Carbon

• Earth's organisms are said to be "carbon-based" because carbon atoms bond so well with the other elements essential to living things. Carbon bonds are not only stable, but reactive, making them ideal for the building blocks of life. Making long, complex molecule chains makes organisms possible, with proteins, nucleic acids (RNA and DNA), carbohydrates and fats. When carbon bonds to carbon, crystal lattices form in familiar structures like diamonds.
  
  

Hydrogen

• Hydrogen is the most common element in the known universe and two parts of the three-part molecule that composes the most common element on Earth's surface: water. Without water, it's hypothesized that no complex life would exist. As an example of the vital role water plays in the sustenance of life, the human body is made up of 55 to 78 percent water.
  
  

Oxygen

• Oxygen is another highly reactive element that, when bonded to itself, forms the aerobic environment that terrestrial organisms need to breathe. It is the third most common element in the known universe. On the planet Earth, it is the most common element, as breathable air and all water contains oxygen. The respiratory acts of organisms diminish and replenish the oxygen levels in the atmosphere in a cyclical, self-sustaining fashion.
  
  

Nitrogen

• Nitrogen exists in all life forms and is essential to biochemistry. It forms part of the respiratory cycle and provides nutrients for plant matter that in turn sustains other living organisms. Nitrogen helps create complex organisms by being essential to the formation of nucleic (especially RNA and DNA) and amino acids.