The Earth is the only planet in the solar system which sustains life. But earth was not the same as it is today. It was a ball of gases when it was first formed about 4.5 billion years ago.
Earth cooled and chemicals of life were synthesized in water - chemical evolution. Primitive atmosphere had gases like CH4, NH3, H2 and CO2 and water vapour. Temperature was extremely high. Earth cooled, water vapour condensed and there were torrential rains. Water collected on the earth’s crust into which were washed down these gases. They combined to yield biomolecules like nitrogenous bases, glucose and amino acids which further formed proteins and nucleic acids.
Biodiversity came to exist through biological evolution for which a valid mechanism termed Natural Selection was provided by Charles Darwin (Father of Modern Evolution) which was later modified in the light of progress in various fields of biology and called Neodarwinism.
Charles Darwin emphasized that all (i) kinds of organisms are related through ancestry and (ii) that Natural Selection is the force which causes evolution. It is not the individual which evolves but the entire population of that area evolves. So, the unit of evolution is a population. It has its own gene pool.
Variation arises in the gene pool through mutation and recombination. Natural selection causes greater reproduction of beneficial variant genes.
Fossils are remains of plants and animals that have been preserved in rocks or oil or amber. A study of fossils tells how life on earth has changed or evolved. Fossils form evidence for Biological evolution.