Archaeologists analyzing food remains on the inside of ancient pottery discovered that 6000 years ago humans seasoned their food with mustard seeds (article). From other food remains in the pottery shards, they can tell that the mustard was added to meat from land animals and marine life as well as starchy plants. Mustard is not a significant source of calories or nutrients, so the seeds were probably a flavor enhancer. Previously, the earliest known use of seasoning was 4500 years ago in Northern India where turmeric and ginger were used.
No actual mustard can be found in the pottery after 6000 years. Scientists studied phytoliths, silica crystals that remain when plants decay, and determined the phytoliths are characteristic of mustard plants.