Origin and Evolution of Life

Origin and Evolution of Life Synopsis

Synopsis

Evolution

Evolution can be defined as the formation of more complex organisms from pre-existing simpler organisms over a certain period. It is a slow, but progressive, natural, sequential development or transformation of animals and plants from ancestors of different forms and functions.
Variation and heredity are the two basic factors of evolution. The selection of variants by the environmental factors forms the basis of evolutionary processes.

Speciation

All living organisms, including plants and animals, exist in the form of different species.
A species is a group of organisms consisting of similar individuals which can breed together and produce fertile offspring.
In plants, a vast diversity is observed with respect to the shape of leaves, colour of flowers etc. Despite this diversity, different plants, e.g. wheat, paddy, sunflower, lotus etc., are considered species of the Plant Kingdom.
In human beings, although diversity is observed with respect to body features such as height and colour, all human beings belong to the same species Homo sapiens.
The process by which a new species develops from the existing species is known as speciation.
The biologist Orator F. Cook coined the term speciation.

Important Factors which Contribute to Speciation

Of these several factors, geographical isolation is the major factor in the speciation of sexually reproducing animals because it interrupts the flow of genes. However, it may not have any effect for self-pollinating or asexually reproducing plant species because they do not depend on other organisms for reproduction.

Evolution by Stages

The number of plants and animals on planet Earth is enormous. This great variety of organisms existing on the Earth initially was quite different from the one present now. They have modified themselves in response to their environment. These changes have occurred gradually in stages and have resulted in the evolution of a new species. The occurrence of different stages of evolution in a species is not because of a single DNA change.

Evolution of Eyes

The eye is an example of a complex homologous organ. It has been created in stages over generations. The primitive organisms which existed on the Earth were slow moving and smaller in size. They did not require a specialised organ for observing any object.
Consider the example of flatworms. Flatworms are small invertebrates. The eyes of flatworms are simple and exist in the form of eyespots. Eyespots are light-sensitive cells which can detect light. As evolution progressed, comparatively larger and mobile organisms evolved. Most of them were predators and required better vision for predation. Hence, from the basic design of eyes, more complex forms evolved. Insects, octopuses and invertebrates have eyes. However, the structure of the eyes differs in each of these organisms. This suggests that they have a separate evolutionary origin. The evolution of eyes is an example of evolution by stages.

Evolution of Feathers

Birds make use of their feathers for flying. However, feathers did not evolve for flight. They evolved as a means of providing insulation to the body in cold weather.
Scientists have discovered fossils of dinosaurs which had feathers for insulation against cold weather. Because the dinosaurs were reptiles, it implies that birds evolved from reptiles.

Evolution by Artificial Selection

Artificial selection is the process in which human preferences have a significant effect on the evolution of a particular species. Example: Humans cultivate wild cabbage as a source of food and have produced different varieties of it by artificial selection. Common vegetables such as cabbage, kale, broccoli, cauliflower and kohlrabi are descendents of wild cabbage.

Cabbage was produced by selecting a short petiole. Broccoli was developed by selecting the arrested flower development of wild cabbage. Another variety, cauliflower, was developed by selecting sterile flowers in the wild cabbage plant. Selection of swollen parts of wild cabbage led to the evolution of yet another variety known as kohlrabi. Similarly, kale was developed by selecting large leaves.
Artificial selection has helped in creating diversity in plants and animals. In agriculture, superior strains of corn, wheat and soya bean have resulted from the process of artificial selection.

Origin of Life

It is believed that the Earth has originated about 4,600 million years ago. It is formed by the condensation and cooling from a cloud of gases and dust. At first, the Earth was very hot and had various gases and vapour of several elements.
With the passage of time, the Earth gradually cooled down and the gases condensed. Thus, a solid crust of the Earth was formed. There were torrential rains for thousands of years, resulting in the formation of large water bodies like oceans.
There were large quantities of hydrogen, nitrogen, water vapour, carbon monoxide, methane and ammonia in the primitive atmosphere.
Continuous series of chemical reactions took place among the gases to form amino acids. Hence, life originated.
Several theories have been put forward to explain the origin of life.

Theory of Special Creation

According to this theory, life was created by some Super Natural Power (God). This theory did not have evidence, hence it was rejected.

Theory of Spontaneous Generation

This theory was put forward at the end of the seventeenth century. It stated that life originated repeatedly from non-living materials in a spontaneous manner.

