Obesity was probably rare in ancient times, with the current increase starting in the Industrial Revolution of the eighteenth century, and becoming much more widespread from about 1950, so concurrent with the increased consumption of carbohydrates from cereals in the Green Revolution. However, dietary components such as oligosaccharides from plants including cereals may improve health following fermentation to short-chain carboxylic acids in the intestine by bacteria which constitute of the microbiome. Such non-digestible and fermentable components of diet, called prebiotics, have been part of the human diet since at least Palaeolithic times, and include components of the cereals domesticated in the Neolithic Revolution. If consumption of these cereals has now increased, why is obesity increasing? One reason could be lowered prebiotic intake combined with increased intake of simple sugars, thus changing the bacteria in the microbiome. Processing of food has played an important role in this change of diet composition. Since obesity is a low-grade inflammation, changing the microbiome by increased consumption of simple carbohydrates and saturated fats may lead to obesity via increased systemic inflammation. Conversely, there is now reasonable evidence that increased dietary prebiotic intake decreases inflammation, improves glucose metabolism and decreases obesity. Would widespread increases in prebiotics in the modern diet, so mimicking Palaeolithic or Neolithic nutrition, decrease the incidence and morbidity of obesity in our communities?