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Before humans started to grow their own crops and domesticate animals, food supplies were far less reliable and famine was an unavoidable part of life. When famine came along, those who had more body fat were more likely to overcome the difficult period than slim people The thin ones would either die in the famine or their health would be affected and they would have fewer children. The ones who weren't underweight would then, in turn, be more likely to produce children who shared this tendency to gain weight when times were good. This genetic trait has been passed on through evolution, so that today many people have a natural tendency to gain weight. However, in the absence of recurrent famines, this is no longer such a good thing, particularly when you consider that people today do far less physical work than they used to. The easy availability of food with a high sugar and fat content, coupled with inadequate physical activity, lies at the root of the general increase in weight in the population.