All animals are dependent upon plants, since plants are the organisms at the base of the food chain,because of their capacity to photosynthesize—that is, to turn water and carbon dioxide into oxygenand sugars, in the presence of sunlight. As life on Earth gradually evolved from simple unicellularorganisms to the variety of organisms we know today, the complexity of interactions betweenplants and animals increased, but plants remained the basis for life on Earth. Plants support all animallife. Humans are no exception to this rule, and we are just as dependent upon plants as anyother animal. We depend on plants not only for their role in producing the oxygen we breathe, butalso for food, shelter, medicines, clothing, and countless other uses.