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But when people did start to survive into older age, it had "far-reaching effects" that led to the development of new tools and art forms.
The advantages that humans enjoyed by having larger families with older relatives could have helped them "out-compete" rivals such as Neanderthals, it is claimed.
A feature in the magazine Scientific American concludes: "The relation between adult survivorship and the emergence of sophisticated new cultural traditions, starting with those of the Upper Paleolithic, was almost certainly a positive feedback process.
"Initially a by-product of some sort of cultural change, longevity became a prerequisite for the unique and complex behaviours that signal modernity. These innovations in turn promoted the importance and survivorship of older adults, which led to the population expansions that had such profound cultural and genetic effects on our predecessors. Older and wiser, indeed."
In the article, Rachel Caspari describes how analysis of the teeth of Neanderthals found in Croatia, who lived about 130,000 years ago, suggests "no one survived past 30".
Because of gaps in the fossil record, she and colleagues tried to estimate when grandparents became common by working out how many individuals from different prehistoric groups reached 30.
They calculated the ratio of older to younger adults - the OY ratio - in fossil samples of 768 individuals spanning 3million years, stretching back from the most primitive australopithecines to modern Europeans of the early Upper Paelolithic, who lived between 30,000 and 20,000 years ago.
The researchers found that for every 10 young adult Neanderthals, who probably died between 15 and 30, there were just four who survived past 30 and so lived long enough to see their children have babies themselves.
Among the modern European group, however, there were 20 potential grandparents for every 10 young adults, concluding that "adult survivorship soared very late in human evolution".
The researchers say they do not know why so many of this group started to live longer, but once they did it helped their societies greatly.
Grandparents were able to contribute "economic and social resources", which in turn led to members of the group having more offspring, but also made families stronger and transmitted "cultural knowledge".
By passing down knowledge of what plants were poisonous or how to sharpen a stone blade, elders would have improved early humans' skills and helped them thrive.
If grandparents back then were like grandparents today, then they loved their grandkids.
By assuming some portion of child care functions, grandparents would have freed time for parents.