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Darwin's theory of evolution explains how life evolved from simpler to more complex forms, and Mendel's tests explain how features are passed down from generation to generation. Neither, however, explains how life arose on earth in the first place.
Origin of earth and origin of the universe:
The origin of life is connected with the origin of earth. The most basic question which led to many active and vigorous discussions among the scientists and researchers is that when and how did the universe evolve?
Out of the numerous theories proposed to answer this question, the Big bang theory clearly explains the origin of the universe.
Big bang theory:
Georges Lemaitre
Georges Lemaitre, a Belgian cosmologist, is the father of the big bang theory.
According to this theory \(15\) billion years ago, the universe had a huge explosion of cosmic material that resulted in hydrogen and other nuclei which organised to form galaxies.
The universe started as a tiny spot of a hot and a massive ball.  The explosion threw the contents within the ball into all directions of space. 
Image illustrating the Big Bang Theory
After this big bang explosion, the universe began to expand and it continues to expand even today. In accordance with time the universe expanded and cooled, leading to the origin of life found on earth.
The sub-atomic particles such as neutrons, protons gave rise to atoms, stars, etc.
The solar system of the milky way was created by the collapse of the gaseous clouds due to the force of its gravity forming atoms and particles. These atoms, dust grains and gaseous discs aggregated to give rise to planets.
Solar system
A galaxy is a complex structure of millions or billions of stars, gas, and dust, kept together by gravity. The beginning of life is closely connected to the beginning of the earth.
Milky way galaxy
There were many theories and opinions about how life began on earth.
Earth is thought to have formed approximately \(4.5\) billion years ago and \(500 \)million years after the earth's origin, life appeared.
They lacked experimental proofs, however. The contemporary idea of origin of life, on the other hand, is largely recognised around the world.