The six most common elements in living organisms, making up over 99% of their mass, are:

1. Oxygen (O): Found in water, carbohydrates, proteins, and fats.

2. Carbon (C): The backbone of all organic molecules, including carbohydrates, lipids, proteins, and nucleic acids.

3. Hydrogen (H): Present in water and most organic molecules.

4. Nitrogen (N): Found in proteins and nucleic acids (DNA and RNA).

5. Calcium (Ca): Important for bones, teeth, and muscle function.

6. Phosphorus (P): Found in nucleic acids, ATP (energy currency), and bones.