Origin of Life
The universe is about 20 billion years old. The earth was formed about 4.5 billion years ago. The atmosphere was not present on early earth. The earth was covered with water vapor, methane, carbon dioxide and ammonia. The UV rays from the sun split water into hydrogen and oxygen. The oxygen reacted with ammonia and methane to form water, carbon dioxide and other gases. When the earth cooled down further, water vapor fell as rain; to form oceans. Life appeared on earth about 4 billion years back.
Early Greek thinkers thought that units of life (called spores) were transferred to different planets including earth. Panspermia is a hypothesis which says that life came from outside the earth, i.e. from another celestial body.
Some thinkers believed that life came out of decaying and rotting matter; like straw, mud, etc. But Louis Pasteur disproved this theory by proving that life comes only from pre-existing life.
Oparin and Haldane proposed that the first form of life came from pre-existing non-living organic molecules; like RNA, protein, etc. They proposed that formation of life was preceded by chemical evolution. Diverse organic molecules were formed form inorganic constituents during the course of chemical evolution.
Experiment by Miller and Urey
They created an atmosphere in laboratory; similar to what existed on the earth about 4 billion years ago. A closed tank was filled with CH4, H2, NH3 and water vapor. They created electric discharge at 800°C in the tank. Miller observed the formation of amino acids. Observations by some others showed formation of sugars, nitrogenous bases, pigments and fats. These experiments proved the hypothesis of Oparin and Haldane. Thus, chemical evolution became an accepted theory of origin of life.
The first non-cellular forms of life could have originated about 3 billion years ago; in the form of giant molecules, e.g. RNA, protein, polysaccharides. The first cellular form of life possibly did not come into existence till about 2000 million years ago. Thus, first form of life gradually evolved from non-living molecules. This is the most accepted theory on origin of life on earth.
Charles Darwin's Theory
Any population has built in variation in characteristics. The characteristics which enable an organism to survive better in natural conditions would enable the organism to outbreed others.
Darwin proposed the concept of survival of the fittest. Here, fitness ultimately means reproductive fitness. Organisms which are better fit in an environment leave more progeny than others.
Darwin proposed the concept of natural selection. More progeny results in better chances of survival. Such organisms are selected by nature.