Theory of Biogenesis

This theory stated that life can arise from pre-existing life and not from non-living matter by abiogenesis.
Louis Pasteur prepared hay infusion (a culture prepared from water collected from a pond or lake) in a swan-necked flask. The infusion was boiled to kill the microbes present in the infusion and to make it sterile. Pasteur reported that when the swan-neck of the flask was broken and the infusion came in contact with air, microbes appeared. Thus, Pasteur proved that life comes only from pre-existing life.

Cosmozoic Theory

This theory states that life came to earth from some heavenly bodies in the form of spores and seeds.

Oparin – Haldane Theory

Abiogenesis is not possible but could have occurred under the conditions prevailing in the reducing atmosphere of the primitive Earth which were favourable for chemical evolution.
High temperature, lightning and intense UV radiation provided energy required for chemical reactions
Living beings arose from simple inorganic and organic compounds by polymerisation.

Miller–Urey Experiment

Stanley Miller and Harold Urey tested the Oparin–Haldane theory in 1953.
They made an apparatus to circulate methane, ammonia, water vapour and hydrogen. All these gases were put in a flask fitted with electrodes. In another flask, water was boiled continuously.
Electrical charges to provide energy similar to lightning were passed for more than a week.
They then collected and analysed the contents of the apparatus. Several amino acids and acids which are found in living organisms were present in the flask.

Organic Evolution

Though life originated by chemical evolution on the primitive earth, it was later replaced by organic evolution.
Changes in the genetic makeup of species in a population as a result of responding to environmental changes is organic evolution.
Organic evolution states “Descent with modification” i.e. present day complex living organisms have evolved from earlier simpler organisms by small but gradual changes which have occurred over millions of years.
Though living organisms show great diversity in size, structure, function and behaviour, they also show basically similar metabolic process indicating some common ancestors.

Evidences for Evolution

A large amount of information has been collected over the last 200 years to support the theory of organic evolution. Such supporting information which helps us in accepting the theory is called evidence.
Several common features of different organisms provide evidence in favour of evolution. The more the characteristics which the two species have in common, the more closely they are related to each other. The more closely they are related, the more recently they would have had a common ancestor. The characteristics which accumulate in a particular generation can be used to trace the evolutionary history.

Morphological Evidence

Morphological evidence of evolution reflects in the form of external features or the appearance of an organism.
Consider different animals such as the sheep, wolf and llama. A common feature is the presence of thick hairy covering called fur on their body surface. This fur serves as the exoskeleton in these animals. This similarity observed among these different animals suggests that they originated and evolved from a common ancestor.

Anatomical Evidence

Anatomical evidence of evolution is usually reflected in the form of structures which appear quite similar in their organisation. The similarities found in different groups of organisms indicate that these organisms must have had a common ancestor.
A human arm, a bull’s leg, a bat’s wing and a whale’s fin are different body parts which are not similar to one another. Their functions are also quite different. The human arm is used for different movements such as picking up a book, throwing a ball etc. The bull uses its legs for activities such as walking, running etc. The bat uses its wings for flying. The whale uses its fins for swimming.
Due to the differences in their functions, the parts show differences in their external structural morphology. However, when we observe the internal structure of bones, we can see certain similarities. This similarity exists between the arrangement of bones and joints. Such similarities suggest that these animals are descendants of a common ancestor.
Different organisms have organs which perform a similar function. These organs which have a similar function but are different in structure and origin are called analogous organs. The tail fin of a lobster and flukes of a whale, wings of a fly and wings of a bird and eyes of arthropods and eyes of vertebrates are analogous organs.
Some organs are fundamentally the same in structure and origin but are modified to perform different functions in different organisms. They are called homologous organs. Examples: Forelimbs of man are adapted for handling, while forelimbs of bat and bird are adapted for flying and those of whales and seals are adapted for swimming.

In plants, the scales of the ovulate or the female cone of pine correspond with the carpel of a flower of higher plants. The scales of the staminate or the male cone of pine correspond with the stamen of a flower.

Vestigial Organs

Organs which are found in reduced or rudimentary condition and do not perform any function in the possessor are called vestigial organs or non-functional organs. They help in understanding the history of evolution and continuity of life.
Man does not have a tail like monkeys, although they both seem to have evolved from a common ancestor. He possesses a rudimentary tail bone composed of four fused vertebrae along with some rudimentary muscles attached to the tail bone. These muscles resemble those which monkeys use for swinging their tail. This evidence emphasises that both man and monkey have a common ancestry.
The other vestigial structures observed in man include the ear muscles, wisdom tooth, coccyx or reduced tail and plica semilunaris.

