All living organisms use the same four types of macromolecules for cellular metabolism and reproduction. Together, they illustrate the commonalties of life on earth. The way they are used in different forms and combinations explains today's variety or biodiversity. Both aspects, sameness and variety, are the result of biological evolution. An interesting link between hierarchical organization and chemistry is the combinatorial nature of living things. With this I mean that cells are made of macromolecules, macromolecules of molecules and molecules of elements.