Why do humans tend to live such a long time? Our closest relatives, the chimpanzees, can last into their mid forties in the wild. Yet somewhere in the last six million years, human lifespans have lengthened dramatically, so that living into our seventies is no big surprise.
The last few weeks have seen some exciting new developments in this area. First, a recent paper featured in The Conversation showed that at all ages, humans are less likely to die than chimps.
Excitingly, however, modern health care, diets and the steady decline in violent deaths have slashed mortality rates of young adults. People in societies like Japan are now almost 200 times less likely to die in at a given age than people of the same age in hunter-gatherer societies.
The dramatic declines in modern mortality rates are almost entirely due to technological developments, but the lengthening of the maximum potential human lifespan since we diverged from the other great apes poses an intriguing evolutionary problem.
Our bodies only function as well as they do because we have a quiver-full of cellular repair and maintenance mechanisms. For example, we have systems that correct mistakes in DNA replication, and others that detect and kill off pre-cancerous clusters of cells.
But natural selection optimises those mechanisms to operate during our expected lifespan. Modern people who live beyond seventy are much more likely to suffer from cancers, dementia and other diseases of old age. Few of our ancestors – even the most recent ones – lived that long, and these late-onset diseases didn’t interfere with their successful reproduction. By the time they got the diseases, our ancestors had already passed their genes on – and that’s why we carry those same genes.
Six million years ago, the diseases of old age probably kicked in before our great-ape ancestors hit forty. The evolution of a longer lifespan involved a steady postponement of ageing. But only if older individuals contribute to the success of their own genes can this actually happen. Continue reading Is human longevity due to grandmothers or older fathers?
Want to live a long time? Who doesn’t? Quite a number of smart people have staked their hopes for long life on restricting their calorie intake. It’s an idea that rests on a not inconsiderable body of evidence. But some new developments suggest it may be a questionable strategy.
The idea that a diet containing a very modest energy content (measured in Calories or Kilojoules) could dramatically prolong lifespan is based on one of the most replicated findings in all of biology.
As long ago as 1935, researchers reported that lab rats fed dramatically restricted diets lived up to twice as long as rats allowed unrestricted access to food. The lifespan extending effects of dietary restriction appear to come about by delaying the onset of ageing and slowing the cellular damage and onset of diseases like cancers that naturally occur with age.
Since then, similar results have been obtained in mice, Rhesus monkeys, zebra fish, vinegar flies (Drosophila melanogaster), the nematode worm (Caenorhybditis elegans) and even yeast. The fact that dietary restriction prolonged lifespan and slowed ageing in such a variety of organisms strongly suggested that the underlying mechanisms are shared by all animals and possibly even all eukaryotes – the large group of complex organisms that includes animals, plants and fungi.
It didn’t take long for people to spot a potential route to slowing ageing and putting off the inevitable. Roy Walford championed what he called CRON – Calorie Restriction with Optimal Nutrition – in Beyond The 120-Year Diet. He argued that cutting dietary energy while maintaining micronutrient intake would prolong normal lifespan to well over a century, reducing the incidence of the major diseases of aging. Continue reading A hunger to live longer