Palaeontology / Study of Fossils

Fossils are the preserved remains or traces of animals, plants and other organisms from the remote past. Fossils may be remains of organisms or their impressions.
Fossils are generally found in the layers of sedimentary rocks. They are formed by a continuous process of burying and decomposition over a certain period. The hard parts of the body such as skeleton, shell, teeth and occasionally the entire animal are found embedded in the sediments. These sediments form rocks.
The rocks containing the remains of plants and animals of the past have been studied carefully. This study of fossils is called palaeontology.
Palaeontology is a branch of science which involves the study of fossils of plants and animals found in sedimentary rocks. It is the only branch which provides direct evidences in favour of organic evolution. It helps us to compare the past with the present so as to establish the changes which have occurred in the course of evolution.
Fossils of invertebrate animals are found in the deepest layers of rocks, whereas fossils of vertebrates, birds and mammals, are found in the recent layers of rocks. This palaeontological evidence suggests that the invertebrates came into existence before the vertebrates and reflects the order in which these animals appeared on the Earth.

How do fossils form layer by layer?

The study of fossils helps us to estimate their age. This can be carried out in two ways. When we dig into the Earth, we find that the fossils closer to the surface are more recent as compared to the fossils found in deeper layers.
The fossils can also be dated by detecting the ratios of different isotopes of the same element in the fossil material. Radiocarbon dating is the most accurate, most studied and most verified of all radiometric dating schemes. When living organisms change into fossils, their rate of radioactive C₁₄ decay decreases slowly. In this way, the age of fossils can be determined with the help of radioactive C₁₄. Because the age of a fossil can be clearly established by the radioactive carbon dating technique, the exact period of formation of a species can also be ascertained.
Ammonite, Trilobite and Dinosaurs are examples of fossils which have been determined with the help of C₁₄ radioactive decay.

Embryological Evidence

The study of development of an organism from the stage of an embryo is called embryology.
During the course of study, embryos of different genera of vertebrates, e.g. fish, tortoise, chick, amphibians, rabbit and human embryos are compared with one another. These animals have different sizes and shapes of adult organisms and differ in their mode of nutrition. However, the embryos of these animals have striking similarities in the early stages. They differ in their later stages of development. The comparison of embryos suggests a common ancestry of these animals. In all the embryos, the development begins with a single cell. The individual group characteristics develop at a later stage in life.

On the basis of observation of comparative embryology, Ernst Haeckel proposed the Recapitulation Theory—‘Ontogeny recapitulates phylogeny’. It is a biological hypothesis which states that in the course of development from the embryo to the adult, animals go through stages which resemble or represent successive stages in the evolution of their remote ancestors.

Evidences from Atavism (Reversion)

Sometimes, in some individuals certain characters appear which were supposed to be present in their ancestors but were lost during the course of development. This phenomenon is known as atavism or reversion.
Atavism proves that animals developing atavistic structures have evolved from ancestors in which these structures were fully developed.

Examples:

Human baby with tail
Cervical fistula
Long and pointed canine teeth represent carnivorous ancestors.
Large and thick body hair reflect our relationship with apes.
Presence of extra nipples (more than two)

Evidences from Physiology and Biochemistry

Different organisms show similarities in physiology and biochemistry. Physiology and biochemistry thus prove that all animals have evolved from some common ancestor.

Protoplasm: Structural and chemical composition of protoplasm is same from Protozoa to Mammalia.
Enzymes: Enzymes perform same functions in all animals like trypsin digests proteins in amoeba as well as in man, amylase digests starch in Porifera as well as Mammalia.
Blood: Chordates show almost same composition of blood.
ATP: This energy rich molecule is formed for biological oxidation in all animals.
Hormones: They are secreted in different vertebrates and perform the same functions.
Hereditary material: Hereditary material is DNA in all organisms and its basic structure is same in all animals.
Cytochrome c: It is a respiratory protein situated in the mitochondria of all organisms.

