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Every year in the course I teach on the 14th-century Black Death, I ask students to imagine themselves as peasants, nuns, or nobles in the Middle Ages. What would their lives have been like if they were confronted with this terrifying disease that killed millions of people in just a few years?
Aside from what they imagine it would be like to encounter the plague, these students often think that during the Middle Ages they would be considered middle-aged or elderly at the age of twenty. Instead of being in the prime of their lives, I think they'll soon be decrepit and dead.
They reflect a common misconception that human longevity is very recent, and that no one has lived much past thirty in the past.
But that's just not true. I am a bioarchaeologist, which means I study human skeletons excavated from archaeological sites to understand what life was like in the past. I'm particularly interested in demography - mortality (deaths), fertility (births) and migration - and how it related to health problems and diseases such as the Black Death hundreds or thousands of years ago. There is physical evidence that many people in the past lived long lives - just as long as some people live today.
Bones record the length of a life
One of the first steps in researching past demography is to estimate how old people were when they died. Bioarchaeologists do this using information about how your bones and teeth change as you age.
For example, I look for changes in the joints in the pelvis that often occur in old age. Observations of these joints in today's people whose ages we know allow us to estimate the ages of people from archaeological sites with joints that look the same.
Another way to estimate age is to use a microscope to count the annual additions of a mineralized tissue called cementum to the teeth. It is similar to counting the rings of a tree to see how many years it has lived. Using such approaches, many studies have documented the existence of people who have lived long lives in the past.
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For example, by examining skeletal remains, anthropologist Meggan Bullock and colleagues found that in the city of Cholula, Mexico, between 900 and 1531, most people who reached adulthood were over 50 years old.
And of course there are many examples from historical archives of people who lived very long lives in the past. For example, the sixth-century Roman emperor Justinian I reportedly died at the age of 83.
Analysis of the tooth development of an ancient anatomically modern Homo sapiens An individual from Morocco suggests that our species has been long-lived for the past 160,000 years.
Clearing up a mathematical misunderstanding
Given the physical and historical evidence that many people in the past lived long lives, why does the misconception persist that everyone was dead by the age of thirty or forty? It stems from confusion about the difference between individual lifespan and life expectancy.
Life expectancy is the average number of years of life remaining for people of a certain age. For example, life expectancy at birth (0 years) is the average lifespan of newborns. Life expectancy at age 25 is how much longer people live on average, given that they have reached the age of 25.
In medieval England, life expectancy at birth for boys born to land-owning families was only 31.3 years. However, life expectancy at age 25 for landowners in medieval England was 25.7 years. This means that people of that era celebrating their 25th birthday would live an average of 50.7 years - 25.7 more years. While 50 may not seem old by today's standards, remember that this is an average; many people would have lived much longer, into their seventies, eighties and even older.
Life expectancy is a population-level statistic that reflects the circumstances and experiences of a wide variety of people with very different health conditions and behaviors; some die at a very young age, others live to be over 100 years old and many whose life spans fall somewhere in between. Life expectancy is not a promise (or threat!) about the lifespan of one person.
What some people don't realize is that low life expectancy at birth for any population usually reflects very high infant mortality. That is a measure of deaths in the first year of life. Given that life expectancy reflects averages for a population, a high number of deaths at a very young age will bias calculations of life expectancy at birth toward younger ages. But generally, many people in those populations who survive the vulnerable infancy and early childhood years can expect to live relatively long lives.
Advances in modern sanitation - which reduce the spread of diarrheal diseases, which are a major killer of infants - and vaccinations can significantly increase life expectancy.
Consider the effect of infant mortality on overall age patterns in two contemporary populations with dramatically different life expectancies at birth.
In Afghanistan, life expectancy at birth is low, just over 53 years, and infant mortality is high, with almost 105 deaths per 1,000 children born.
In Singapore, life expectancy at birth is much higher, at over 86 years, and infant mortality is very low: fewer than two children die for every thousand children born. In both countries people survive to a very old age. But because so many more people die at a very young age in Afghanistan, proportionately fewer people survive into old age.
Living a long life has long been possible
It is incorrect to view longevity as a notable and unique feature of the "modern" era.
Knowing that people in the past often lived long lives may help you feel more connected to the past. For example, you can imagine multi-generational households and gatherings where grandparents in Neolithic China or medieval England bounce their grandchildren on their knees and tell them stories about their own childhood decades ago. Maybe you have more in common with people who lived long ago than you thought.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Sharon DeWitte, University of South Carolina Read more: Sharon DeWitte receives funding from the National Science Foundation.