Biochemistry and the Study of Evolution

PEOPLE COMMITTED to the evolutionary explanation of the history of life on earth often compare the evolution of living things to the growth of a tree. Originally there is only one shoot; the first twigs produced from it correspond to the initial divergence of two or more species from the single form of life originally present on the earth. . .

-president of Avondale College in Australia at the time this article was written

PEOPLE COMMITTED to the evolutionary explanation of the history of life on earth often compare the evolution of living things to the growth of a tree. Originally there is only one shoot; the first twigs produced from it correspond to the initial divergence of two or more species from the single form of life originally present on the earth. The process continues and the differences accumulate. Fairly soon there are major divisions in the trunk (the plant kingdom, animal kingdom, bacteria kingdom, et cetera), and the major branches of each correspond to the major groups of living things present at that stage of evolutionary history. Each limb bears branches, and each branch bears smaller branches and, finally, twigs. Each twig (each species) can be classified by naming in order the various larger twigs and branches from which it grew.

Ultimately, in evolutionary theory, the tree attains its present size. The major branches from each of the main divisions of the trunk typify the major unit of classification of plants and animals, called a phylum. 1

People who question the evolutionary account still find it useful to picture animal and plant species by the twig-branch-tree analogy, but they call it a taxonomic tree rather than a phylogenetic tree. In other words, the tree is used for displaying similarities and classifying species, but not for saying anything about the "genesis of the phyla." At this point the proponent of evolution interrupts to say that the fact that species can be so conveniently represented on a tree is indeed very strong evidence that they grew like a tree. To substantiate his claim he turns to the detailed comparisons made by physiologists and anatomists be tween living animals and to the comparisons made by paleontologists between living animals and fossils.

With this as background, we deal here with the recent claim that biochemical studies of living animals also support the hypothesis of an evolutionary origin of the twigs on the tree, substantiating the claims made by paleontologists. The biochemists claim that information about the structure of proteins in living cells supplements what is known from studies of physiology and anatomy and actually enables a scientist to deduce the evolutionary history of a species without needing to know anything about its fossil precursors. Much publicity has been given to this field of research and to the supposed confirmation of the theory of evolution provided by it.

Our appraisal of the results of research in biochemical evolution leads us to the conclusion that there are still two ways to account for the origin of species and that biochemical studies of evolution provide no excuse whatever for excluding divine creation as one of the alternatives. Indeed, many parts of the evidence are difficult to accommodate to any naturalistic theory.

Protein Sequences

Proteins comprise the vast variety of the enzymes responsible for living processes in cells and they are crucial to our understanding of living organisms. Other important proteins are the antibodies of the blood as well as many of the hormones, and many of the structural materials used in living cells. Composed of hundreds, sometimes even thousands, of links, each protein chain is folded into a structure uniquely determined by the kinks and twists and ceilings of the amino acids that comprise it. To perform their functions precisely —without simultaneous precision in hundreds of enzymes, life in even the simplest cell would be impossible—it is necessary that the twenty different kinds of amino acids used in nature be precisely incorporated into enzyme chains. Provided that the specifications have been followed exactly and the amino acids linked together in the correct sequence, the protein will assume its proper structure and perform its proper function.

It has been a triumph of biological science that molecular biologists have been able to dis cover the actual mechanisms used by cells to record the specifications for the structure of proteins, and then for automatically decoding this information to control the synthesis of proteins with the exact sequences required for their function as enzymes, hormones, et cetera.

The sequence of amino acids is now known for a very large number of enzymes and other proteins found in nature. The first complete amino acid sequence for a protein was worked out by the British chemist Sanger for the hormone insulin. This protein has fifty-one amino acids in its chain, but sequence studies on very much larger molecules are commonplace today. 2

Protein sequences are usually determined today by using enzymes to split a protein chain into small fragments. The fragments are further digested in a way that splits off the amino acids one by one and permits them to be identified in turn. Automatic machines greatly reduce the labor required, but the work is still exceedingly tedious and demanding. The number of laboratories that currently report protein sequences is an indication of the importance that the scientific community attaches to this information. On the one hand, there is a desire to under stand what it is in the structure of an enzyme that gives it its remark able efficacy in catalyzing the chemical reactions of the cell, and, on the other hand, there is the urge to understand the supposed evolutionary pathways by which the proteins of present-day species were produced from the proteins belonging to earlier forms of life.

