The complete word-for-word universality of the genetic dictionary is, for the taxonomist, too much of a good thing. C)nce it has told us that all living things are cousins, it cannot tell us which pairs are closer cousins than others. But other molecular information can, because here we find variable degrees of resemblance rather than total identity. The product of the genetic translating machinery, remember, is protein molecules. Each protein molecule is a sentence, a chain of amino acid words from the dictionary. We can read these sentences, either in their translated protein form or in their original DNA form. Though all living things share the same dictionary, they don't all make the same sentences with their shared dictionary. This offers us the opportunity to work out varying degrees of cousinship. The protein sentences, though different in detail, are often similar in overall pattern. For any pair of organisms, we can always find sentences that are sufficiently similar to be obviously slightly 'garbled' versions of the same ancestral sentence. We have already seen this in the example of the minor differences between the histone sequences of cows and peas.
Taxonomists can now compare molecular sentences exactly as they might compare skulls or leg bones. Closely similar protein or DNA sentences can be assumed to come from close cousins; more different sentences from more distant cousins. These sentences are all constructed from the universal dictionary of no more than 64 words. The beauty of modern molecular biology is that we can measure the difference between two animals exactly, as the precise number of words by which their versions of a particular sentence differ. In the terms of the genetic hyperspace of Chapter 3, we can measure exactly how many steps separate one animal from another, at least with respect to a particular protein molecule.
