Living organisms possess distinct characteristics, display organizational and functional unity, entail a mechanism of origin and evolution of diversities and maintain a balanced relationship with nature. Most aspects of evolution indicate that the knowledge of the past has become essential for fully understanding the present. Life since its beginning on earth had changed through time. The history life comprises of two aspects, one is the origin of life on earth and the other is the mechanism involved in the gradual changes and adaptations of living organisms through time which is known as the evolution of life.
Origin of life is linked with the origin of earth. The Big Bang Theory explains the Origin of the Universe. It proposes that the universe had an explosive beginning. (Big Bang) and originated 15 billion years ago. The universe comprised of stars, clouds of gas and dust which form the galaxies. The solar system was probably created when the gaseous clouds started to collapse due to the force of its own gravity forming atoms and particles. Atoms, dust grains and gaseous disc aggregated to form clumps and gave rise to planets. This forms the solar system of the milkey way galaxy. Life appeared 500 million years after the formation of earth.
Theories:
Special Creation: This idea embodies that life on Earth is a divine creation and also attributes to supernatural event at a particular time in the past. It also emphasizes that life has not changed ever since its origin.
According to this theory, life originated spontaneously from lifeless matter. It was believed that fishes originated from mud, frogs from moist soil and insects from decaying matter.
It was speculated by Louis Pasteur (1862) that life originates from pre-existing life. He showed that pre-sterilized flasks kept closed airtight, with killed yeast, did not give rise to any life form, while in another flask kept open to air living organisms arose from killed yeast.
Some scientists still believe that life came from outer space. This states that units of life called spores were transferred to different planets, including earth. This is still an idea of some astronomers.
This idea was developed by Haldane (1929). They proposed that with the conditions prevailing on earth, life arose by a series of sequential chemical reactions. The first form of life could have come from pre-existing non-living inorganic molecules which gave rise to formation of diverse organic molecules which are transformed into colloid system to produce life. The modern concept of chemical evolution regarding origin of life was accepted.
Evolution can be better understood only by observing the interrelationship between the existing organisms and also relating the similarities with the extinct organisms. These evidences support the concept that all organisms have evolved from common ancestors.
The comparative study of morphology and anatomy of animals, reveal that they possess common set of characteristics. The homologous organs are those which have inherited from common ancestors with similar developmental pattern in embryos. The fore limbs of mammals are homologous structures. A human hand, a front leg of a cat, the flippest of a whale and a bat’s wing look dissimilar and adapted for different functions. Their mode of development and basic structure of bone are similar. The analogous organs look similar and perform similar functions, but they have different origin and developmental pattern. The function of the wings of a bat, the wings of a bird and the wings of an insect are similar, but their basic structures are different. The degenerated and non-functional organs of animals are called vestigial organs. Some vestigial organs in man are vermiform appendix, nictitating membrane, caudal vertebra, coccyx etc. Atavism is the reappearance of ancestral characters in some individuals. e.g. Presence of rudimentary tail in newborn babies, presence of thick hair on the human body.
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