Kitchen Counter Comparative Anatomy

The experiments used to investigate the avian flight system involve flying birds in wind tunnels, surgically altering bones, cutting ligaments, and de-innervating muscles, which prevents their function. But you can perform similar experiments yourself in your own kitchen with a sharp knife and a raw chicken wing. (The experiment works better with an entire raw bird because sometimes the wings in packages are broken.)

Any wing is divided into three segments of about the same length folded into a Z. The segment with the pointed end is the wing tip. The bone in it is the fused hand and finger bones of the bird, called the carpometacarpus. If you look closely, you can see that it has two parts of different lengths. The longer part is the fused second and third fingers. The primary feathers attach to these fingers; the hand and its feathers provide the principal lift and propulsive force of the wing (Rayner 1988, Norberg 1990). The shorter part is the first finger, or thumb. This digit carries a feather called the alula, which helps to control the aerodynamics during low-speed flying, much as do the front flaps of an airplane (Norberg 1990).

If you continue up the arm from the hand, the next segment is the forearm, composed of the radius and the ulna. The radius is the top bone on the same side of the arm as the thumb; it is thin and does not provide the principal mechanical strength of the forearm. The larger bone of the forearm is the ulna, to which the secondary wing feathers attach. The third and most robust segment is the humerus. The principal flight muscles, which raise and lower the arm as well as extend and retract the forearm, attach to this segment. (This is why it has the most meat.)

Those are the principal segments of the wing; to see how they work together with the muscles, take the wing, hold it at both ends, and stretch it out. The big triangular flap of skin stretched between the humerus and forearm is called the propatagium. It serves as the leading edge of the wing and contains the propatagial ligaments. These ligaments help to keep the leading edge taut during flight and aid in the automatic extension of the hand as described previously (Shufeldt 1898, Vazquez 1993). If you hold the arm by the middle segment and pull the humerus away from it, you will notice that the hand automatically extends as well. Push back on the humerus, and the hand flexes back against the forearm. You have just witnessed the automatic, coordinated flexion and extension of the hand with the forearm. This motion is important for the bird's wing during flight.

Now you are ready to begin experimenting. Start by cutting the propatagium and the patagial ligaments. Does the wing still automatically open and fold? If you have done it right, it will. Therefore, the patagial ligaments are not necessary for automatic flexion and extension.

To continue, next skin the wing to reveal the muscles and ligaments beneath. After removing the skin, you can start to investigate the muscles that flex the wing. The large muscle on the front of the humerus is the M. biceps. This muscle flexes the elbow, triggering the kinematic chain of automatic wrist flexion. The fleshy muscle that runs down the back of the humerus and attaches to the ulna's olecranon process, the bony extension of the proximal end of the ulna that is closest to the humerus, is the M. triceps. This muscle extends the elbow. Pull on this muscle, and watch the forearm and hand straighten; pull on the biceps, and watch them flex. These are the muscles that the bird uses to flex and extend its arm during flight.

Now look at the muscles of the forearm, the large muscle on the top of the radius and ulna. This is the M. extensor metacarpi radialis, which originates from the distal end of the humerus, near the biceps muscle, follows along the radius, and inserts onto the extensor process of the first metacarpal. If you cut this muscle, you will find that the hand will no longer extend with the elbow, but it will still automatically flex with the elbow. Next, examine the flexor muscles. The M. extensor metacarpi ulnaris is the large muscle that originates on the outer side of the distal end of the humerus, proceeds along the ulna, and inserts onto the back of the hand. The M. flexor carpi ulnaris is the large muscle that originates on the inner side of the distal end of the humerus and inserts onto the ulnar side of the wrist. If you cut both of these muscles, you will see that the arm can no longer automatically flex very well. Finally, if you remove the radiale, or shorten the radius, but leave the muscles, the hand will no longer flex or extend with the elbow.

So having made a mess in your kitchen and ruined a perfectly good potential hot wing, you can conclude that the avian flight system is irreducibly complex and thus—according to Behe's argument—could not possibly have evolved. Since all these components are necessary for flight, then flight was not possible before they were all assembled. Therefore, says Behe, they could not have been selected for flight.

But this assumption relies on the notion that these components evolved specifically for flight. What if they did not? What if these structures originally evolved for some other function and only later exapted for flight?

How could such a claim be investigated and tested? One major source of data is the fossil record. By looking at the history of avian flight, we can see how the flight apparatus was assembled, not for flight but for predation and insulation. Limbs and feathers were employed together to function in a rudimentary form of flight, later modified into the highly refined form that we see today.

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