On our planet, plants utilize the energy of sunlight to manufacture sugars and other essential organic compounds by a process known as photosynthesis. Photosynthesis converts radiant energy from the Sun into a form of stored chemical energy that plants can use. But along the way, photosynthesis also generates an important waste product: oxygen. In other words, the same process that supplies the energy plants need, just happens to also produce a waste product that supplies energy to humans and many other animals.
For more information about photosynthesis, and how it makes human life and the life of other organisms possible, view The Privileged Species or read Chapter 6, “The Vital Gases” in Michael Denton’s book Nature’s Destiny.