Throughout hominid history, it was exceedingly rare for individuals to live more than 30 years. Paleoanthropologists can examine teeth to estimate how old a hominid was when it died, based on which teeth are erupted, how worn down they are, and the amount of a tissue called dentin. Anthropologist Rachel Caspari of Central Michigan University used teeth to identify the ratio of old to young people in Australopithecenes from 3 million to 1.5 million years ago, early Homo species from 2 million to 500,000 years ago, and Neanderthals from 130,000 years ago. Old people—old here means older than 30 (sorry)—were a vanishingly small part of the population. When she looked at modern humans from the Upper Paleolithic, about 30,000 years ago, though, she found the ratio reversed—there were twice as many adults who died after age 30 as those who died young.

The Upper Paleolithic is also when modern humans really started flourishing. That’s one of the times the population boomed and humans created complex art, used symbols, and colonized even inhospitable environments. (The modern humans she studied lived in Europe during some of the bitterest millennia of the last Ice Age.) Caspari says it wasn’t a biological change that allowed people to start living reliably to their 30s and beyond. (When she looked at other populations of Neanderthals and Homo sapiens that lived in the same place and time, the two different species had similar proportions of old people, suggesting the change was not genetic.) Instead, it was culture. Something about how people were living made it possible to survive into old age, maybe the way they found or stored food or built shelters, who knows. That’s all lost—pretty much all we have of them is teeth—but once humans found a way to keep old people around, everything changed.