By Dr David Whitehouse
BBC News Online science editor
"Grandparents" started to be much more common about 30,000 years ago, a study of fossilised teeth has revealed.
Are grannies the key to survival
Researchers at the Universities of Michigan and California looked at the ratio of older and younger adult teeth found at sites up to 100,000 years old.
Finding more older teeth in the Upper Palaeolithic suggests the grandparent role - being on hand to help out more - became more common at that time.
The research is in the Proceedings of the US National Academy of Sciences.
After studying more than 750 fossilised teeth anthropologists Rachel Caspari and Sang-Hee Lee noticed they were finding more specimens from older adults in more recent sites.
They defined "old" to be at least double the age of reproductive maturation, which is also the time when the third molars erupt.
By calculating the age of old-to-young individuals in the samples from successive time periods, the researchers found that the number of old people surviving quadrupled about 30,000 years ago.
The scientists say the presence of grandparents confers an important evolutionary advantage.
Grannie knows best
"We believe this trend contributed importantly to population expansions and cultural innovations that are associated with modernity," the researchers say.
It is also thought that living longer strengthened social relationships and kinship bonds, as grandparents educated extended families.
"Significant longevity came late in human evolution and its advantages must have compensated somehow for the disabilities and diseases of older age," say the authors.
"There has been a lot of speculation about what gave modern humans their evolutionary advantage," says Dr Caspari. "This research provides a simple explanation for which there is now concrete evidence: modern humans were older and wiser."