Today air is composed of nitrogen and oxygen. When life first began hydrogen was the dominant gas in the atmosphere. Oxygen, so vital for growth and metabolism, was dissolved in the seas. It was the evolution of plants, not man, which released this oxygen into the atmosphere. Plants use sunlight, water, and carbon dioxide to form simple sugars. This process is called photosynthesis. The water molecule is split during the reaction releasing the oxygen. We humans are totally dependent on this property in plants for two reasons. Since we cannot use sunlight to form our own foods we must eat plants or animals that have eaten plants for energy, and since we must breathe oxygen we rely on plants to return the oxygen to the atmosphere.
Our dependence on water is easy to understand as it was the first provider of all materials for life, and it was the home for the first cell which developed in that primordial sea. As the cells developed from the early "live" molecules, it is believed they were able to construct a membrane or protective covering around themselves which enabled them to hoard/hold the nutrients and minerals in short supply and reject the ones they did not need. Today, living cells in our bodies have retained this property and work to maintain a distinct distribution of minerals inside and outside the membrane.
Cell fluids (the cytoplasm) contain potassium and magnesium ions while the fluids that surround them contain sodium and chloride ions and calcium. In a sense, we "brought the sea with us."
More than two-thirds of the human body is composed of water distributed among the cells, the tissue fluids that surround them, and the blood plasma. It is the tissue fluids and the blood plasma which so closely resemble the composition of the sea .
Today we can trace a much slower development from those first molecules and single cells to the vastly complicated multimillion-celled creature that is present-day mankind. Apart from their dependence on the sun and the sea those early cells were independent creatures. They were very small and could absorb, digest, and excrete all they needed from their surroundings by simple diffusion. They could use oxygen to metabolize their food, they could react to stimuli such as light and heat, and they could grow, divide, and reproduce. They were a jack-of-all-trade and had, within their cell, all that constituted life.
Water is the basis of all life and that includes your body. Your muscles that move your body are 75% water; your blood that transport nutrients is 82% water; your lungs that provide your oxygen are 90% water; your brain that is the control center of your body is 76% water; even your bones are 25% water.
Our health is truly dependent on the quality and quantity of the water we drink.