The elemental composition of living matter differs markedly from the relative abundance of elements in the earth's. Hydrogen,oxygen,carbon,and nitrogen constitute more than 99%of the atoms in the human body,with most of the H and O occurring as H2O.The elemental composition of living matter differs markedly from the relative abundance of elements in the earth’s. Hydrogen, oxygen, carbon, and nitrogen constitute more than 99% of the atoms in the human body, with most of the H and O occurring as H2O