有一种理论叫“姥姥奶奶假说 （grandmother hypothesis） ”

Grandmas Made Humans Live Longer

“The grandmother hypothesis says that when grandmothers help feed their grandchildren after weaning, their daughters can produce more children at shorter intervals; the children become younger at weaning but older when they first can feed themselves and when they reach adulthood; and women end up with postmenopausal lifespans just like ours.”