Merriam-Webster defines chemotaxonomy as the method of
biological classification based on similarities and dissimilarity in the structure of certain
compounds among the
organisms being classified. Advocates argue that, as
proteins are more closely controlled by
genes and less subjected to
natural selection than the
anatomical features, they are more reliable indicators of
genetic relationships. The compounds studied most are proteins,
amino acids,
nucleic acids,
peptides etc.
Physiology is the study of working of
organs in a
living being. Since working of the organs involves
chemicals of the
body, these compounds are called
biochemical evidences. The study of
morphological change has shown that there are changes in the
structure of animals which result in
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 and
Victor Plouvier were among the pioneers of chemotaxonomy.
Biochemical products
The
body of any
animal in the animal kingdom is made up of a number of
chemicals. Of these, only a few biochemical products have been taken into consideration to derive
evidence for evolution.
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Protoplasm: Every living
cell, from a
bacterium to an
elephant, from
grasses to the
blue whale, has protoplasm. 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 living things have a common origin point or a
common ancestor, which in turn had protoplasm. Its
complexity increased due to changes in the mode of life and
habitat.
- Nucleic acids:
DNA and
RNA are the two types of nucleic acids present in all living organisms. They are present in the
chromosomes. The structure of these acids has been 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, and
thymine. RNA contains
uracil instead of thymine. It has been proved in the
laboratory that a single strand of DNA of one
species can match with the other
strand from another species. If the
alleles of the strands of any two species are close, then it can be concluded that these two species are more closely related.
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Digestive enzymes are
chemical compounds that help in
digestion. Proteins are always digested by a 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 are fundamentally the same. Likewise,
carbohydrates are always digested by
amylase, and
fats by
lipase.
- 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. It can thus be comfortably concluded that the similarity of the end products is due to
common ancestry.
-
Hormones are
secretions from
ductless glands called the
endocrine glands like the
thyroid,
pituitary,
adrenal, etc. Their chemical nature is the 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 supplemented from another human being. It can be extracted from any
mammal and
injected into humans for normal metabolism to take place. Likewise,
insulin is secreted from the
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. As there is a fundamental relationship among these animals, such exchange of hormones or
glands is possible.
- 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 fifth day of development
excretes
ammonia; from its 5th to 9th day,
urea; and thereafter,
uric acid. Based on these findings, Baldwin sought a biochemical recapitulation in the development of
vertebrates with reference to
nitrogenous excretory products.
-
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 (PA) in
invertebrates and
phosphocreatine (PC) in vertebrates. Among the
echinoderms and
prochordates, some have PA and others PC. Only a few have both PA and PC. Biochemically, these two groups are related. This is the most basic proof that the first
chordate animals should have been derived only from echinoderm-like
ancestors.
-
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 that primitive members of most of the
phyla lived in the
sea in
Paleozoic times. It is clear that the first life appeared only in the sea and then evolved onto land. A further point of interest is that the body fluids of most animals contain less
magnesium and more
potassium than the
water of the present-day
ocean. In the past, the ocean contained less magnesium and more potassium. Animals' bodies accumulated more of these
minerals due to the structure of DNA, and this characteristic remains so today. When the first
life forms appeared in the sea, they acquired the composition of the contemporary sea water, and retained it even after their evolution onto land, as it was a favorable trait.
-
Opsins: In the vertebrates,
vision is controlled by two very distinct types of opsins,
porphyropsin and
rhodopsin. They are present in the
rods of the
retina.
Fresh water fishes have porphyropsin;
marine ones and land vertebrates have rhodopsin. In
amphibians, a tadpole living in fresh water has porphyropsin, and the adult frog, which lives on land most of the time, has rhodopsin. In
catadromous fish, which migrate from fresh water to the sea, the porphyropsin is replaced by rhodopsin. In an
anadromous fish, which migrates from the sea to freshwater, the rhodopsin is replaced by porphyropsin. These examples show the freshwater origin of vertebrates. They then deviated into two
lines, one leading to marine life and the other to terrestrial life.
-
Serological evidence: In recent years,[
when?] experiments made in the composition of
blood offer good evidence for evolution. It has been found that 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 of doing so; the method employed by
George Nuttall is called the
precipitation method. In this method,
anti-serum of the involved animals has to be prepared. For human study,
human blood is collected and allowed to
clot. Then, the
serum is separated from the
erythrocytes. A
rabbit is then injected with a small amount of serum at regular intervals, which is allowed to
incubate for a few days. This forms
antibodies in the rabbit's body. The rabbit's blood is then
drawn and clotted. The serum separated from the
red blood cells 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 animal like
dogs,
cats, or
cows, no precipitate appears. It can thus be concluded that humans are more closely related to monkeys and apes. As a result, it has been determined that
lizards are closely related to
snakes,
horses to
donkeys, dogs to cats, etc. This systematic position of
Limulus was controversial for a long time, but has been found to show that human serum is more closely related to
arachnids than to
crustaceans.
The field of biochemistry has greatly developed since
Darwin's time, and this
serological study is one of the most recent pieces of evidence of evolution. A number of biochemical products like nucleic acids, enzymes, hormones and phosphagens clearly show 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 the aquatic ancestry of vertebrates, and the nature of visual pigments points out the fresh water ancestry of land vertebrates. Serological tests indicate relationships within these animal phyla.
Paleontology
When only fragments of fossils, or some
biomarkers remain in a rock or oil deposit, the class of organisms that produced it can often be determined using
Fourier transform infrared spectroscopy
[1]
References
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^ Chen, Y., Zou, C., Mastalerz, M., Hu, S., Gasaway, C., & Tao, X. (2015). Applications of Micro-Fourier Transform Infrared Spectroscopy (FTIR) in the Geological Sciences—A Review. International Journal of Molecular Sciences,16(12), 30223-30250.
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