Is there a theoretical upper limit to human life? © Moron Eel

Is there a theoretical upper limit to human life?

Have you reached your Hayflick limit?

Asked by: Lewis Rylands, Liverpool

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Most of the cells in your body have a natural lifespan, called the ‘Hayflick limit’. This is because chromosomes have biological bookends, called telomeres, that get shorter each time the cell divides. When the telomere shrinks past a certain point, the cell won’t divide any more and eventually dies. Statistical projections, based on the typical rates that our different cells divide, suggest an upper limit to our lifespan of around 120. This corresponds pretty well with what we observe: the longest lived person was Jeanne Calment, who died aged 122, and only a handful of others have made it past 110.

Cell lifespan isn’t fixed, though. When organs are transplanted into a younger body, the cells in the older organ live as long as the body into which they are transplanted. This may be because their telomeres grow longer again. Skin cells, sperm and some white blood cells can do this using the enzyme telomerase, and manipulating telomerase levels for other cells in lab animals does sometimes extend their lifespan. But it also seems to increase their cancer rates. In fact, there is evidence that cell ageing mechanisms evolved to protect multicellular organisms from cancer, so mortality might be inescapable.

What cells in the human body live the longest? © Getty Images

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