We have grandmothers to thank for longevity, monogamous mating, and the tendency for older men to go after younger girls, according to a University of Utah anthropologist. Kristen Hawkes made waves with her “Grandmother Hypothesis,” which claims that long human lifespans can be chalked up to the role grandmas played in raising their grandkids. Expanding on her theory, her latest study, which will be published in the September 7 edition of the Proceedings of the National Academy of Sciences, suggests that our evolutionary grandmothers’ influence led to a surplus of older-yet-still-fertile men who figured the best way to pass on their genes was to hook up with younger, childbearing females.

The idea behind Hawkes’ longevity argument is that if grandmothers were around to take care of grandkids, mothers were free to pop out more babies, sooner. They became such an integral part of child-rearing that evolution favored a grandma that was longer-lived — and if her offspring survived, chances are they would be carrying those same genes for a long life.

Now, through computer simulations of the evolution of societies with and without grandma’s influence, Hawkes found that human’s longevity led to a surplus of older, fertile men who, naturally, grew competitive over younger females that might pass on their genes. Guys with a preference for younger females were more likely to leave descendants.

For older dudes, guarding their mates from other horny suitors became necessary. This protective behavior, Hawkes suggests, is what led to “pair bonding” — a term referring to the mostly exclusive (but not necessarily long-term) relationships found between men and women.

The hypothesis runs counter to the idea that pair bonding stemmed from women being dependent on male hunters to support them. For Hawkes, when it comes down to discussing long life — and long, sometimes-creepy relationships — everything comes back to grandma.

Photos via https://www.flickr.com/photos/stevendepolo/, Unsplash / Jeremy Wong