The callitrichines are known for twinning and for a communal rearing system in which all or most group members help care for the offspring. The origin of twinning has been the subject of much speculation. In this study predictions from earlier hypotheses are tested on the basis of two alternative phylogenetic trees. From this analysis we infer that helping behavior and male care preceded the origin of twinning, and that these traits did not coevolve with, but might have been important prerequisites for twinning in callitrichines. Small body size does not necessarily result in twinning, although it might still have been a prerequisite for its evolution. Gum feeding was an ecological change which evolved along with twinning. If nutrition was a limiting factor in the number of offspring produced, then the use of a new feeding resource could have been crucial for the origin of twinning in callitrichines. According to one of the two alternative solutions inferred by the total evidence tree, and in accordance with the morphological tree, semi-annual breeding appears in the marmosets together with specialization in gum feeding. The fact that gums are available for these monkeys all year may have facilitated semiannual breeding. We suggest that the exploitation of gums as a feeding resource could have been the decisive factor in the increase of the reproductive rate by twinning and by semi-annual breeding.