This image shows a Hazda couple and child in northern Tanzania in 1985. University of Utah anthropologist Kristen Hawkes and colleagues' research on Hazda led them to formulate the 'grandmother hypothesis' that grandmothering among early human relatives made the human lifespan evolve so it is much longer than lifespans of other apes. Now, a new study credits grandmothering for the human tendency to form couples or pair bonds. Photo credit: James F. O'Connell, University of Utah
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