Why Darwin Matters_ The Case Against Intelligent Design - Michael Shermer [34]
Although an organ may not have been originally formed for some special purpose, if it now serves for this end we are justified in saying that it is specially contrived for it. On the same principle, if a man were to make a machine for some special purpose, but were to use old wheels, springs, and pulleys, only slightly altered, the whole machine, with all its parts, might be said to be specially contrived for that purpose. Thus throughout nature almost every part of each living being has probably served, in a slightly modified condition, for diverse purposes, and has acted in the living machinery of many ancient and distinct specific forms.30
This solution is called exaptation, in which a feature that originally evolved for one purpose is coopted for a different purpose.31 The incipient stages in wing evolution had uses other than for aerodynamic flight—half wings were not poorly developed wings, they were well-developed something elses—perhaps thermoregulating devices. The first feathers in the fossil record, for example, are hairlike and resemble the insulating down of modern bird chicks.32 Since modern birds probably descended from bipedal therapod dinosaurs, wings with feathers could have been employed for regulating heat—holding them close to the body would retain heat, stretching them out would release heat.33 In the Galápagos Islands I have seen flightless cormorants returning to shore after diving for sea food, upon which they stretch out their stubby little wings with desultory feathers to dry them out and collect heat from the sun. In this case, wings evolved from flight tools to thermoregulation devices. In evolution, structures can be adapted for one function and evolve into use for another function and may have multiple functions at one time.
Another function for wings and feathers recently discovered is as an aid to running—some modern birds flap their wings to gain traction when running up steep inclines, even enabling them to climb straight up a 90-degree vertical structure.34 Yet another function for incipient wings on bipedal dinosaurs was grasping. The most famous transitional fossil in evolutionary history, Archaeopteryx, has wings whose surface area is large enough to support its body in flight, asymmetrical feathers capable of attaining lift, and a shoulder that allows enough flexibility for an adequate upstroke of the wing necessary for flight. Nevertheless, Archaeopteryx retains many dinosaur features, including a functional grasping hand, for which the “wing” was probably originally adapted, and only later exapted for flight.35
Similar reasoning explains the incipient stages in the evolution of eyes, the flagellum motor, and the other structures claimed by Intelligent Design advocates to be inexplicable through evolutionary theory. For the human eye, it is not true that it is irreducibly complex, where the removal of any part results in blindness. Any form of light detection is better than none—lots of people are visually impaired by a variety of different diseases and injuries, yet they are able to utilize their restricted visual capacity to some degree, and would certainly prefer this to blindness. And natural selection did not create the human eye out of a warehouse of used parts lying around with nothing to do, any more than Boeing created the 777 without the ten million halting jerks and starts beginning with the Wright brothers.
As for the bacterial flagellum, although it is a remarkable structure, it comes in many varieties of complexity and functionality. Bacteria in general may be subdivided into eubacteria and archae-bacteria; the former are