Pre-print, Working paper: The labour input among pre-historic foragers was normally rewarded within the same day of the effort. For the first farmers, by contrast, labour input and its rewards could be far apart. However, the patience was worthwhile: population growth rates among early agriculturalists were up to 60 times higher than those of their foraging counterparts. It is well-known from the biological science that humans differ with respect to metabolism. This study argues that rates of metabolism well-suited for the many hours of labour input required for farming gained an evolutionary advantage with the advent of agriculture. This theory helps shedding light on the puzzles why farming was adopted despite its high labour costs, and why people of agricultural societies work more than their foraging counterparts.