‘On the Origin of Species’, a book written by Charles Darwin in 1859, is the foundational book of evolutionary biology. It explains how life forms, related through a common descent, evolve by the gradual process of evolution, leading to speciation (formation of new species), opposing the idea of creationism (the doctrine which assumes that living things are created by some divine intelligence).
So where does the life first come from? The Theory of Abiogenesis (more) explains that when the conditions in the primitive earth were perfect (electrical currents due to thunderstorms, medium, temperature and radiation), the protein (an organic compound) combined in a way that produced amino acid, a precursor of life. This impression that living things can be developed from organic compounds (non-living things) has also been proved by Miller-Urey experiment. Those primitive units of amino acid had self-replicating property. Now, along with the passage of time, these units struggled to adapt the conditions of the changing earth. Those which were not able to change and respond according to the conditions of the environment periled. Those which underwent changes accordingly, survived. There were competitions among these units as well and guided by the so called ‘survival of the fittest’, few who could struggle and fit to the environment better, survived. These adaptational requirements when integrated over a very long period of time, caused changes in the gene pool of the organism, making it a new species. Some creatures that were isolated geographically, underwent evolution in a different environment and finally evolved as a new species.
Since ancient time, people were at ease to accept the idea that we along with other life forms were created by the god because that would terminate further queries into this mystery like field. But the modern science which is a rational inquiry into the nature of things, has falsified this notion and revealed that we are the descendants of Saharan mammals who managed to evolve better especially in cognitive abilities.