The main constituent of human bones is calcium. Calcium plays a vital role in maintaining bone health and strength. It helps in the formation and maintenance of bone structure. It is responsible for providing rigidity and hardness to the bones. Calcium is also involved in important functions such as muscle contraction, nerve functioning, and blood clotting. Phosphorus, zinc, and iron are also important for overall health, but they are not the main constituents of human bones. Phosphorus is another essential mineral that works closely with calcium in bone formation. Zinc is important for growth and development, while iron is essential for transporting oxygen in the blood. However, neither of these minerals is the primary constituent of human bones.