The initial fitness benefits of group living are considered to be the greatest hurdle to the evolution of sociality ¹ , and evolutionary theory predicts that these benefits need to arise at very small group sizes ² . Such benefits are thought to emerge partly from scaling effects that increase efficiency as group size increases ^3-5 . In social insects and other taxa, the benefits of group living have been proposed to stem from division of labour ^5-8 , which is characterized by between-individual variability and within-individual consistency (specialization) in task performance. However, at the onset of sociality groups were probably small and composed of similar individuals with potentially redundant-rather than complementary-function ¹ . Self-organization theory suggests that division of labour can emerge even in relatively small, simple groups ^9,10 . However, empirical data on the effects of group size on division of labour and on fitness remain equivocal ⁶ . Here we use long-term automated behavioural tracking in clonal ant colonies, combined with mathematical modelling, to show that increases in the size of social groups can generate division of labour among extremely similar workers, in groups as small as six individuals. These early effects on behaviour were associated with large increases in homeostasis-the maintenance of stable conditions in the colony ¹¹ -and per capita fitness. Our model suggests that increases in homeostasis are primarily driven by increases in group size itself, and to a smaller extent by a higher division of labour. Our results indicate that division of labour, increased homeostasis and higher fitness can emerge naturally in social groups that are small and homogeneous, and that scaling effects associated with increasing group size can thus promote social cohesion at the incipient stages of group living.