Debunking earlier theories, anthropologists now show that unlike in the past when the entire village came to raise a child, older children are helping rear younger siblings in the family nowadays.
Mothers underwent a remarkable transition from the time when they needed support of their young at weaning and received no help from others to the present when mothers often have multiple kids who help rear other children, the study showed.
The team found that early in that transition, it was a mother's older children who helped to raise her younger children and only with more modern life histories did mothers also need the cooperation of other adults.
Before the transition, according to Kramer, mothers likely nursed children until the age of five to six, did not nutritionally support children after weaning and received no help in raising the children.
"Human mothers are interesting. They are unlike mothers of many other species because they feed their children after weaning and others help them raise their children," Kramer noted.
Deep in the past, mothers likely received no help and consequently had much lower rates of fertility and lost many children.
Not only do mothers work hard to care for their young, but so do her older children, grandmothers, fathers and other relatives.
But this was not always the case, the authors concluded.
Kramer's study found that it was a mother's own children who were the most reliable as helpers during transition.
Most earlier hypotheses by other anthropologists about who helped mothers in ancient societies point to other adults.
The findings appeared in the Journal of Human Evolution.