How the emergence of grandmothers helped build monogamy
You may remember the saving moment when granny arrived on the scene, a few days after the first baby decided (your) sleep was overrated. Grannies still know how to hold and soothe a baby, and many of them cannot think of anything that makes them happier. Grannies also, conveniently, biologically need sleep a little less than younger women.
While in modern times it can feel like they save young parents' lives, in ancient times, they actually did save lives. Families who had a grandmother around, that is, a woman who would no longer bear children but could look forward to another twenty-odd years of life, had better survival chances. Granny could look after older children while mummy had a new baby at the breast, and daddy was free to go hunting. Families whose genes supported such a lifecycle, i.e. the end of female fertility during healthy years, were favored by evolution.
As a recent PNAS article explains in more detail, an increase in life histories involving grandmothering had another beneficial side effect. In societies with active grandmothers as described above, fertile males would naturally outnumber fertile females. As mentioned previously here and here, this also means, (fertile) women had higher bargaining power than men. Men needed to compete for fertile females; the latter could choose and thereby call the shots.
In a world where women call the shots (also see here), a couple of things happen to relationships and family life. You have empirically a higher incidence of monogamous marriage, higher earnings for men and higher wealth for young parents. - All symptoms of men working hard to obtain the favors of women.
In short, the emergence of grandmas has very likely helped couples form a strong and stable bond (as desired by -the newly powerful- women.)
With this in mind, a warm thanks to all grandmothers, and well wishes to all grandparents, on the occasion of National Grandparents' Day, 13th September!