The study of history of lifeforms on earth is, called evolutionary biology. Evolution is a process that results in heritable changes which spread over many generations, leading to diversity of organisms on earth. In simple words, evolution is the sequence of gradual changes from simple life forms to complex life firms over millions of years
Origin of Life
It is considered a unique event in the history of our universe.
Origin of Universe: -
✅It is about 20 billion years old. It contains huge galaxies.
✅ Galaxies conations stars and clausa of gas and dust.
✅ The origin of universe is explained by big bang theory given by Abbe Lemaitre in 1931.
✅The Big Bang Theory states that the universe arose from, a huge explosion. As the universe expanded, temperature came down and hydrogen and helium were formed later. The galaxies were then formed due to condensation of gases under gravitation.
Origin of earth: -
It was supposed to be formed about 4.5 billion years back in the solar system of the milky way galaxy.
✅Water vapor, methane, carbon dioxide and ammonia released from molten masses covered the surface.
✅UV rays from the sun broke up water molecule into hydrogen and oxygen and lighter hydrogen escaped.
✅ Oxygen combined with ammonia and methane to form water, carbon dioxide and other compounds.
✅ Subsequently Ozone layer formed. As earth cooled, the water vapour fell as rain to fill depression and formed oceans.
✅ Life appeared 500 million (about 4 billion years back) years after the formation of earth.
Theories of Origin of Life: -
Various theories of origin of life were given by different thinkers and scientists. These are as follows: -
✅Theory of special creation states that God created life by his divine act of creation. 1t was proposed by Father Suarez.
✅Theory of panspermia/cosmogonic states that the spores or panspermia came from outer space and developed into living fronts. It was given by Arrhenius.
✅ Theory of spontaneous generation states that life originated from decaying and rotting matter like straw, mud, etc. This theory is also known as theory of abiogenesis or auto biogenesis.
✅Louis Pasteur rejected the theory of spontaneous generation and demonstrated that life came from pre-existing life.
✅ In his experiment, he kept killed yeast cells in presterilized flask and another flask open into air. The life did not evolve in the former but new living organisn1s evolved in the second flask.
✅Theory of chemical evolution or Oparin-Haldane theory states that life originated from pre-existing non-living organic molecules and that formation of life was preceded by chemical evolution. The condition s on the earth that favored chemical evolution were very high ten1perature, volcaruc storms and reducing atmosphere that contained CH4, NH3, water vapor, etc.
Experimental Evidence of Chemical Evolution: -
Miller's experiment provided experimental evidence for chemical evolution.
✅The experiment was carried out by SL Miller and HC Urey in 1953.
✅They took a closed flask containing CH4, H2, NH 3 in the ratio of 2: 2: land water vapor at 800°C and created electric discharge. These conditions were similar to those in primitive atmosphere.
✅ After a week, formation of amino acids was observed. Complex molecules like sugars, nitrogen bases, pigments and fats were seen in the flask by other scientists.
✅ Miller and Urey supported the idea of abiogenesis followed by chemical evolution. Analysis of the meteorite also revealed the presence of similar compounds.
✅ Chemical evolution of life was more or less accepted.
Origin of First Cell: -
✅ First non-cellular lifeforms originated three million years ago. These molecules were like RNA, protein and polysaccharides inside a self-replicating metabolic capsule which later reproduced its molecules.
✅ Cellular lifeform first evolved about 2000 million years ago. These were single-celled and formed in aquatic environment.
✅This form of abiogenesis, i.e., the first form of life arose slowly through evolutionary forces from non-living molecules, was accepted by many scientists.
Evidence of Evolution: -
Evidence that suggests that evolution has occurred on earth are derived from: -
(i) Palaeontology
(ii) comparative anatomy and morphology
(iii) Biochemical/Physiology
(iv) Biogeography
(v) Embryology
Paleontological Evidence: -
(i) Paleontology is the study of fossils. The fossils are the remains of past organisms preserved in sedimentary rocks.
(ii) Rocks form sediments and a cross-section of earth's crust indicates the arrangement of sediments one over the other during the long history of earth.
(iii) Different aged rock sediments contain fossils of different lifeforms, who died during the formation of particular sediment.
(iv) Some organisms appear similar to modern organisms. They represent extinct organisms like dinosaurs.
(v) A study of fossils in different sedimentary layers indicates the geological period in which they existed. Fossils which are obtained from old rocks arc of simple ty pe, while which were obtained from new rocks are of complex type.
(vi) The study showed that lifeforms varied over time and certain lifeforms are restricted to certain geological time scale. Hence, new forms of life have evolved at different times in the history of earth.
Comparative Anatomy and Morphological Evidences; -
These studies high light the similarities and differences among the organisms of today and those that existed years ago. The evidence come from comparative study of external and internal structure.
