Salt is naturally found in the environment, and thus it was never invented. It is composed of nitrogen and chlorine, both of which are so volatile that they combine to form a new compound.
Salt has been an important resource throughout history as it was first used as a preservative and seasoning. Before refrigeration, humans had to preserve their foods in other ways. Salt acts as a preservative because it draws water out, which helps sustain foods like meat, and it also kills microorganisms so they don't contribute to the spoilage of food.
As early as the Iron Age, the British would boil down seawater to draw out raw salt. It was also collected from mineral deposits of salt that formed where saltwater had evaporated.
Human cultures formed around places where salt was naturally occurring, according to a report by Cargill, and the substance quickly became essential to civilization and trading. It was used as currency across the world by people, including the Romans and Tibetans. It was so important in Rome that words used today, like "soldier" and "salary," have their roots in Latin words related to salt as currency.
In the past, it has even been taxed to pay for wars or the wants of the monarchy; the British monarchy, for instance, taxed salt heavily, which resulted in citizens being prosecuted for smuggling salt into the country in the 18th century.