Sellayah and colleagues have postulated an 'Out of Africa' theory to explain the evolutionary origins of obesity. The theory cites diverse ethnic based differences in obesity susceptibility in western civilizations to contend that, neither the thrifty or drifty gene hypotheses can explain the demographics of the modern obesity crisis. Although the arguments against these patterns arising due to 'drift' are unclear. Sellayah et al. argue that ethnic groups whose ancestors were adapted to hot climates have low metabolic rates due to lack of thermogenic capacity, whereas those groups whose ancestors were cold-adapted were endowed with greater thermogenic capacity and higher metabolic rates. Sellayah and colleagues provide evidence of thermogenic capacity, metabolic rates and obesity prevalence in various indigenous populations in support of their argument. Contrasting this analysis however a study of the spatial distribution of obesity across the mainland USA showed that once the effects of poverty and race were accounted for there was no association between ambient temperature and obesity rates.