Evidences of Evolution

The doctrine of organic evolution is supported by following evidences:

1. Structural evidence: The most extensive evidence in support of evolution comes from the study of structural similarities. Related groups of organisms possess homologous structure because of common ancestry. Among the vertebrates from the frog to man, the vertebral column, eyes, ears, brain and the digestive system are constructed on the same basic plan. Some homologous structures suggest a common ancestry. In plants, the structural similarities of roots, stems, leaves, flowers show the evolutionary tendencies of plants. Morphological similarities in the type of venation, shape of corolla, cohesion or adhesion of stamens among a group of plants suggest a common ancestry.

2. Anatomical Evidence: The study of comparative anatomy of plants and animals also provides strong evidence for organic evolution. the forelimbs of several vertebrates such as those of amphibians, lizards, bats, birds and men are all constructed on the same basic plan and include the same bones in the same position. But they have become morphologically different in the course of evolution as a result of modifications to suit different requirements. The organs in different groups of organisms that are similar in structure and origin but not necessarily in function are called homologous organs. The wings of bats, birds, reptiles and insects have the same function and are superficially alike, but they are quite different in origin and internal structure. They have arisen in the evolutionary process through adaptations of quite different organisms to similar modes of life. The structures which have superficial similarity due to similar functions but having a different evolutionary origin are known as analogous organs. Similarly, the study of types of Xylem, development of stele, nature of tracheids and vessels among the higher cryptograms, gymnosperms and angiosperms provide additional support to the theory of evolution.

3. Palaeontological Evidence: Palaeontology is the study of fossils remains of ancient plants and animals. Fossils are defined as "the remains of ancient plants and animals preserved in the rocks or different layers of earth". The study of fossils provides most convincing and direct evidence of evolution. They bear sound evidence regarding the existence of different types of plants and animals in different geological eras of the earth. They help us to trace the facts correlated with their origin and evolution, relationship, increasing complexity, etc. The surface of the earth consists of layers or strata of rocks formed in different ways in different periods. The earlier formed rocks show fossils of most primitive forms of life, while later formed rocks show fossils of more complex and advanced types. Thus, the fossil evidences reveal a gradual development of organisms with time. The fossil records are, however, incomplete due to various reasons and as such they reveal several wide gaps in the evolutionary history of plants and animals. General survey of the fossil plants ans animals of different geological eras of the earth support the doctrine of organic evolution. Animals are better preserved as fossils as compared to plants and helps in drawing correct conclusions about the possible origin of certain modern vertebrates.

Types of Fossils: In general, the fossils are classified into the following types:

1. Unaltered: In this type, whole bodies of extinct organisms have been found frozen in ice at the poles. About 25,000 years old frozen elephant-like woolly mammoths were found buried in ice in Siberia. Their flesh was so well-preserved that it could be fed to dogs.
2. Petrified: These are formed as a result of infiltration of some 20 minerals in form of carbohydrates, sulphates, silicates, phosphates etc. in organisms tissue. These are rare type of fossils but the best for the study of structural details. In this type, the organic parts of body gets partly or completely replaced by mineral deposits. Replacement of organic parts by mineral deposits is called petrification.
3. Impressions: These are formed when the organism or the parts of the organism come in contact with a soft clay. The organism material disorganises and leaves impression on the soil. The soil later on hardens into rock. These are useful in the study of external features of various parts of organisms.
4. Compressions: These fossils are formed as a result of burial of organisms or its parts in the sediments. The buried parts become flat due to overlying pressure of the sediments. it is useful in the study of the external morphology.
5. Casts or Moulds: these are formed when an organism gets submerged in water containing lime. Sediments get deposited in the form of a hard coat around the whole body or its parts. The surrounding material hardens to form a structure called the mouls. In most cases, the buried organisms have been completely destroyed, but the moulds have retained true copies of their shapes. Moulds exhibit the same external configurations of the organisms.
6. Amber: It is a resinous excretion of certain fossil coniferous trees. The exudate accumulated on the forest floor, gradually hardens to take the shape of amber. Several small plants and animals get preserved in this resin, therefore, amber is considered as a type of fossil.

Determination of Age: Age of fossils is determined either by radioactive carbon dating technique or by the amount of lead in a rock.

A) Radioactive Carbon Dating Technique: Radioactive carbon has been used to determine the age of recently found fossils. It is applicable only to those specimens, which have carbon. It is known that radioactive carbon has a half life of 5568 years.

B) The Amount of Lead in a Rock: The element uranium changes into lead through a long process of transformation. The rate of this change is independent of the conditions under which it takes place. it is calculated that 7,600,000 gm of uranium yield 1 gm of lead per year. Thus, the age of a piece of rock containing uranium and lead can be determined.

Importance of Fossil:

1. Most primitive forms of life are found in the oldest rocks.
2. There is a succession of higher and more complex forms of fossils from older to recent strata of rocks.
3. In many cases, a group arose in one era or period and remained scarce. However, in the next period or era, it became dominant due to adaptive radiation.
4. Different fossils are formed at different time periods.
5. The past forms of life are not exactly similar to any of the living forms.
6. Large groups became extinct but after establishment of all the major phylum, some species of each phylum remained upto the present.

