what is evolution?

what is evolution?


All the life forms that we see around us like plants, animals, or microorganisms comprise the biodiversity on earth. These all life forms have a long history of changes and the study of the history of life forms on earth is called Evolutionary biology.

Evolution in simple language is the orderly change from one form to another. It means the act of unfolding or unrolling.


The origin or beginning of life is a unique event in the history of the universe.

The universe is always expanding and is a very vast region of space. It is possibly infinite in volume. Millions and Billions of galaxies are present in it which form a clustered together to form this huge universe. The universe contains heavenly bodies, like the stars, planets, and rocky bodies bounded together by gravitational force and a cloud of gas and dust.

The distance between these heavenly bodies present in the space is called “STELLAR DISTANCES” which is measured in ‘LIGHT YEARS’.

    **    Light year is the distance traveled by light in a vacuum in one year. It is also a unit of measurement of length used only when the distances to be measured are in millions OR billions of kilometers.


The Big Bang Theory

  • The origin of the universe is the most important phenomenon in the origin of life. The Big Bang theory attempts to explain the origin of the universe. It states that the universe is very old, and its origin took place almost 20 Billion (20×109) years ago. It states that a single huge explosion i.e. Big Bang took place in the space which was so powerful that is unimaginable in physical terms.
  • Due to this explosion, the universe expanded in volume and the temperature of space calm down. Slowly Hydrogen and Helium gases were formed, which condensed due to the gravitational forces present in the surroundings. All this resulted in the formation of billions of galaxies, about which we have already discussed.
  • In the solar system of the milky way, galaxy earth was supposed to have been formed about 4.5 billion years ago.
  • The gaseous envelope or atmosphere was not present on the early earth. Due to volcanic eruptions, the molten masses were released which slowly covered the surface of the earth. From these molten masses, various gases were released into the surroundings like Methane (CH4), Ammonia (NH3), Carbon dioxide (CO2), water vapor,
  • The ultraviolet (UV) radiations present in the sunlight acted upon water and broke up water into hydrogen and water and lighter H2
  • Oxygen readily combined with other gases like Ammonia and Methane to form again water and carbon dioxide and other gases. Therefore, free oxygen (like today’s environment) was not present on the early earth. Therefore, the early atmosphere was reducing, unlike today’s oxidizing atmosphere.
  • It was present in the form of compounds like water, carbon dioxide, etc. Oxygen also participated in the formation of the ozone layer which acts as a shield to save our present atmosphere from harmful UV radiations.
  • The water vapor condensed to form water drops which fell as rain and slowly filled the depressions present on the surface of the earth which is supposed to be the origin of large oceans, seen today.
  • But during all these changes, life took approximately 500 million (500×106) years to appear or we can say that life appeared 500 Million years after the formation of earth or almost 4 Billion (4×109) years back from now (as the earth was formed 4.5 billion years ago).

Many theories of evolution have been put forward by the scientist to support the origin of life which we shall discuss in the next topic.

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