As you read of the many contemporary threats to life and limb, from pollution to shooting rampages, you may long for a simpler and safer yesteryear. But if the conditions of the past prevailed today, you would probably be dead. The average age of a New York Times subscriber is around 55, and for most of human history, life expectancy was around 30. At least one of your children would probably be dead, too. Until a couple of centuries ago, more than a quarter of children died before their first birthday, around half before their fifth. In “Extra Life,” Steven Johnson, a writer of popular books on science and technology, tells the stories behind what he calls, in an understatement, “one of the greatest achievements in the history of our species.” Starting in the second half of the 19th century, the average life span began to climb rapidly, giving humans not just extra life, but an extra life. In rich countries, life expectancy at birth hit 40 by 1880, 50 by 1900, 60 by 1930, 70 by 1960, and 80 by 2010. The rest of the world is catching up. Global life expectancy in 2019 was 72.6 years, higher than that of any country, rich or poor, in 1950. People in the shortest-lived countries today will, on average, outlive those of your grandparents’ generation. “Life expectancy” is a statistical abstraction — the average number of years a newborn would live if prevailing mortality rates remained unchanged — and much of its rise was propelled by the sparing of children, whose deaths mix a lot of small numbers into the average. Today, more than 99 percent of children in rich countries survive, and more than 96 percent worldwide. But it’s not just children: People who clear any obstacles in the gantlet of life have been granted more time ahead. In 1950, a person who had reached the age of 70 could look forward to another nine years; today, another 16.Steven JohnsonCredit...Nutopia Ltd. The multiplication of our life spans, Johnson notes, is underappreciated and under-chronicled. We revere war heroes and astronauts, not a demographic trend. Worse, we misunderstand which factors are likely to kill us or save us. Lewis Fry Richardson, an early quantifier of violence, discovered in 1953 that deadly quarrels of all magnitudes, from homicides to world wars, account for less than 2 percent of all deaths. “This is less than one might have guessed from the large amount of attention which quarrels attract. Those who enjoy wars can excuse their taste by saying that wars after all are much less deadly than disease.”Presumably this innumeracy about the true causes of death has lessened in the past year, which has awakened us to an unfortunate constant of the human condition. To a germ, you and I are big yummy mounds of gingerbread, there for the eating — and in the arms race between pathogens’ weapons and our defenses, they can evolve faster than we can. The greatest life extenders in Johnson’s history were feats of human ingenuity that made this a fairer fight. Of the eight innovations that have saved the most lives, as Johnson sees it, six are defenses against infectious disease.The sin of ingratitude is said to condemn one to the ninth circle of hell, and that’s where we may be headed for our attitudes toward the granters of extra life. In the list of inventions that saved lives by the hundreds of millions, we find antibiotics (squandered to fatten chickens in factory farms), blood transfusions (condemned as sinful by the devout), and chlorination and pasteurization (often mistrusted as unnatural). Among those that saved lives by the billions, we find the lowly toilet and sewer (metaphors for the contemptible), artificial fertilizer (a devil for Whole Foods shoppers) and vaccines (perhaps the greatest invention in history, and the target of head-smackingly stupid resistance).Johnson also devotes a chapter to devices such as seatbelts that shield our fragile bodies from yet another lethal hazard, moving objects, and another to the successful battle against starvation. But he singles out not just tonics and gadgets, but also lifesaving ideas. Vital statistics, literally data about life, were necessary to better understand what kills people in great numbers from what generates headlines, and to distinguish the actual causes from bunkum and superstition. In a famous example, John Snow traced London cholera deaths to a neighborhood pump, proving in 1854 that the disease was transmitted by contaminated water rather than fetid miasmas.And the nerdy “randomized controlled trial” exposed the uselessness of the many nostrums, snake oils and sometimes poisons that people were ingesting as supposed cures. As Oliver Wendell Holmes wrote in 1860, if all the drugs then in use “could be sunk to the bottom of the sea, it would be all the better for mankind — and all the worse for the fishes.” Johnson reminds us that we owe the lion’s share of our extra life to public health, not individualized medicine. Until the widespread use of antibiotics in World War II, doctors were barely more effective than their barber-surgeon forebears.It’s been a long time since the history of technology has been recounted as the triumph of plucky heroes, and Johnson’s stories reflect today’s more sophisticated understanding. No one person can take all the credit. Most every new discovery was anticipated by some unsung tinkerer or folk practice, and would have died in the cradle but for persuaders and activists who championed the innovation and policies and institutions that allowed it to be implemented.Sometimes the anti-heroism goes too far — Norman Borlaug, whose Green Revolution saved a billion lives, is unmentioned. But altogether, Johnson is a fine storyteller. Among his cast of characters are John Graunt (1620–74), the British haberdasher who studied mortality reports as a hobby and thereby invented epidemiology; Joseph Bazalgette (1819–91), the man behind “one of the 19th century’s greatest engineering achievements,” which you probably did not guess was the London sewers; “Moldy Mary” Hunt (1910–91), the Peoria bacteriologist who scoured fruit markets for the perfect rotten cantaloupe, the one with a strain of mold that enabled the mass production of penicillin; John Stapp (1910–99), who strapped himself into his invention, the rocket sled, and safely decelerated from 628 miles per hour to 0 in 1.4 seconds; and Dilip Mahalanabis, 86, the Indian pediatrician who discovered that a bit of salt and sugar dissolved in clean water could stop fatal diarrhea and thereby saved the lives of nearly 60 million people.Human interest aside, “Extra Life” is an important book. Johnson shakes us out of our damnable ingratitude and explains features of modernity that are reviled by sectors of the right and left: government regulation, processed food, high-tech farming, big data and bureaucracies like the Food and Drug Administration, the Centers for Disease Control and Prevention and the World Health Organization. He is open about their shortcomings and dangers. But much depends on whether we see them as evils that must be abolished or as lifesavers with flaws that must be mitigated.Because after all, nothing is more important than extra life. It is the prevention of the unthinkable agony of losing a child, billions of times over. It is the enrichment of our pool of wisdom and expertise, with octogenarians who lead the House of Representatives and the National Institute of Allergy and Infectious Diseases. And it is a prolongation of the precious gift of consciousness, with all its moments of pleasure and enlightenment.L’chaim!