Any student who took Anthropology 101 with me should remember our lecture over the dawn of agriculture and civilization. In spite of thought to the contrary, civilization did not bring with it toilet paper and toothpaste. In fact, agriculture and its accompanying sedentary (vs nomadic) lifestyle had an extremely negative impact on our overall health, quality of life, and life-span.
Along with raising crops, the human diet became extremely limited – most early peoples lived on only two or three staple crops. Your grandmother was right, variety is the spice of life – limited food stuffs mean limited nutrients, vitamin deficiencies, and (with the rise of grain agriculture) a destruction of our dental health. When we look at the skeletons of early agricultural peoples, their bones are marred with distinctions of malnutrition, vitamin deficiencies, dental caries (fancy word for extremely large and painful cavities), and we shrunk (several inches shorter than our hunter-gatherer counter-parts). We also worked a lot harder – both physically and in terms of hours.
For more on recent findings about this topic, check out this Science Daily Article.