Commentator Alan Lightman reflects on the life span of humans and the ways in which the environment affects the length of our lives.
CURWOOD: As time marches by, a sense of one’s own mortality sinks in. Memory may go first, followed by other increasingly persistent signs of aging. Writer and physicist Alan Lightman is all too aware of his own mortality, and wonders if there is an environmental basis for the limit of the human life span.
LIGHTMAN: I’ve reached the age where I see new gray hairs and craggy wrinkles in the mirror each week. My gait is not quick like it used to be. Once my skin was smooth. Life stretched in front of me like the ocean.
I remember an essay by the physician and writer Louis Thomas in which he said that the human body just seems to fall apart after 75 or 100 years. Even without any particular disease, we just crumble after awhile. And that got me to thinking. Why do we live about 100 years? Why isn’t the human life span a few days? Or a million years?
A big hint, it seems to me, is that most animals on earth live about the same length of time. An average mouse lives about four years; an elephant lives about 70. Although these life spans may seem far apart, in cosmic terms, there isn’t much difference between a few years and a hundred.
So, why do all animals live about the same length of time? What do we all have in common?
One factor we share is that we all depend, directly or indirectly, on plants. For all vegetation on earth, the year is an especially important unit of time. Plants must live at least one year in order to bury their roots and seeds, push through the earth in warm air, bear fruit, and then sleep again through the winter.
Since we animals are part of the food chain, a full cycle of plant life--in other words, a year--is also a minimum length of life span for us. But a relatively small number of years is also a maximum.
After we have lived long enough to reproduce, we’ve done our job as far as Mother Nature is concerned. Like an engineer creating a bridge, Nature designs with economy. So, the year is the yardstick by which Mother Nature measures all life.
But Mother Nature is part of a much larger design. A year is the time for the earth to orbit the sun. And that time is set within limits by the pull of gravity, the mass of the sun, and the boiling point of water. The fundamental laws of physics require that the length of the year on a habitable planet, and thus the length of an average animal’s life, couldn’t be vastly different from what it is.
As I stare again in the mirror, I feel an odd kinship with all living things, and even nonliving things. It seems both beautiful and strange that the grand pageant of our lives--from birth, to passionate courtship, to parenthood, to old age--is somehow determined by the charge of an electron, the mass of a proton, the size of the quantum.
[MUSIC: Paul Horn “Head Dance” “Altitude of the Sun” CBS (1975)]
CURWOOD: Alan Lightman is author of “Einstein’s Dreams” and teaches writing at the Massachusetts Institute of Technology.
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