Molecular Mechanism of Evolution

From what has been said above, it is clear that different species of plants and animals are different in function and form because of the differences in their proteins and in the genes that code for these proteins. All organisms use the same genetic code for storing information about protein sequences and all use the same mechanism for translating the code when proteins are synthesized. Consequently, it is very clear that any explanation of progressive differences between organisms must begin at this point.

Mutations, errors in gene code script caused by substitution of one code letter for another so as to change the meaning of a code word from one amino acid to an other, are the raw material of evolution.

To be specific, each amino acid is coded by a set of three DMA base-pairs called a "codon." Three typical examples are:

Glycine GGA
Alanine GCA
Serine TCA

where the letters G, C, T, A, refer to the base-pairs of which the DNA is made. Mutations occur when one base pair is exchanged for another in the DNA of the genes; the amino acids above are seen to be exchanged for each other in a protein by single mutations. Mutations that delete base pairs are also known. Mutations probably occur during the copying process—rarely enough to prevent their consequences from being disastrous to the species but still frequently enough for their effects to be noticed.

As yet, mutated codons for amino acids are very difficult to locate in the genes, as sequence studies on DNA are not easily carried out, but their effects on proteins are immediately apparent as soon as amino acid sequences are reported. Here, then, is the molecular mechanism of evolution: the changes in anatomy and physiology on which natural selection operates are due to changes in protein function due to changes in protein structure due to changes in protein sequence due to mutations.

Comparisons of Protein Sequences

Protein sequences for different enzymes in the same animal are decidedly different, of course, but very similar for the same protein from different animals. Table 1 shows a comparison of some sequence date for hemoglobins. Hemoglobin is the oxygencarrying pigment of the blood of many species and is one of the most easily studied of all proteins. Its function is crucial to the survival of the organism.

The data in Table 1 are typical. As expected, small differences are superimposed on a common general pattern. The reason for the expectation is that the protein is doing the same job in each case and naturally needs a specific structure in order to do a specific job.

An enzyme is thought to function by providing a cleft of precise dimensions to accommodate the molecule it is designed to affect. Changes in the sequence of amino acids in the region of the "active site" are unlikely to preserve its precise dimensions. Therefore, mutations that lead to the substitution of one amino acid for another in parts of the protein chain near the active site are usually too drastic for the animal to survive and they are rarely seen. Changes elsewhere in a large protein molecule are much more common, however, and although the effects of these mutated genes in producing modified proteins are almost universally harmful the animals often survive, especially when protected from the intense competition of the wild. Some proteins are exceedingly sensitive to mutations, others are more capable of change.

Generally speaking, protein sequence differences are greater between animals that are far apart on the tree of life than in those that are relatively close together. The point is illustrated in both Figure 2 and Table 1. There are two explanations for this. The requirements for the effective functioning of an enzyme will vary slightly from one animal to another but the greatest differences will be found between animals whose physiological requirements differ most. Thus, for example, although all hemoglobins are required to carry oxygen from the lungs to the tissues of an animal via the blood vessels, the conditions under which this is done are much more different between sharks and snakes than they are between snakes and lizards.

Since all parts of a. protein molecule, not only the active site, are involved in the effective functioning of the molecule, it is perfectly reasonable to see large variations between different proteins of the same kind as provisions for carrying out delicate tasks under different conditions. It is true that the animals that lie farthest apart on the evolutionary tree also show the greatest differences when their protein sequences are examined. But this information, by itself, does nothing more than merely confirm that the animals are different and that the 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.

In next month's conclusion to this study we will demonstrate that 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. In this way it can be seen that divine creation is the only plausible explanation for their origin.


REFERENCES

1 The terms for the taxonomic groupings used by biologists for classifying organisms are, in order:

Kingdom

Phylum (plural: phyla)

Class

Order

Family

Genus (plural: genera)

Species

2 A frequently revised compilation of protein sequences is found in Atlas of Protein Sequence and Structure, M. O. Dayhoff (Editor), 5th Edition, 1972, National Biomedical Research Foundation, Washington, D.C. This book places great stress on the evolutionary significance of protein sequence data and contains numerous chapters that explain the way in which those differences might be accounted for by evolutionary mechanisms.


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-president of Avondale College in Australia at the time this article was written

November 1973

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