Robert Dorit in American Scientist
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.