The theory of Evolution! (simple)
The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms, these simple life forms first developed more than 3 billion years ago (the earth is 4.5 billion years old).
The fundamental theory behind evolution is that it happens because of Natural Selection (survival of the fittest). Natural Selection is basically when certain individuals in a species adapt to be suited to their environment, which means they are more likely to survive and reproduce (have children). These individuals in a species show a wide range of variation (differences), this variation is because of a difference in genes. These different genes are basically mutations, for example in Xmen (a popular film franchise) the mutants have different genes to humans and therefore have 'powers', these 'powers' make them stronger than normal humans. The genes that allow this certain group of individuals to survive are passed down to their offspring (children). The individuals that don't adapt to their environment are less likely to survive and reproduce, meaning that the evolved individuals will become the dominant ones in that species.
(Note: If there is anything you would like me to add or change please do say so, and if you want more info on evolution I advise you to read Charles Darwin's, The Origin of Species or look up evolution online. Also some of this information is not mine as I did use a few websites as a reference. Finally if you're wondering why i decided to write about the theory of evolution it is because I did it today in class and it really interested me so I decided to share it with you guys! )