Excerpt:
Salt is physiologically absolutely necessary for human life, but in the past prior to the Industrial Revolution the known mineral salt sources were limited so much so, that its supply was a critical demographic power factor for most communities, until industrial means of extraction from brines were devised. It was only available as visible and exposed rock outcrops in arid regions, or as dried out salt cake on the shores of some seas and salt lakes. In areas with wet climates, the protruding salt dissolved making it almost impossible to discover. It is probably this, more than for any other reason, that many of the great civilisations first developed near deserts and desert climates, for example the Mediterranean region, at the edges of the "arid" zones.