Basis for all living things every living thing contains this element

Carbon, all living things use carbon because we are all carbon based live forms. It could also be oxygen or carbon dioxide because we all use those to some extent. But we are all made of this element. Fun fact, it's also the most common element in the world

All living things contain carbon so an argument could be made that carbon was the basis. Or it could be oxygen because all living things in one way require oxygen to live, even plants and fish who pull oxygen from the water molecules.