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.
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?