Evolution of Horse:

The evolution of horse (Equus) began in the Eocene epoch about 60 million years ago. They appear as Eohippus about 11 inch in height, the size of a fox but with a longer head. They have four toes in front feet and three in hind feet. The third digit is somewhat longer than the others. Eophippus was a forest dweller and browser, feeding on soft vegetation. The lines of descent perhaps passed from Eohippus through Mesohippus, Miohippus, Parahippus, Merychippus, Pliohippus to Equus. The major changes occurring in the course of evolution of the modern horse are: (a) a general increase in body size, (b) the increase of length and mobility of neck, (c) a reduction in the number of toes from four to one, (d) specialization of teeth for grazing habit, (e) fusion of metatarsals and metacarpals.

Archaeopteryx:
It is the fossil bird of the Jurassic period (about 140 million years) and Mesozoic era. It is a connecting link between birds and reptiles. It has the size of a crow and in certain respects is quite like reptiles.

1. Like birds it bears wings with feathers, but also had scales like reptiles.
2. It had beak like bird but Jaw is provided with teeth like reptiles.
3. Its skeleton framework also resembles with that of reptiles.
4. Its forelimbs were modified into wings like birds but had claw on its wing like reptiles.
5. It had feathery tail like birds but had tail vertebrae like reptiles.

4. Evidence from Physiology and Biochemistry: Study of biochemistry provides some important and dependable evidences to support the idea of evolution.

A) Protoplasm Chemistry: Biochemical analysis of the living organisms shows that their protoplasm consists of the same substances like proteins, lipids, carbohydrates, nucleic acids and water, although they show a great diversity in the form and function. This would suggest that during evolution the most fundamental property of living things remained intact, while variations in certain essential areas appeared according to the use and needs.

B) Chromosome Chemistry: Similarly common structure of chromosome among the diverse groups of organisms suggest a common origin of most living organisms.

C) Enzyme Similarities: Similar enzymes such as trypsin is found in animals ranging from protozoa to mammals, whereas amylase is found from sponges to mammals.

D) Haemin Crystals: The haemoglobin in blood precipitates in crystals, when treated with glacial acetic acid. These crystals are called haemin crystals. Crystals of all members of a species are similar in shape, size and structure, but differ from those of other species. The degree of difference agree with the classification of animals and confirm their phylogenetic relationships.

E) Hormonal Similarities: All vertebrates show a close similarity in their hormones. For example, thyroid hormone is commonly found in all vertebrates. The deficiency of this hormone in frogs can be corrected by feeding them om mammalian thyroid tissues. This suggests a common ancestry of vertebrates.

F) Serological test: It is used in measuring the degree of relationship among different groups of animals and is helpful in establishing their affinities. The thin liquid that remains from blood on clotting is called serum. The serum, in fact is the plasma containing specific proteins. If the blood serum of an animal is injected into the blood stream of other animal, antibodies against the antigens of the injected blood are produced. The foreign inoculated protein is refered to as antigen. These antibodies destroy the antigens of injected blood serum. Antigen-antibody reaction can be carried out in a test-tube also. The reaction shows a soft white precipitate. The precipitate is called precipitin and this test is called precipitation test. The relative degrees of precipitations of antigens of the sera from a group of animals give an idea of the degree of closeness or affinity between them. If precipitate results with more diluted antigen of one animal against the test animal, the animals are more closely related and precipitate results with less diluted antigen shows less close relation between the animals. Such serological tests have shown that man is more related to chimpanzee than to apes, gorilla and baboon. Similarly cats, dogs and bears are closely related. Serological tests can also be done in plants.

5. Evidence from genetics: In search of new varieties or species, man is involved in selecting and breeding animals and plants for his own use. The same selection and interbreeding might have occurred and is occurring on large scale in nature and new species are evolved. Thus, the selection and breeding of domesticated animals and plants for the past several thousand years provide models as to how some evolutionary processes operate in nature. Organisms change with the ever changing environment and many of these variations are heritable. The most important genetic phenomena that played a key role in evolution are recombination and mutation. They generate diversity on which natural selection works.

6. Evidence from Vestigial Organs: The reduced and functionally useless structures of plants and animals are called vestigial organs or rudimentary organs. They were functional and useful in the ancestral forms, but due to changes in environment and mode of life, they gradually became reduced and functionless. A large number of such vestigial organs are found in man. the muscles of the ear which move the pinna in certain other mammals like horse are found in man in reduced condition. the vermiform appendix which is well-developed in herbivorous mammals, is present as a small diverticulum in man. A fold of the skin in the inner corner of the human eye represents the nictitating membrane of reptiles and birds. In the kiwi, a flightless bird of New Zealand, the feathers are reduced to almost hair-like structures and the wings are reduced and become useless organs of flight. The bones and muscles of the wing, however are present in vestigial form. Similarly, the well developed tails of the majority of mammals is represented in the primates by its rudiments in the form of a few caudal vertebrae. Wisdom teeth of man is another similar example of vestigial organ. Snakes in general lack external limbs and girdles but the python possess the rudiments of these structures. In plants the abortive stamens of Labiatae and abortive pistillodes of male flowers of Cucurbitaceae are well known examples of rudimentary structures. Similarly, in some members of the family Compositae, the ray florets have an abortive pistil devoid of any stigma.