Salt, also referred to as table salt or by its chemical formula NaCl, is an ionic compound made of sodium and chloride ions. All life has evolved to depend on its chemical properties to survive. It has been used by humans for thousands of years, from food preservation to seasoning. Salt's ability to preserve food was a founding contributor to the development of civilization. It helped eliminate dependence on seasonal availability of food, and made it possible to transport food over large distances. However, salt was often difficult to obtain, so it was a highly valued trade item, and was considered a form of currency by certain peoples. Many salt roads, such as the via Salaria in Italy, had been established by the Bronze Age.