Salt is a fundamental nutrient. It helps preserve food, but it also also essential for life. Because of the way our tongues work, salt also often causes other flavors to taste more strongly.
Pepper became common in the 15th and 16th centuries when direct trade opened between Europe and the East. Why? Because of all the spices, pepper was one of the cheapest. A strong flavor plus low cost equaled a common spice.
Salt is most likely oldest,cheapest and easiest to obtain spice regardless of where you are. Also is the easiest to use food preservative (very,very important factor). All other spices have to be imported from southern countries or from very specific locations where it can be grown. Than makes them far more expensive. They were used as currency for a very long time.
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