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Merriam-Webster defines chemotaxonomy as the method of biological classification based on similarities in the structure of certain compounds among the organisms being classified. Advocates argue that, because proteins are more closely controlled by genes and less subjected to natural selection than are anatomical features, they are more reliable indicators of genetic relationships. The compounds studied in the most cases are proteins, amino acids, nucleic acids, peptides etc.

Physiology is nothing but the study of working of organs in a living being. Since working of the organs involves chemicals of the body, these evidences are otherwise called biochemical evidences. The study of morphological changed has shown that there are changes in the structure of animals; leading to evolution. When changes take place in the structure of a living organism, they will naturally be accompanied by changes in the physiological or biochemical processes.

John Griffith Vaughan was one of the pioneers of chemotaxonomy.

Biochemical Products: The body of any animal in the animal kingdom is made up of number of chemicals. Of these, only a few biochemical products have been taken into consideration to derive evidence for evolution.

1. Protoplasm: Every living cell, from a bacterium to an elephant, from grasses to the blue whale has protoplasm in their cells. Though the complexity and constituents of the protoplasm increases from lower to higher living organism, the basic compound is always the protoplasm. Evolutionary Significance: From this evidence, it is clear that all ling things have a common origin point or a common ancestor, which had protoplasm and its complexity increased due to change in the mode of life and habitat.

2. Nucleic Acids: DNA and RNA are the two types of nucleic acids present in all living organisms. They are present in the chromosomes. When the structure of these acids are studied, it is found to be similar in all animals. DNA always has two chains forming a double helix and each chain is made up of nucleotides. Each nucleotide has a pentose sugar, a phosphate group and nitrogenous bases like adenine, guanine, cytosine, thymine. RNA has uracil instead thymine. It has been experimentally proved that a single strand of one species can match with the other strand from another species. If the alleles of the strands of any two species are more close, then it can be concluded that these two species are more closely related.

3. Digestive Enzymes: Enzymes are chemical compounds that help in digestion. Proteins are always digested by particular type of enzymes like pepsin, trypsin, etc., in all animals from a single celled amoeba to a human being. The complexity in the composition of these enzymes increases from lower to higher organisms but basically they are same. Likewise, carbohydrates are always digested by amylase and fats by lipase.

4. End Products of Digestion: Irrespective of the type of animal, the end products of protein, carbohydrates and fats are amino acids, simple sugars and fatty acids respectively. Evolutionary Significance: We can comfortably conclude that the similarity of the end product is due to common ancestry.

5. Hormones: Hormones are secretions from ductless glands called the endocrine glands like thyroid, pituitary, adrenal etc. Their chemical nature is same in all animals. For example, thyroxine is secreted from the thyroid gland, irrespective of what the animal is. It is used to control metabolism in all animals. If a human being is deficient in thyroxine, it is not mandatory that this hormone should be given from another human being. It can be extracted from any mammal and injected in humans for normal metabolism to take place. Likewise, insulin is secreted from pancreas.

If the thyroid gland from a tadpole is removed and replaced with a bovine thyroid gland, normal metabolism will take place and the tadpole will metamorphose into a frog. Evolutionary Significance: Thus we can say that unless there is a fundamental relationship among these animals, such exchange of hormones or glands is impossible.

6. Nitrogenous Excretory Products: Mainly three types of nitrogenous waste is excreted by living organisms; ammonia is a characteristics of aquatic life form, urea is formed by the land and water dwellers, uric acid is excreted by terrestrial life forms. A frog, in its tadpole stage excretes ammonia just like a fish. When it turns into an adult frog and moves to land, it excretes urea instead of ammonia. Thus an aquatic ancestry to land animal is established.

A chick on up to its 5th day of development excretes ammonia, from 5th to 9th day, urea and 9th day onwards, uric acid. Based on these findings, Baldwin sought a biochemical recapitulation in the development of vertebrates with reference to nitrogenous excretory products.

7. Phosphagens: Phosphagens are energy reservoirs of animals. They are present in the muscles. They supply energy for the synthesis of ATP. Generally, there are two types of phosphagens in animals, phosphoarginine or PA in invertebrates and phosphocreatine or PC in vertebrates. Among the echinoderms and prochordates, some have PA and others PC. Only a few has both PA and PC. Evolutionary Significance: We can thus conclude that biochemically these two groups are related and this is the first proof that first chordate animal should have been derived only from echinoderm like ancestors.

8. Body fluid of animals: When the body fluids of both aquatic and terrestrial animals are analyzed, it shows that they resemble sea water in their ionic composition. There is ample evidence to show that primitive members of most of the phyla lived in the sea in the Palaeozoic time. Thus it is clear that the first life appeared only in sea and then they invaded land. A further point of interest is that the body fluids of most animals contain less Mg and more K unlike the water of the present day ocean. In the past the ocean contained less Mg and more K. Due to geological changes, it acquired more Mg and less K and it remains so today. Since the first life form appeared in the sea, they acquired the composition of the then sea water and retained it even after their evolution on land since it was favorable for them.

9. Visual Pigments: In the vertebrates, vision is controlled by two very distinct types of visual pigments, porphyropsin and rhodopsin. They are present in the rods of the retina. Fresh water fishes have porphyropsin and marine ones and land vertebrates have rhodopsin. In amphibians, a tadpole living in fresh water has porphyropsin and the adult frog, that lives on the land for most of the time has rhodopsin. In catadromous fish, which migrates from freshwater to sea, the porphyropsin is replaced by rhodopsin and in an anadromous fish which migrates from the sea to freshwater, the rhodopsin is replaced by porphyropsin. Evolutionary Significance: All these examples clearly show freshwater origin of the vertebrates. Then they deviated into two lines, one line leading to the marine life and the other to the terrestrial life.

10 Serological Evidence: In the recent years, experiments made in the composition of blood offer good evidence for evolution. It has been found the blood can be transmitted only between animals that are closely related. The degree of relationship between these animals is determined by what is known as the serological evidence. There are various methods and the method employed by George Nuttall is explained. This is called the precipitation method. Precipitation Method: In this method, anti-serum of the animals have to be prepared. Suppose the relationship of human beings has to be tested with other animals, then anti-human serum has to first prepared. Human blood is collected and allowed to clot. Then the serum is separated from the RBC. A rabbit is injected with little amount of serum at regular intervals and allowed to incubate for a few days. This forms antibody in the rabbit's body. Now the rabbit's blood is drawn and clotted. The serum separated from the RBC is called the anti-human serum.

When such a serum is treated with that of blood of monkeys or apes, a clear white precipitate is formed. When the serum is treated with the blood of any other animals like dog, cat or cow, no precipitate appears.

Thus it can be concluded that humans are more closely related to monkeys and apes. Evolutionary Significance: By preparing anti-serum of various vertebrates, the evolutionary relationship can be determined. Thus we are affirmative that lizards are closely related to snakes, horses to donkeys, dogs to cats, etc. The systematic position of Limulus was controversial for a long time. With the help of this method, it has been found that it is closely related to arachnids than to crustaceans.

Conclusion: The field of biochemistry has developed since Darwin's time and this serological study is one of the most recent evidence of evolution. A number of biochemical products like nucleic acids, enzymes, hormones and phosphagens clearly shows the relationship of all life forms. The composition of body fluid has shown that the first life originated in the oceans. The presence of nitrogenous waste products reveal aquatic ancestry of vertebrates and the nature of visual pigments points out fresh water ancestry to land vertebrates. Serological tests indicate relationships within an animal phyla. These evidences, unavailable to Darwin at that time, has extended our knowledge of evolution.

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