Despite there being 118 known elements, only a handful of them are known to be found in living organisms. Indeed, the immense complexity of life is made up almost entirely of four elements: carbon, hydrogen, oxygen and nitrogen; approximately 99 percent of the human body is made up of these elements.
All known living organisms on Earth are carbon-based organisms. Carbon is essential to life because of its ability to hold up to four stable bonds at a time, meaning it can form a larger variety of molecules and compounds than any other element found in living organisms, and thus it is often in the middle of a complex chain of elements. Because of this feature, carbon is found in all essential fats, proteins and is the basis for DNA and RNA.
Much like carbon, hydrogen is ubiquitous to molecules that form the basic components of life. Indeed, hydrogen is essential because it easily bonds with carbon. This is because hydrogen effectively serves as a linking element between the carbon base of a chain and other elements; hydrogen's high level of electronegativity is what allows it to play this role. Frequently hydrogen will end up linking carbon to more hydrogen atoms, and this continuing chain of hydrogen atoms is what creates the level of complexity needed to create an organic molecule (e.g., a fat or protein).
Oxygen serves several purposes in a living organism. Much like hydrogen, it combines easily with carbon, and because a neutral atom of oxygen has eight electrons, an oxygen atom easily combines with more hydrogen atoms to create a complex chain when forming fats or proteins. Additionally, oxygen (along with hydrogen) is found in water, which is essential for a living organism because many chemical reactions within a living organism occur in water, and water is also the primary median in which cells live.
Despite only making up about 1 percent of a human being's total atoms, nitrogen is essential to both human and other organic life because it, alongside carbon, is found in all proteins. A protein is a compound that is used in cells to signal what actions the cell needs to take; effectively, proteins translate the passive codes of the DNA into actions. Nitrogen is also similar to oxygen in its ability to bond with several hydrogen atoms; a neutral nitrogen atom has seven electrons.