Although the differences between humans and chimps may be quantitative only, the two species as adults do not look much alike and their adaptive differences are, to say the least, profound (no monkey, despite the common metaphor, will ever type—much less write—the Iliad). Yet King and Wilson (1975), reviewing evidence for the astoundingly small difference in structural genes between the two species, have found that the average human polypeptide is more than 99 percent identical with its counterpart in chimps. Moreover, much of the difference can be attributed to redundancies in the genetic code or to variation in nontranscribed regions (pp. 114–115). For 44 structural loci, the average genetic distance between chimps and humans is less than the average distance between sibling species barely, if at all, distinguishable in morphology—and far less than the distance between any measured pair of congeneric species.
What, then, is at the root of our profound separation? King and Wilson argue convincingly that the decisive differences must involve the evolution of regulation; small changes in the timing of development can have manifold effects upon a final product: "Small differences in the timing of activation or in the level of activity of a single gene could in principle influence considerably the systems controlling embryonic development. The organismal differences between chimpanzees and humans would then result chiefly from genetic changes in a few regulatory systems, while amino acid substitutions in general would rarely be a key factor in major adaptive shifts" (p. 114). Differences in regulation may evolve by point mutations of regulatory