In a recent article "Why Aging isn't Inevitable" (http://nautil.us/issue/36/aging/why-aging-isnt-inevitable), the author suggested that the the "Grandmother Hypothesis" sounds reasonable, "but a number of demographic researchers have found that when they do the numbers, it’s hard to make it work".

He suggests an alternate theory: "An older, “retired” segment of the population, we argued, serves to keep the population stable over cycles of feast and famine."