BIOLOGY Volume 3 - A Guide to General Biology - 2004

26. EVOLUTION, OR THE HISTORY OF LIFE ON EARTH

26.7. Evidence for Evolution

26.7.8. Comparative Biochemistry

As more precise Methods of biochemical analysis were developed, this field of research became a source of new evidence supporting The Theory of evolution. The presence of identical substances across a wide range of organisms points to potential biochemical Homology, akin to morphological homology at the level of Organs and Tissues. It bears emphasizing once again that these findings merely reinforce other arguments in favor of evolution; by themselves, they cannot serve as conclusive proof. The majority of comparative biochemical studies focus on the Introduction/19.html">Primary Structure of widely distributed Proteins, such as cytochrome c and Hemoglobin, and more recently, Nucleic Acids, particularly ribosomal RNA. Minor alterations in METABOLISM/28.html">The Genetic Code caused by Gene Mutations lead to subtle Changes in the overall STRUCTURE OF THE corresponding proteins or RNA. This provides a basis for elucidating phylogenetic relationships, operating on the assumption that fewer structural differences in a given substance imply fewer accumulated mutations, and thus a closer evolutionary relationship between the organisms containing it. Major differences in molecular structure reflect major differences in DNA. It follows that such differences are predictably found between organisms with relatively few morphological homologies.

Cytochromes are respiratory proteins found in Cell/35.html">Mitochondria that participate in Electron transport along the Respiratory Chain (see Section 9.3.5). Cytochrome c is one such protein involved in this pathway. It is a complex protein consisting of an iron-containing prosthetic group surrounded by a polypeptide chain of 104–112 Amino Acids, depending on the species. Modern computerized mass spectrometry methods have made it possible to determine the primary structure of the cytochrome c polypeptide chain in numerous organisms, including certain Bacteria, Fungi, wheat, the blowfly Chrysomyia, the silkworm, tuna, penguin, kangaroo, and several primates. Across the subjects investigated in this manner (21 organisms in total), the Amino acid sequences proved to be remarkably similar. In 20 of these organisms—ranging from the fungus Candida to humans—the amino acids at positions 78–88 are identical (Table 26.6). The amino acid sequences of cytochrome c in humans and chimpanzees are identical, and they differ from that of the rhesus macaque by a single amino acid. Phylogenetic trees of plants and animals derived from modern data on cytochrome c amino acid sequences closely match those constructed on The basis of morphological homologies.

Class="center">Table 26.6. Amino acid sequences of cytochrome c in 21 species (after Dayhoff M. O., Eck R. V., 1967-1968, Atlas of protein sequence and structure, National Biomedical Research Foundation, Silver Spring, Md.)

Amino acid designations

A — Alanine

C — Cysteine

D — aspartic acid

E — glutamic acid

F — phenylalanine

G — Glycine

H — Histidine

I — isoleucine

K — Lysine

L — leucine

M — Methionine

N — asparagine

P — Proline

Q — glutamine

R — Arginine

S — Serine

T — Threonine

V — valine

W — Tryptophan

Y — Tyrosine

Similar results were obtained in studies of globins—hemoglobin and Myoglobin—which are involved in oxygen TRANSPORT AND STORAGE. The degree of sequence similarity among hemoglobin molecules in four primate species is shown in Table 26.7. Evolutionary relationships among various globins, inferred from their amino acid sequences (along with the organisms in which they occur), are presented in Fig. 26.18. Differences in the amino acid sequences of cytochrome c and these globins apparently arose As a result of mutations in ancestral genes.

Table 26.7. Differences between polypeptide chains of hemoglobin in four primate species

Fig. 26.18. Putative origin of myoglobin and globin polypeptide chains in vertebrates. Humans possess chains of all five types. (After V. M. Ingram, 1963, Hemoglobins in genetics and evolution, Columbia University Press.)

Immunological studies also provide evidence for Phylogenetic relationships among organisms. When Blood Serum proteins are introduced into the blood of animals lacking these globins, they act as Antigens, i.e., they stimulate the Organism to produce corresponding Antibodies, resulting in an Antigen-Antibody Reaction. This Immune Response is based on the recipient animal's ability to recognize the presence of foreign proteins in the serum. Human serum injected into rabbits sensitizes them and induces The formation of antibodies against human serum proteins. If human serum is added to a sample of sensitized rabbit serum after some time, antigen-antibody complexes form and precipitate out of solution, The amount of which can be measured. When sera from various animals are added to samples of rabbit serum containing anti-human serum antibodies, varying amounts of precipitate are formed. Assuming that the amount of precipitate is directly proportional to the amount of "foreign protein," this method can be used to estimate the degree of relatedness among different animal groups (Table 26.8).

Table 26.8. Amounts of precipitate formed upon addition of various mammalian sera to rabbit serum containing antibodies against human serum (the amount of precipitate formed with human serum is taken as 100%)

The comparative serological method is widely used to confirm phylogenetic relationships. For example, zoologists debated for a long time about the taxonomic placement of the horseshoe crab (Limulus). When antigens from various Arthropods were added to serum against horseshoe crab antigens, the largest amounts of precipitate were produced by antigens from arachnids, which include spiders and scorpions. This result reinforced existing morphological data, and the horseshoe crab is now confidently classified under the class Arachnida. Similar studies have resolved many ambiguities in mammalian phylogenetic relationships.

The validity of dividing animals into protostomes and deuterostomes has been confirmed by studies of phosphate-containing energy reserves in Muscle tissue involved in ATP synthesis. The Muscles of protostomes, which include Annelids, Mollusks, and arthropods, contain arginine phosphate, whereas the muscles of deuterostomes, i.e., Echinoderms and Chordates, contain creatine phosphate.

Finally, another example of biochemical homology is the presence in vertebrates of similar or even identical Hormones performing a wide range of different Functions. For instance, a pituitary hormone similar to mammalian prolactin has been found in representatives of all vertebrate groups. Although available data indicate that prolactin elicits as many as 90 different effects, they can be divided into two broad categories: one group of effects is related to reproduction, and the other to osmoregulation (Table 26.9).

Table 26.9. Actions of prolactin in vertebrates

26.7.9. Conclusion

The neo-Darwinian theory of evolution is built upon data from a vast array of sources and is supported by numerous entirely independent observations. To scientists, such evidence serves as the most compelling proof of the theory's validity. While the evolutionary theory has achieved widespread acceptance, much work remains to refine it and apply it to all observable situations.

All scientific explanations, hypotheses, and theories concerning The history of life are provisional, and as long as humans maintain objectivity in their search for truth, they will remain so.

Because The problem of evolution is central to The Study of biology, it would be unpardonable to conclude this chapter without indicating its place in our overall system of knowledge about nature. This is best done by quoting the passage with which Darwin concludes *On THE ORIGIN OF Species*:

"There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on According to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."