tails which in turn has allowed them to attain exploitation of a wide range of lift forces during flight (Gatesy & Dial, 16). This means that the birds of today are certainly more graceful and capable flyers than Archaeopteryx but irrelevant of its ability, it is still the earliest known creature almost definitely capable of a significant degree of powered flight.
Burgers, P and LM Chiappe, 1, "The wing of Archaeopteryx as a primary thrust generator.", Nature, 60-6
Feduccia A, 1, "Evidence from claw geometry indicating arboreal habitats of Archyopteryx.", Science, 570-7, New Haven.
Feduccia A, 16, "The origin and evolution of birds", Yale University Press, New Haven.
Galis, F, 001, "Digit identity and digit numbering indirect support for the descent of birds from theropod dinosaurs", Trends in Ecology and Evolution, 1616
Gatesy, SM and KP Dial, 16, "From frond to fan Archyopteryx and the Evolution of short-tailed birds", Evolution, 5007-048
Padian, K and LM Chiappe, 18,"The Origin and early evolution of birds", Biological Review, 71-4
Quiang, J, PJ Currie, MA Norell and J Shu-An, 18, "Two feathered dinosaurs from northeasetern China", Nature, 75-761
Stahl, BJ, 174 "Vertebrate History", McGrawHill
Xu X, HH Zhou and RO Prum, 001, "Branched integumental structures in Sinornithosaurus and the origin of feathers.", Nature, 41000-04
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