The original question was: What does it take to convince you guys? Surely the latest find of a feathered dinosaur Kulindadromeus zabaikalicus, is enough to convince any intelligent researcher that dinosaurs evolved into birds? What’s stopping you? Is it just your religious bigotry?”
Answer by Andrew MacHutchon
To tackle this question we must first ask: Why can birds fly and other creatures cannot? The answer is found in the optimum structure and function of all systems in the bird, not just feathers.
If we theoretically start with a dinosaur’s supposed cousins i.e. a lizard or crocodile, then add wings and feathers, we may produce an animal with the capacity for short range gliding flight, but such a creature would not be able to take off and actively fly, because the structure and functions needed for flight are simply not there.
We all know an ‘intelligently designed’ aeroplane can take off, as well as fly, so what’s crucial to its flight? The key elements of aeroplane flight turn out to be good design, aerodynamics and thermodynamics, balanced against the effects of gravity, where all systems are functionally acting and interacting at the same time
Likewise, successful high performance, high altitude, and long range flight in birds has been discovered to depend on multiple specialised and interactive design feature, where all necessary features must be present at the same time. Good power to mass ratio, good musculoskeletal mechanics, feathers, sophisticated physiological and biochemical systems to deliver energy. At the head of these requirements is a neurological system that enables hunting for food, navigation, migration, social interactions and breeding.
Efficient delivery of Oxygen is essential for sustained high performance function. Consider the Pathway for Oxygen. Conversion of food into energy requires delivery of oxygen all the way from the mouth to the mitochondria in the cells. That pathway comprises trachea and other air passages, lungs, appropriate heart structure, blood circulation, gas transfer to peripheral organs, and correct biochemical arrangements in the cells to use both oxygen and food for energy. There must likewise be a suitable pathway for removal of carbon dioxide and other waste materials.
The anatomy and physiology of bird respiratory systems have some common features shared by non-flying creatures. The unique aspect is how these are combined to facilitate the level of performance that makes sustained flight possible.
Non-flying creatures with a trachea, bronchial passages and lungs, mostly have alveoli (thin walled air sacs) for gas transfer between air and blood. Air moves in and out of alveoli in a tidal movement, following the same path in and out. The result is that inspired Oxygen meets expired CO2, the latter diluting the former. This works well for humans living at sea level, sustaining long life, but is not satisfactory for high altitude.
Flying birds have radically different respiratory (breathing) anatomy and physiology.
Respiratory System
Movement of air in an out of the bird lung is by a mechanism that differs substantially from non-flying animals. In humans, the work of breathing is shared by three groups of muscles. Diaphragm 45%, Intercostal and Abdominal wall 45%, Accessory (neck muscles) 10%.
Birds have highly articulated ribs, a moveable sternum, and no diaphragm. The work of breathing is done by intercostal and abdominal wall muscles.
Air takes a different path between the beak and the lungs so that inspired Oxygen does not conflict with expired CO2. The most fascinating and unique part of bird respiratory function is that they do not have alveoli, but instead have a linear micro-tubular counter-current system (parabronchi) for gas transfer. This means air flows through fine tubes close to thin walled blood vessels. The air and the blood flow in opposite directions, which enables rapid, efficient flow of oxygen into the blood, and carbon dioxide from the blood into the air. This is similar to gas exchange units that are used in heart lung bypass machines (CPB) for heart surgery, or Extra Corporeal Membrane Oxygenator (ECMO) that are occasionally used for life support in patients with acute lung injury. The mechanism of breathing in birds is different from land animals.
Are there any examples of parabronchi in non-flying animals? Yes, Alligators have parabronchial lung structure (C. G. Farmer, et al. “Unidirectional Airflow in the Lungs of Alligators”, Science 327, 338 (2010)), so there is some evidence that alligators have also a continuous flow air breathing system. However the big difference between reptile breathing and bird breathing is that birds have no diaphragm. They use the hinged rib structure and push its sternum bone in and out as described above.
Heart and Circulation
But it gets worse… not only is there the fundamental difference that birds have no diaphragms, reptiles do not have the correct type of heart to sustain the intense activity of flight.
