Science Friction_ Where the Known Meets the Unknown - Michael Shermer [107]
Evolutionary biologist Jerry Coyne, in a review of Behe’s book in Nature (September 1996), explained that biochemical pathways such as those claimed by Behe to be impossible to explain without an intelligent designer, “did not evolve by the sequential addition of steps to pathways that became functional only at the end,” as Behe argues. “Instead, they have been rigged up with pieces co-opted from other pathways, duplicated genes, and early multi-functional enzymes.” Behe, for example, claims that the blood-clotting process could not have come about through gradual evolution. Coyne shows that, in fact, thrombin “is one of the key proteins in blood clotting, but also acts in cell division, and is related to the digestive enzyme trypsin.”
This is the same answer given to the nineteenth-century antievolution argument that wings could not have evolved gradually—of what use is half a wing? The answer is that the incipient stages in wing development had uses other than for aerodynamic flight; in other words, half wings were not poorly developed wings, they were well-developed something elses. Likewise with the incipient stages in the evolution of blood clotting, the flagellum motor, and the other structures claimed by IDers to be inexplicable through evolutionary theory. The principle can be illustrated simply in figure 11.1.
As 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 with any number of different diseases and injuries to the eyes, yet they are able to utilize their restricted visual capacity to some degree and would certainly prefer this to blindness. No one asks for partial vision, but if that is what you get, then like all life-forms throughout natural history you learn to cope in order to survive.
There is a deeper answer to the example of the evolution of the eye, and that is that 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 747 without the ten million halting jerks and starts beginning with the Wright Brothers. Natural selection simply does not work that way. The human eye is the result of a long and complex pathway that goes back hundreds of millions of years: a simple eyespot where a handful of light-sensitive cells provides information to the organism about an important source of the light—the sun; a recessed eyespot where a small surface indentation filled with light-sensitive cells provides additional data in the form of direction; a deep-recession eyespot where additional cells at greater depth provide more accurate information about the environment; a pinhole camera eye that is actually able to focus an image on the back of a deeply recessed layer of light-sensitive cells; a pinhole lens eye that is actually able to focus the image; a complex eye found in such modern mammals as humans.
Figure 11.1. Co-opting nature. At each stage of an evolutionary sequence a particular structure, or series of structures, such as the eye, may serve one function, only to be co-opted later for some other use. The end product may appear to be designed for that final function, but it was not because evolution does not look ahead to the future.
We should also note that the world is not always so intelligently designed, and the human eye is a prime example. The configuration of the retina is in three layers, with the light-sensitive rods and cones at the bottom, facing away from the light, and underneath a layer of bipolar, horizontal, and amacrine cells, themselves underneath a layer of ganglion cells that help carry the transduced light signal from the eye to the brain in the form of neural impulses. And this entire structure sits beneath a layer of blood vessels. (See figure 11.2.) For optimal vision, why would an intelligent designer have built an eye backward