Many of today’s families enjoy the babysitting duties most grandmothers contribute willingly and lovingly. An anthropologist from Utah suggests grandmothers have contributed much more to human evolution than feeding babies and changing diapers. Her “grandmother hypothesis” suggests monogamy and longevity exist today because grandmothers of long, long ago stopped being fertile decades before they reached the end of their natural lifespans. That by losing their own fertility to menopause, grandmothers actually generated greater numbers of offspring in their bloodlines and made it possible for humans to fall in love and live 80+ years today.

Grandmother Hypothesis

Anthropologist Kristen Hawkes of the University of Utah says her grandmother hypothesis contradicts the traditional theory that mate-bonding behaviors (monogamy) evolved from a trade-off between providing food and claiming descendants. The common theory is that males paired with one female by feeding her and her children so he could father her children and advance his genetic contribution to future generations.

Hawkes theorizes it was grandmotherly child-rearing assistance, not the food itself, that allowed females to have more babies sooner than without grandma’s help. When grandma took over feeding duties for weaned grandchildren, her daughter was free to have more babies with shorter intervals in between.

Chimpanzees and Modern-Day Hunter-Gatherers

Hawkes tested her theory using computer-simulated models based on how two specific populations would evolve over 30,000 years:

• Chimpanzees, humans closest genetic relatives. Chimps are promiscuous and do not form male-female bonds. Grandmother chimps don’t babysit their children’s offspring.
• Modern-day hunter-gatherer societies, which do form male-female monogamous bonds and include grandmothers that do tend to their children’s children.

The Effect of Menopause

• When Hawkes ran her computer models with grandmothers who retained fertility all their lives (no menopause), both societies evolved into female-dominant societies like today’s chimp societies. Females eventually outnumbered males, as is typical of a chimpanzee colony.
  
  
• When Hawkes’ computer-simulated grandmothers did experience menopause, the ratio of fertile males to fertile females skyrocketed, to an eventual stable ratio of 156 fertile males to 100 fertile females.
  
  
• When the fertile male population is so much larger than the fertile female population, males become much more guarded of their female mate. As the natural human lifespan lengthened over time, the number of older fertile males in a population grew even larger and competition for fertile females became more fierce.

Romance Blossomed

Promiscuous males fathered fewer children than males who bonded within a closed relationship with a female or a family containing several fertile females. Over time, the male’s mate-guarding behaviors became more complex than survival and procreation; affection and romance blossomed and fertile couples fell in love.

Longevity and Grandmother’s Genes

As the role of grandmothers became more valuable to the survival of small children, grandmothers started living longer after menopause. They became as valuable to the society infertile as when they were fertile. Since they lived longer, they could take care of more grandchildren, freeing up their children to have more children in quicker succession.

Human longevity increased with each generation as grandmother’s genes for long lifespan were passed forward. Hawkes’ computer-simulated society with infertile grandmothers eventually reached lifespans of 70 or 80 years, as happens in real life today.


Sources:

1. Coxworth, James E., Peter S. Kim, John S. McQueen, and Kristen Hawkes. "Grandmothering life histories and human pair bonding." PNAS / Proceedings of the National Academy of Sciences of the United States of America (2015). Web. 10 Sep. 2015.
2. Kelly, Diane. "Menopause May Be the Evolutionary Origin of Romantic Relationships — And Cheating." Gizmodo / Throb. Gizmodo, 8 Sep. 2015. Web. 10 Sep. 2015.