From the Into to this article from 2012:

“According to a study published today in the journal Proceedings of the Royal Society B, the answer is grandmothers. ‘Grandmothering was the initial step toward making us who we are,’ says senior author Kristen Hawkes, an anthropologist at the University of Utah. In 1997 Hawkes proposed the ‘grandmother hypothesis,’ a theory that explains menopause by citing the under-appreciated evolutionary value of grandmothering. Hawkes says that grandmothering helped us to develop ‘a whole array of social capacities that are then the foundation for the evolution of other distinctly human traits, including pair bonding, bigger brains, learning new skills and our tendency for cooperation.'”