Consider now the circulation. Most non-flying reptiles have a three-chambered heart. There are no examples of successful flight with a three chambered heart, because with that type of circulation, the blood delivered to the lower body is not fully oxygenated. Connecting a parabronhial lung to a three chambered heart, whilst the combination may offer some theoretical advantage, is not compatible with flight due to outflow of poorly oxygenated blood from the heart.
Alligators (and all crocodilians) have a four chambered heart, but they have shunting of blood between the main arteries from each side of the heart. This system allows them to vary the oxygen mix in their blood as their metabolic rate changes, and enables them to use blood circulation for heat distribution. This system is quite different from a bird (or mammal) circulatory system. Birds have a four chambered heart, with no shunting. This means they always maintain high oxygen blood in their general circulation, which is essential for maintaining a constantly high metabolic rate. (See News Physiol Sci 17: 241_245, 2002; doi:10.1152/nips.01397.2002 for more details)
Flying birds all have a four chambered heart, like humans. Blood passes through the lungs, then left atrium, left ventricle, to the aorta and systemic circulation. Blood returning from the body organs passes through the vena cava, right atrium, right ventricle, to the pulmonary artery, thence to the lungs.
Whereas the land reptile heart with three chambers could be considered an intermediate between fish (two chambers) and mammalian heart (four chambers), there are no known intermediate forms between the two types of lung. Intermediate or transitional forms would likely be incompatible with life.
Structure and Function of other parts of bird anatomy.
Consider the bat. Bats are mammals. They have an alveolar lung but with morphological differences giving them much higher gas exchange capacity per mass of lung tissue than is found in other mammals. And unlike birds they have a muscular diaphragm like humans. (J.N. Maina and A.S. King, “Correlations between structure and function in the design of the bat lung: a morphometric study”, Jnl. Exp. Biol. 111, 43-61, 1984, PDF here)
One of the major differences in bird morphology to reptiles is that they have what is called the acoracoid process. This involves an additional powerful muscle, one end of which is attached to the humerus bone (supporting the wing) and such that the ligament is threaded round the coracoid (the shoulder bone), and the other end of which is attached to the sternum close to where the pectoralis major muscle is also attached. Flapping flight is thus performed by these two muscles operating in tandem. The downstroke is obtained when the pectoralis major muscle is shortened and the ligament the pulls down on the underside of the humerus bone. The upstroke is obtained when the supracoracoideus muscle is activated and pulls on the ligament which goes round the coracoid bone rather like a pulley mechanism. This is unique to flapping bird flight. And is dealt with in detail in the paper by the dino to bird sceptic Ruben – see Quick, D.E. and Ruben, J.A.C. Cardio-pulmonary anatomy in theropod dinosaurs: implications from extant archosaurs, Journal of Morphology, doi: 10.1002/jmor.10752, 2009 .
In summary the supracoracoideus is a special muscle of flight that is unique to birds. It is responsible for powering the upstroke and pitch control of the wing. Bats lack such a supracoracoideus muscle so cannot take off from ground level. They can only initiate flight from a perch above ground.
Bats have folds of skin as their load bearing aerodynamic surfaces and do not have feathers. While bats are successful in their normal limited environment, they are not known for long distance high altitude migratory flight.
Conclusion
Birds with the optimum respiratory system circulatory system, especially lightened hollow bone skeleton, a full set of flight muscles including the supracoracoideus, and a covering of feathers, are capable of amazing aerial manoeuvres, short take off, tight turns, hover, migration, and safe landing. Man has only been able to imitate this performance with huge expenditure of heat and noise. Reptiles have a different morphology, breathing and heart systems, and are not the precursors to birds.
Overall, the interlinked structure and function of all parts of bird anatomy provide compelling evidence against the dinosaur-to-bird theory, and are marvellously consistent with Genesis: God deliberately and intelligently created the flying things as fully formed functional creatures, and called them good! Religious bigotry has nothing to do with it.
Related Question
Don’t feathered dinosaurs prove that birds evolved from dinosaurs? Answer here.
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