Homology: -
✔ In an organism, the homology indicates common ancestry.
✔ The organs with same structural design and origin hut different functions arc called homologous organs. For example, forelimbs of some animals like whales, bats and cheetah have similar anatomical structure, such as humerus, radius, ulna, carpals, metacarpals and phalanges. But these forelimbs perform different functions in these animals. Other examples of homology arc vertebrate heart and brain.
✔ In plants, thorns and tendrils of Bougainvillea and Cucurbita represent homology.
✔ Homology is based on divergent evolution. The same structure developed along different directions due to adaptations to different needs. The condition is called divergent evolution.
Analogy: -
✔It refers to a situation exactly opposite to homology.
✔ Organs which are anatomically different but functionally similar are called analogous organs. For example, wings of butterfly and birds. In both, wings perform same function, but they have different origin and structure.
✔Analogous organs arc a res ult of convergent evolution. It is the evolution in which d 'ferent structures evolve for same function and hence, have similarity. It can be said that above organisms had different structures, but they came in the same environment and evolved to perform same function.
✔Other examples of' analogy are eyes of Octopus and mammals, flippers of penguins and dolphins. In plants, sweet potato (root modification) and potato (stem modification) are analogous, structures.
Vestigial Organs: -
Like homologous organs, these organ s also provide evidence for organic evolution. These are degenerate, non-functional and rudimentary organs to the possessor, while correspond to fully developed and functional organs of related organisms.
✔There are about 90 vestigial organs in the human body. Some of them are tail bone (coccyx), wisdom teeth, nictitating membrane, vermiform appendix, etc.
✔ Some examples from other animals are hip girdles and bones of the hindlimbs in some whales and certain snakes and wings of f1ightless birds.
Biochemical Evidences: -
✔ The metabolic processes in organisms are similar with some new materials and end product. For example, energy released by oxidation is stored in ATP, which then provide power for energy requiring process.
✔Molecular homology is the similarity among animals at the molecular level. For example, human DNA differs in only 1.8% of its base pairs from chimpanzee's DNA and there is no difference between the two in the amino acid sequence for the protein cytochrome-c.
✔In the same way, enzymes, proteins and genes perform same functions in different organisms. So, it indicates common ancestry among organisms.
Biogeographical Evidence: -
The species restricted to a region, develop unique features. Also, species present in far separated regions show similarity of ancestor. This can be explained with the help of following processes.
Adaptive Radiation: -
It is an evolutionary process in which an ancestral stock gives rise to new species that are adapted to new habitats and new ways of life. In other words, adaptive radiation is evolution of organisms, starting from a point which then radiate or spreads to other habitats. Examples are: -
(i) Darwin's finches These were small black birds, which Darwin observed in Galapagos Island.
(a) He observed many varieties of finches in the same Island.
(b) All varieties of finches had evolved from original seed-eating finches.
(c) There was alternation in beaks enabling to become insectivorous and some vegetarian. All these varieties evolved from original species due to different type of food available at different Island.
(ii) Marsupials of Australia A number of marsupials (mammals with pouch), Different from each other evolved form an ancestral stock, all within the Australian Island continent.
Convergent Evolution: -
It is the occurrence of more than one adaptive radiation in an isolated geographical. area with different habitats. Examples are: -
✔Wings of insect, bird and bat.
✔Spiny anteater and scaly anteater belong to different orders of class Mammalia. They have acquired similar adaptations for food, e.g., leg ants, termites and insects.
Parallel Evolution: -
It refers to independent development of similar characters in two animal groups of common ancestry living in similar habitats of different continents. Examples are: -
(iii) Placental mammals in Australia show parallel evolution as they have evolved from marsupial mammals. All these closely resemble and look similar to a corresponding marsupial.
Embryological Evidence: -
The study of comparative embryology shows common pattern of development.
- The principles of embryonic development were given by Von Baer.
- Ernst Haeckel propounded the theory of recapitulation or Biogenetic law, which states that an individual organism in its development (ontogeny) tends to repeat the stages passed through by its ancestors (phylogeny), i.e., ontogeny recapitulates phylogeny.
- This means that the life history of animals reflects its evolutionary history. For example, during the life history, Frog's tadpole larva resembles fishes, the ancestors of amphibia. The presence of gill clefts in all vertebrate embryos including human provides strong embryological evidence.
Anthropogenic Evidence: -
Excess use of herbicides, pesticides, etc. has resulted in selection of resistant varieties in a lesser time scale. This is also true for microbes against which antibiotics or drugs have been used. All this evidence reflects that 'Evolution is a stochastic process based on chance events in nature and chance mutation in the organisms.
Artificial Selection: -
Human has created many varieties of plants and many breeds of animals for his use by domestication of wild varieties, selection and carrying out intensive breeding programs. So, if human can create new species why not nature.
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