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The limits to human lifespan must be respected

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Lengthening our lives will come at a cost.

Why do animals grow old and die at characteristic ages? Even if maintained in peak condition and not eaten by your cat, your hamster is unlikely to make it much past its second birthday. And your cat might live for ten times that. Yet neither cat nor hamster will ever match the average healthy human for longevity.

Human age limit claim sparks debate

A study published online in Nature this week uses demographic data to reveal a lifespan that human beings cannot exceed, simply by virtue of being human. It’s like running, as an accompanying News and Views article points out. Elite athletes might shave a few milliseconds off the world record for the 100-metre sprint, but they’ll never run the same distance in, say, five seconds, or two. Human beings are simply not made that way. The same is true for longevity. The consequences of myriad factors related to our genetics, metabolism, reproduction and development, all shaped over millions of years of evolution, means that few humans will make it past their 120th birthdays. The name of Jeanne Calment, who died in 1997 at the age of 122, is likely to remain as long in the memory in the Methuselah stakes as that of Usain Bolt on the Olympic track.

Maximum lifespan is a bald measure of years accumulated. It is not the same as life expectancy, which is an actuarial measure of how long one is expected to live from birth, or indeed from any given age. Life expectancy at birth has increased in most countries over the past century, not because people have longer lifespans, but mainly because infectious disease does not kill as many infants as it once did. Factors such as poverty and warfare conspire to decrease life expectancy. Although life expectancy at birth has risen steadily for both men and women in France since 1900, for example, there are dramatic and poignant drops that coincide with the two world wars.

In Britain in the early twentieth century, many children still died from infectious diseases, and men would die shortly after retiring from physically demanding jobs. The National Health Service was the political response. It has become, in some ways, the victim of its own success. People live longer than they did even a few decades ago, and die (eventually) of different (and more expensive) complaints. As any beginning medical student is soon taught, gerontology is far from a dying discipline. So if we owe our increases in life expectancy to better public health, nutrition, sanitation and vaccination, is it not fair to ask whether more-effective treatments for diseases such as cancer, Parkinson’s disease and Alzheimer’s might also yield dividends in maximum lifespan? Will 120th birthday parties become routine, outmatched by a small yet increasing number of sesquicentenarians? The demographic data say no. People are living longer, and the population as a whole is greying, but the rate of increase in the number of centenarians is slowing, and might even have peaked.

Could it be possible, in some science-fictional future, to break free from the bonds of human life expectancy and increase lifespan indefinitely? An unquenchable desire for eternal life has preoccupied humanity from the earliest times, as attested by the earliest passages of the Bible, the Gilgamesh epic and many other stories from our past. Perhaps the chilliest evocation of mortality comes in Bede’s seventh-century Ecclesiastical History of the English People, in which a chieftain remarks that the ‘few moments of comfort’ offered by human life are as the brief flight of a sparrow through a warm and lighted mead hall, in through one door, and out through the other, back into a dark, storm-tossed and demon-haunted night of which we know nothing. No wonder we’d all like a little more light. Technological solutions might one day transcend the limitations of the human body, but transcend them they must — mere extension is already yielding diminishing returns.

The risks of transcendence are twofold. First, it might be that to extend our lives beyond our normal span, we must somehow become other than human. After all, what would a 50-year-old hamster be like? The unintended consequences of immortality are graphically and grimly illustrated in Aldous Huxley’s 1939 novel After Many A Summer, in which people fed on a life-extending diet of carp intestines live for centuries — at the cost of turning into witless apes. Second, there is a risk that life wouldn’t really be that much longer — it would only feel like it.

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Taylor Bennet (CC BY-NC 2.0)

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Evidence for a limit to human lifespan 2016-Oct-05

Human age limit claim sparks debate 2016-Oct-05

Ageing: Dietary protection for genes 2016-Aug-24

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The limits to human lifespan must be respected. Nature 538, 6 (2016). https://doi.org/10.1038/538006a

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