If Covid-19 had swept Britain in 1800, the chances are that no one would have noticed. The clinical signatures of plague, smallpox and cholera, for example, are hard to miss, but you need a laboratory to diagnose coronavirus. Nor is it (relatively speaking) that deadly: its case-fatality rate is scarcely enough to feature on the Richter scale of pestilence and its victims are overwhelmingly the elderly. In 1800, however, life expectancy was 40.

The big difference between people now and two centuries ago is that we feel so much more secure – or we did until Covid shattered the illusion of control. My great-great-grandfather, a seaman, was about to get married in 1800. Each voyage was as dangerous to him as childbirth was to his wife. While they lived, 90 per cent of people died before the age of 60. Now, it is the other way around.

We are the most fortunate generation ever. If you don’t believe it, the prophet Isaiah’s vision of the New Jerusalem is a useful starting point. ‘Never again’, he said, ‘will there be in it an infant who lives but a few days, or an old man who does not live out his years: he who dies at a hundred will be thought a mere youth: he who fails to reach a hundred will be accursed’. By these criteria, we are doing rather well. Neonatal mortality is down to 2.8 per thousand births in the UK, and ONS projected in 2010 that 20.8 per cent of boys and 26.1 per cent of girls born in 2043 would see their 100th birthday.

The change in our fortunes dates to the Anthropocene – the man-made era. This came about in the blink of an eye, in evolutionary terms: my great-great-grandfather was just about to start his family when it began. The outcome of the Anthropocene was what the Nobel-winning economist Robert Fogel called ‘the Great Escape’ from plague and famine, which began on the Atlantic seaboard and took several generations to work its way through western populations. Then it went global, and with startling speed.

Economists use measures such as height and longevity to mark the scorecard of progress, and they often see these measures as passive consequences of increased well-being. It seems self-evident that people grow bigger if you feed them better, and that they will live longer if you shield them from infection. But the change in our biology actually reflects something more fundamental. Our genotype (our complement of genes), is no different from that of our grandparents, but our genes are active participants in a dialogue with their environment, a dialogue that begins at conception and ends with our final breath. The interaction of genotype with environment known as the ‘phenomenon type’, or phenotype.

The concept of a lifelong and networked dialogue between the human body and its environment is relatively recent. Fogel called the interplay between prosperity, living conditions and biological outcomes the technophysiological (or technophysio) revolution. He defined it as ‘a synergism between technological and physiological improvements that has produced a form of human evolution that is biological and not genetic, rapid, culturally transmitted and not necessarily stable’. Since ‘technophysio’ is quite a mouthful, I call it the phenotypic transition.

It doesn’t take long to observe this transition at work. Koreans, for example, were traditionally short: men averaged 161 cm (63 inches) and women 149 cm (59 inches) in the 19th century. Little had changed when the war of 1950-53 divided them between a free enterprise economy and a repressive totalitarian regime. Those born before 1950 grew to much the same size on either side of the border. Not so in 2002, when a United Nations survey found that pre-school children in North Korea were 13 cm shorter and seven kg lighter than in the South. Adult North Koreans were no taller than before, but South Korean women set an international growth record of 20.2 cm and now enjoyed the fourth longest life expectancy in the world. A line on the map had translated into a difference in biology.

There were quantitative and qualitative aspects to this global transition, and the quantitative came first. Robert Malthus, a contemporary of my great-great-grandfather, predicted (correctly) that prosperity would cause a geometric increase in population, but he was wrong to assume that food production would be limited to arithmetical progression.

Food imports fuelled the expansion of Europe, and people of European descent grew from around 20 per cent of the global population in 1800 to 33 per cent in 1933. The increase still threatened to swamp food production when Sir William Crookes, distinguished scientist and researcher into the paranormal, addressed the British Association in 1898. He estimated that there were currently 517 million people of European descent, and he projected an increase to 819 million in 1941. But would there be wheat to feed them? Famine seemed inevitable unless, as he pointed out, we could learn to harvest nitrogen from the air we breathe. Should this prove possible, he predicted that ‘the chemist will step in and postpone the day of famine to so distant a period that we, and our sons and grandsons, may legitimately live without undue solicitude for the future’.

He did not have long to wait. Fritz Haber showed Carl Bosch that he could fix atmospheric nitrogen in 1909, and nitrate fertilisers powered the Green Revolution of the 20th century. Some two-three billion people are alive today courtesy of Haber-Bosch. Food production started to outpace population growth for the first time in history. This resulted in a rising gross national product, a smaller fraction of disposable income devoted to food, and an increasing proportion of expenditure on former luxury items such as meat and dairy products.

Numbers apart, more food and better living conditions brought about a qualitative change in our phenotype which amounted to reconfiguration of our entire growth programme, not to mention epigenetic effects that are transmitted from one generation to the next. The consequences of this phenotypic transition surround us on every side, most obviously in the long legs of young people. Taller mothers have bigger babies, but increased growth in the long bones of the legs, prefigured in the womb, contributes to their final height.

Other changes are less visible. Life expectancy in the UK increased at a rate of three months per calendar year of birth in the 20th century, for example. The increase was largely due to the decline of infectious disease in the first half of the century, but it then became apparent that old people were also living longer. Some people really do age faster than others – everyday experience is confirmed by longitudinal studies of biomarkers of ageing – and the pace of ageing is strongly affected by our early environment.

Hard as it may be to believe after a year of Covid, meliorists such as Steven Pinker are right to point out that things are getting better, and long may this continue. ‘Intellectuals’ are derided for their pessimism, but the real pessimists are politicians who promise a return to an imaginary past rather than looking to the future. The future is menacing and unknown, but what we do today will determine the human capital we transmit to future generations.