“WHAT makes us modern is the contribution of older folks.” A contrary view, perhaps, in today’s society, with its focus on youth culture, modernity and fashion. But the appearance of grandparents may have facilitated the birth of modern civilisation.
Anthropologist Rachel Caspari of the University of Michigan in Ann Arbor reached that conclusion after studying the age of adults living in human societies over the past few million years. The study is the first to show that a dramatic increase in the number of adults surviving to an older age coincided with the appearance of artistic, inventive modern humans in the Early Upper Palaeolithic around 17,000 to 30,000 years ago.
The finding also provides the first statistical evidence supporting the so-called “grandmother hypothesis”, the idea that grandparents accelerate the appearance of successful, civilised communities by helping to rear more children, passing down valuable knowledge and experience, and promoting kinship across the generations.
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Caspari and her colleague Sang-Hee Lee of the University of California, Riverside, examined teeth from 768 ancient skulls collected over 15 years from sites around the world. By examining the extent of wear and tear on the teeth, and the appearance of wisdom teeth – a sign of sexual maturity – the researchers could estimate the age of each individual. For instance, newly mature adults have barely worn wisdom teeth compared to those of adults twice their age.
They found that the proportion of hominids that survived to twice the age of sexual maturity, the age at which they could theoretically become grandparents, rose steadily but slowly for almost 3 million years. For instance, of 353 australopithecine skulls studied, just 37 were old enough to be grandparents – a ratio of about 1 in 10. These early ancestors lived between 1 and 3 million years ago, and although bipedal, they were short, with relatively small brains. Roughly 1 million years ago, Homo species appeared, with brains twice the size of their forbears. Around 1 in 4 of these individuals were potential grandparents.
By the time Neanderthals thrived 130,000 to 30,000 years ago, older members were exerting their influence, with 4 out of 10 individuals surviving into later life. Neanderthals were much more advanced than their forbears. “They were bright, had burials, had very large brains and were very similar to us, although with high brow-ridges,” Caspari says.
But the biggest change came with the appearance of Stone Age humans, who began making tools in the Early Upper Palaeolithic. This group had a disproportionate population of potential grandparents, around two for every younger adult (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas. 0402857101). That is four times the rate seen among Neanderthals, and 17 times the rate in australopithecines (see Graphic). “We were really surprised by the increase. It was a eureka moment,” Caspari says.
“I’m flabbergasted to see the magnitude of the difference between Upper Palaeolithic people and Neanderthals,” says Kristen Hawkes, an anthropologist at the University of Utah, Salt Lake City. Her colleague James O’Connell says Caspari’s findings support his own theory that it was modern humans’ ability to survive through population expansion, rather than superior mental ability or language skills, that allowed them to displace Neanderthals.