Theories of Evolution

There were various hypothesis that have been put forward to explain the theories of evolution but these lacked scientific explanations and hence were discarded.
Four modern theories have been put forward to explain the mode of evolution are-

Theory of Lamarckism

According to the theory of inheritance of acquired characters, “the changes in structure or function of any organ acquired during the life-time of an individual in response to changes in the surrounding environment are inherited by offspring and keep on adding over a period of time”.
This theory states that characters are acquired by animals in two ways,

-The effects of environment

-Use and disuse of body parts.

For example, the long neck of giraffe is explained by Lamarck on the same principle. Giraffe, which lived in the dry and arid deserts of Africa, tried to reach the foliage high up on the trees to eat them as there was no vegetation on the ground. In the process its neck and forelegs got stretched a bit and this was inherited to the next generation. Then in the next generation same efforts were continued. Gradually through many successive generations, we got giraffe having such a long and forelegs.

Lamarck’s idea of the use and disuse of body parts and the inheritance of acquired characters was not accepted by the scientists. It was disproved by August Wiesmann. He put forward the theory of ‘Theory of Continuity of Germplasm’. According to this theory, each animal consists of germ cells and somatic cells. He suggested that acquired characters are restricted to somatic cells alone and do not influence germ cells which cannot be inherited. He showed that even after cutting the tail of rats for several generations, no rat was born without a tail.

The Theory of Lamarckism was revived with new facts which led to the theory of Neo- Lamarckism which stated that ‘the acquired characters which become incorporated in the germplasm are heritable and accumulate generation after generation resulting in the origin of new species.

Darwin’s Theory of Natural Selection

Darwin put forth the Law of Natural Selection involving the struggle for existence and the survival of the fittest.
He published his theory of evolution in a book called The Origin of Species in 1859. Darwin’s theory is a unifying theory of life science and explains the diversity of life.
But there were evidences against this theory which led to criticism of Darwin’s theory such as

De Vries’ Mutation Theory

Mutation theory for the origin of new species was proposed by Hugo de Vries in 1901.
According to this theory, “new species arise from the pre- existing ones in a single generation by the sudden appearance of distinct and discontinuous heritable changes”.
He observed sudden appearance of seven new varieties of Evening Primrose (Oenothara lamarckiana) which grew wild near his garden.
The various features of Mutation theory includes-

There were various evidences to support DeVries’ theory which are as follows-

Appearance of a short legged Ancon sheep to normal long- legged parents in 1871. This mutation was beneficial to the farmers as these sheep could not jump over the low stone fences.
Appearance of hornless Hereford cattle from normal horned parents in 1889 in single generation.
Birth of hairless cats, dogs and mice from normal parents.
Formation of seedless variety of banana.

This theory also came into criticism following the evidences against the theory-

Mutation theory cannot explain phenomenon of mimicry and protective colouration which is called coevolution.
Most mutations are recessive and hence are not expressed. Their utility for the progeny cannot be determined.
Oenothara lamarckiana was not the normal homozygous plant. It was a hybrid. So the results obtained by him were random and not true.

Modern Synthetic Theory of Evolution

The modern concept of evolution is a modification and elaboration of Darwin’s theory of natural selection which is called Neo- Darwinism.
It was elaborated by Huxley in 1942, Dobzhansky in 1937, Muller in 1949, Fisher in 1958.
The synthetic theory was explained by G.L Stebbin in his book “Process of Organic Evolution”.
According to the modern concept, genetic variations appearing among the members of a population bring about evolution under the influence of natural selection and reproductive isolation.
So, there are four factors in the synthetic theory of evolution- genetic variations, natural selection, speciation and reproductive isolation.

Weismann’s Germ Plasm Theory

August Weismann (1834-1914) was a neo-Darwinian biologist who proposed the germplasm theory in his book Das Keimplasma.
This theory is based on the concept that living bodies are formed of two fundamentally differ¬ent kinds of material - the germplasm (protoplasm of germ cells such as sperms and ova) and the somatoplasm (protoplasm of somatic or body cells).
The germplasm is responsible for the development of reproductive parts and remains in the organism, sealed and secluded from the somatoplasm which develops out of the germplasm.
In course of time, the germplasm give rise to the offspring in which again the germplasm remains separate from the new somatoplasm that it develops. Thus, the germplam is a continuous stream from generation to generation while the somatoplasm is formed anew at every generation and is discontinuous.
In bisexual plants and animals, at every generation the germplasm stream becomes a confluence of the maternal and the paternal streams.

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Origin and Evolution of Life