Phytoplankton and zooplankton have been producing vitamin D for more than 500 million years. While the function of vitamin D in the physiology of lower non-vertebrate organisms is not well understood, it is known that most vertebrates need vitamin D to develop and maintain a healthy mineralized skeleton. However, recent findings have demonstrated that 1,25(OH)2D, the biologically-active vitamin D metabolite, exerts a multitude of important physiological effects independently of the regulation of calcium and bone metabolism. These new functions of vitamin D include protection against cancer and other diseases in various tissues. In this review, current knowledge of an additional new function of the cutaneous photosynthesis of vitamin D, that has recently emerged, is summarized: the role of vitamin D as an evolutionary highly-conserved endocrine system that protects the skin and other tissues against environmental hazards, including ionizing and UV-radiation, microbial infections and oxidative stress, is discussed.