Which substance is considered a building block for all living things?

The answer is carbon. Organic chemistry is the chemistry of carbon and hydrogen. The proteins, made of aminoacis are fundamental are the base of the chemical compounds and reactions that occur in the living organisms. Proteins are long chains of aminoacids. Aminoacids are made of carbon, hydrogen and oxygen. Carbon is the element that permits to build the long molecules thanks to its ability to form several (up to four) bonds with many elements, including other atoms of carbon. Is this ability to have multiple single bonds, or double and triple bonds what confer carbon the possibility to build so many different compounds being considered a building block of living things.