The study of the teeth of various hominid species from 3 million years ago to the present day reveals how average human lifespan has dramatically changed just twice in our evolutionary history. The first time was as recently as 30,000 years ago – prior to this, it was extremely rare for humans to live more than 30 years. The second time has been in the past 150 years, when the average lifespan has doubled in most parts of the world.
Anthropologist Rachel Caspari has spent years examining the teeth of various hominid species in existence from up to 3 million years ago to estimate how old individuals were when they died, based on which teeth were erupted, how worn down they were, and the amount of dentin present under the tooth enamel. From this data, she has estimated the ratio of young to old in populations of Australopithecenes which existed from 3 million to 1.5 million years ago, other early Homo species from 2 million to 500,000 years ago, Neanderthals from 500,000 years ago, and modern humans from 200,000 years ago.
Throughout most of this time, the over 30s were an exceedingly small part of hominid populations. It was only around 30,000 years ago that the ratio reversed in populations of modern humans, when twice as many adults died after age 30 as those who died young. This coincides with the Upper Paleolithic Revolution, or the Great Leap Forward in modern human behaviour which led to more complex social structures and culture.
Professor Caspari believes it was this, rather than genetic adaptations, that led to increased human lifespan. Behavioural modernity may have improved the way shelters were constructed and food was hunted, gathered and stored, and may have also changed the way that older people were viewed by growing communities. Older people brought the experience, skills and wisdom needed for flourishing societies which lived particularly close to nature.
The significance of the 30 year threshold to old age was that this was when sexually reproducing humans could typically become grandparents, and a key consequence of 3 generations of a family being alive together was the ability to pass on that knowledge that would otherwise have been lost. Living long enough to take care of their grandchildren gave modern humans a clear evolutionary advantage, as it meant that those grandchildren were more likely to survive.
The second great change in human evolutionary history, the doubling of average lifespan in the past 150 years, was the direct consequence of plummeting infant mortality in societies which have experienced improved nutrition and medical advances, and which allow children to experience longer childhoods.