The Undiscovered Country
The statistics of death show leaps in modern life expectancy but fail to answer the question: Why do we die?
I do not often picture my own death. When I do, however, the details vary. I can picture my death as being unexpected: a stroke, a heart attack, an auto accident. Some kind of testament to life's unpredictable character. But I prefer to picture the other sort of exit, one far in the future, where I have worn out my body without overstaying my welcome. Either way, I know that my story will end—I just don't know how.
We may be the only species that is aware of its own mortality. Yet despite death's central role in shaping human self-consciousness, mortality remains an elusive biological phenomenon. Fate and accident shape some ends, but these things aside, we cannot answer what seems like a straightforward question: Why do we die? The question, of course, is not really simple, nor does it yield a single answer. We will each die in our own way. But an answer collected from individual stories is not what we are after. We are, instead, seeking a more general explanation, rooted in material cause, which accounts for the patterns of human mortality. For now, we will leave out deaths that come from external causes—accidents or acts of violence—for they tell us little about the biological underpinnings of mortality.
The pattern of death has changed through history. We can infer something about this pattern from fossils of early Homo sapiens. Judging by the condition of their skeletons and the extent of tooth wear, their life expectancy has been estimated at around 25 years. Tens of thousands of years later, as written records and gravestones become available, our ability to estimate life expectancy takes a major leap forward. The story told by these later records is dramatic: In the past 1,000 years, life expectancies and, presumably, their underlying causes have fundamentally changed. In the United States, for instance, the Social Security Administration has predicted life expectancies for the year 2050 will reach 77 years for men and 83 years for women.
So what are we to make of so drastic a change, a tripling of life expectancy at birth in 50 generations? It's a common misconception that a 30-year-old in the year 1000 would have been both old and rare. However, a 30-year life expectancy doesn't mean that at all. Many 50-year-olds walked the roads in the year 1000. Life expectancy at birth is an artificial construct, a statistical measure that averages survivorship across all age groups and that predicts how long an infant born today is likely to live given the death rates in effect today. That less-than-intuitive definition is worth pausing for, because its implications are profound. In effect, in the year 1000 your chances of making it to a ripe old age depended above all on surviving the first years of your life. If you turned 5, you were quite likely to turn 60. Life expectancy a millennium ago was determined primarily by rates of infant mortality. Over the next 800 years, and most particularly over the past 100 years, gains in life expectancy have come primarily from greater survival of infants and small children, often because of public sanitation and better treatment and prevention of infectious disease. A sad caveat: 27 of 136 countries in the world still have life expectancies at birth of under 50 years. For the half billion people in those countries, the gains in infant and child survival have yet to take deep root, or else have been offset by the dramatic increases in the mortality of young adults brought about by war and the HIV pandemic.
More recent increases in life expectancy in the developed world no longer stem from better infant and child survival. Increased life expectancy now comes about through two unrelated causes: a decrease in the number of children (which reduces the impact of child mortality on the calculation of life expectancy at birth) and an increase in the survival of older adults. Today, our life expectancy is greater not because we make it to adolescence, but because we are actually living longer, and the survivorship of older adults now has a far greater influence on the calculation of life expectancy.