An Example of the Logic of "Chance" The Complex Structure and Systems in the Cell
The Problem of the Origin of Proteins Left-handed Proteins The Indispensability of the Peptide Link Zero Probability Is There a Trial-and-Error Mechanism in Nature?
The Evolutionary Argument about the Origin of Life Miller's Experiment

 The Indispensability of the Peptide Link

The difficulties the theory of evolution is unable to overcome with regard to the development of a single protein are not limited to those we have recounted so far. It is not enough for amino acids to be arranged in the correct numbers, sequences, and required three-dimensional structures. The formation of a protein also requires that amino acid molecules with more than one arm be linked to each other only in certain ways. Such a bond is called a "peptide bond." Amino acids can make different bonds with each other; but proteins are made up of those-and only those-amino acids which are joined by peptide bonds.

A comparison will clarify this point. Suppose that all the parts of a car were complete and correctly assembled, with the sole exception that one of the wheels was fastened in place not with the usual nuts and bolts, but with a piece of wire, in such a way that its hub faced the ground. It would be impossible for such a car to move even the shortest distance, no matter how complex its technology or how powerful its engine. At first glance, everything would seem to be in the right place, but the faulty attachment of even one wheel would make the entire car useless. In the same way, in a protein molecule the joining of even one amino acid to another with a bond other than a peptide bond would make the entire molecule useless.

Research has shown that amino acids combining at random combine with a peptide bond only 50 percent of the time, and that the rest of the time different bonds that are not present in proteins emerge. To function properly, each amino acid making up a protein must be joined to others only with a peptide bond, in the same way that it likewise must be chosen only from among left-handed forms.

The probability of this happening is the same as the probability of each protein's being left-handed. That is, when we consider a protein made up of 400 amino acids, the probability of all amino acids combining among themselves with only peptide bonds is 1 in 2399.