Biochemistry and the Study of Evolution (part 2)

IN PART one of this series, it was pointed out that those who question the evolutionary account use the same tree figure to demonstrate the classification of plants and animals as evolutionists do. However, they call it a "taxonomic tree" rather than a "phylogenetic tree" and use it for displaying similarities, but not for demonstrating lines of development.

Recently it has been claimed that biochemical studies of living animals support the hypothesis of an evolutionary origin of the twigs on the tree. Protein sequences are especially used to support this concept. Generally speaking, protein sequence differences are greater between animals that are far apart on the tree of life than those that are relatively close together. This information by itself, however, does nothing more than merely confirm that the animals are different and that conditions under which their enzymes operate are different. It says nothing about how they became different. The facts are consistent with both the evolutionary and the creationist models.

Evolutionary Pathways

Examination of the way in which amino-acid sequences of particular proteins differ from one species to another had led some scientists to postulate the kind of sequences their ancestors may have had. It is possible to work out the number of mutations that convert the sequence of one protein into another. (It is rather like a word game in which one word is trans formed into another by changing one letter at a time.)

Knowing the "minimum mutation distances" between a group of animals for some specified protein, it is straightforward to produce a diagram connecting these species together by lines whose lengths are proportional to the "mutation distances." By assuming that the organisms evolved from a common ancestor, and that the mutations responsible occurred at a constant rate, the diagram becomes a phylogenetic tree, and the point in the middle of the longest line that can be drawn connecting any two species can be called the "point of earliest time." Figure 1 is such a tree, postulated for the evolution of the hemoglobins. Biochemists have expressed satisfaction that the trees for these proteins and others are similar in pattern to those constructed from comparisons of fossils. ¹

Enough has been said already to make it clear that such evidence would only be useful for confirming the evolutionary hypothesis if it were reasonably clear already that evolution had occurred. In fact, although protein sequences do indicate the nature of the changes that must have occurred if evolution took place, they still give no better answer to the question Did evolution occur? than information from comparisons of living animals (with their "missing ancestors") or from sequences of fossils (with "missing links," as well as "missing ancestors").

Differences Too Great

Two further points remain for comment. What we now know about hemoglobins and other proteins whose amino-acid sequences have been compared in different species requires each of them to be viewed as a complex and delicate piece of machinery, responding to the constraints and restraints of the vast number of other enzymes and metabolites (and waste products and poisons!) simultaneously active in the cell. The amino acids in every part of the protein contribute in one way or another to the diverse and detailed interplay of functions that it performs, and it is incredible that mutations affecting them could produce an actual advantage of the kind demanded by the mutation/selection theory of evolution. Each protein used by a species is finely adapted to its own way of life, internal and external, and the differences between the species are too great to be stepped by mutations without disaster for the species.

A cursory description of the structure and function of hemoglobin is evidence in favor of the uniqueness of the form of the protein found in each important taxonomic group. The protein consists of four subunits of the kind illustrated in figure 2, and although the molecular weight of 64,000 seems large for the trans port of only four molecules of oxygen (molecular weight 128) it is clear that every part of each of the four chains is intimately involved in the task. Widely separated portions of the chains have vital effects on the ability of the iron atom to attach oxygen and release it to the tissues without being oxidized itself. Oxygen re lease and uptake is finely con trolled by the interaction between the four subunits and depends on subtle contacts among 380 different atoms from 106 different amino acids; the design makes the molecule a scavenger for oxygen in the lungs, but allows it to release oxygen easily to the tissues.

Other external amino acids in the molecule respond to excessive oxygen demand by a special kind of molecular "flip"; the design of the rest of the molecule allows this movement to produce concerted changes in position of a whole set of other amino acids and, ultimately, in the iron atom, facilitating the release of the oxygen molecule attached to it. Still other parts of the molecule are designed to attach carbon dioxide, the combustion product of active cells, and return it to the lungs for exhalation. Since every one of these functions must be properly adapted to the unique requirements of the internal and external environment of each species, it is quite difficult to envisage the step-by-step development of one hemoglobin from another without serious loss of function on the way.

The study of hemoglobin abnormalities is pertinent at this point. Improved methods of treatment for inherited blood diseases caused by mutations affecting hemoglobin allow people to survive who once would have died; sequence studies on their hemoglobins readily reveal the sites of the mutations in the protein chains. Over a hundred abnormal hemoglobins have now been identified, with "sickle cell anemia" the best known. The effects are less serious in the case of "conservative mutations," where substitution occurs be tween amino acids of very similar properties, but they are serious enough to indicate that every amino acid has a contribution to make to the critical function of the protein. If abnormal hemoglobins are any guide, it seems impossible to account for the hemoglobins of different species by means of mutations. The mutations produce disabilities, but no observable selective advantage. ²

Conclusion

Protein-sequence studies con firm the long-standing observation of creationists that the evidence for evolution is restricted to minor modifications in structures and functions that have always existed. Whatever can be concluded about the differences between hemoglobins of different vertebrates, the molecule is readily recognized as hemoglobin in each case. No evidence from protein sequences about the origin of oxygen transport or of any of its elaborations has yet been found. The same is true of all other proteins studied. And the reason is clear. Unique sequences of amino acids capable of operating within such close tolerances as the enzymes of living cells could not possibly originate by the same process of mutation and selection that allows for minor variations in them once they have originated. ³ Divine Creation is the only plausible explanation for their origin. Taxonomic trees are silent about phylogeny.