Dr. Kristen Hawkes, University of Utah – Grandmothering and the Human Lifespan

Jan 1, 2013

In today’s Academic Minute, Dr. Kristen Hawkes of the University of Utah reveals how the grandmothering impulse has contributed to the length of the human lifespan. 

Kristen Hawkes is Distinguished Professor of Anthropology at the University of Utah where her research is focused on human life history evolution. Her current approach is guided by the hypothesis that grandmothering is a fundamental shift in our genus underlying a suite of key features that distinguish humans from other great apes. She holds a Ph.D. from the University of Washington.

About Dr. Hawkes

Dr. Kristen Hawkes – Grandmothering and the Human Lifespan

Childbearing ends at the same age in women and other female great apes. But other apes grow frail and usually die during the childbearing years while we stay healthy well beyond menopause. Compared with our ape cousins, human children wean earlier but take longer to become adults.

The Grandmother Hypothesis proposes that the timing of these life events evolved as the climate dried in ancient Africa, and fruits that little kids could easily pick became less available.  Before that kids would have been feeding themselves after weaning like other ape children do.  But as the climate changed, mothers who didn’t follow the retreating forests had to feed their children longer.  When ancestral grandmothers helped, their daughters could have next babies sooner.  Grandmothering favored longer lifespans because longer-lived grandmothers left more descendants.

To see whether grandmothering could really change life history that way we built mathematical simulations that started with an ape-like model: that is much shorter lifespans so very few grandmothers in the beginning.  But those grandmothers made increased longevity beneficial to both sexes and chimpanzee-like life histories evolved into human-like ones.

Our simulations didn’t include brains, learning or fathers’ help – things often thought to explain human life history. The simulations show grandmothering alone could have done it, and set the stage for other distinctive human features.  

Social life is changed by grandmothering because moms then have new babies before the older ones are independent.  That means kids no longer have their mother’s undivided attention; and that means infants better at actively engaging it themselves survive better.  Grandmothering not only gave us longer lives but maybe our appetites for social engagement and cooperation as well.