Not many people know this, but in the mid nineteenth century the speed of natural selection accelerated. Or rather, the ability of genetic cells to mutate improved for no apparent reason. But since that time, a reason has become apparent: the increase in the human cell’s selection ability coincided exactly with the invention of anaesthetic.

This may seem at first sight an odd correlation, but it isn’t. Anaesthesia changed everything in terms of human survival of disease. In the 160 years since their invention as an effective form of pain removal, anaesthetics have increased the survival rate in invasive surgery from 3 in 10 to 98 out of 100. Over time, the survivors who would otherwise have died produced thousands of millions of humans. This development – and antibiotics eighty years later – produced the overpopulated planet on which we live today. Better diet and more efficient agriculture have in turn ensured more babies survive, and most adults live far longer lives than they would’ve done a century ago.

But as all these improvements took place, the power and range of natural selection and gene mutation continued to grow. Today, children have noticeably different genetic structures to their parents.

Natural selection is basically evolution’s psychopathic hitman. It ensures that weaker gene-lines are eradicated, and the stronger ones thrive. But as medicine has become increasingly adept at allowing defective genes to survive and reproduce, natural selection’s task has become both broader and more pressing. It is clear that whatever drives this process has responded: an astonishing array of new viruses, diseases and sensitivities have appeared to replace the old ones. The more medicine addresses these, the more new ones appear.

The most astonishing example of this is our sensitivity as a species to salt – a situation that has become a medical obsession in recent years. But three years ago, I met a geneticist who told me that the sensitivity appeared in the late 1930s. Before that, it didn’t exist. Those who have fatal sensitivity have, it seems, been identified as possessing weaker genes which must not be reproduced.

There are a number of fascinating questions raised by this finding. First, why introduce a sensitivity to something essential for human survival? Second, the speed with which the anomoly appeared defies most theories of evolution. Several key changes in gene structure have been observed taking 300,000 years to mature. This one popped out of the woodwork overnight. And third, what drives this selection – given (as per the condundrum of a few nights ago) there is no brain present to guide it?

The obvious conclusion – that the more we try to cure our species, the more natural selection keeps trying to correct our waywardness – is arresting.

What does anyone else think?