Edward Jenner vaccinating a boy, painted by E.-E. Hillemacher. Vaccination was one of the first tools in the revolution that doubled human lifespans.

Look at a world map of life expectancy today and the gradient is stark: 84 years and more in Japan, Switzerland and Australia, down through the 70s across Latin America, China and India, to the low 60s and 50s in a band across the Sahel and Central Africa. It reads like a map of injustice, and it is. But it hides a fact so large it is easy to miss entirely: the worst-off country on today's map does better than the best-off country did for almost all of human history.

For millennia, life expectancy at birth everywhere on Earth hovered between about 25 and 35 years. Ancient Rome, Song China, Renaissance Italy, Enlightenment France — it made little difference. This was not because nobody grew old; plenty of people reached seventy. The average was dragged down by a slaughter of the very young. Something like a quarter of babies died before their first birthday, and close to half of all children died before fifteen. A family that raised six children typically buried two or three others. Human societies differed in art, wealth and power, but in this one statistic they were all alike.

The escape began, quietly, in a corner of northwestern Europe around 1800. It was not primarily a story of doctors and hospitals — for most of the nineteenth century, a hospital was somewhere you went to die. The first great gains came from duller things: sewers and clean water that separated drinking supplies from human waste; cheaper food and better transport that ended the old cycle of local famine; soap; and vaccination, beginning with Jenner's smallpox vaccine of 1796, which disarmed one of the great child-killers. Germ theory, accepted only in the 1870s and 1880s, turned these scattered practices into a program. By 1900, life expectancy in the richest countries had crawled up to around 50 — unprecedented, and yet the revolution had barely started.

The twentieth century turned a European head start into a global transformation, and here the story becomes genuinely surprising. It took Britain roughly 150 years to go from a life expectancy of 40 to one of 70. South Korea did it in about 35 years. So, roughly, did China. Antibiotics, mass vaccination campaigns, oral rehydration therapy — a sugar-and-salt solution that costs pennies and has saved tens of millions of children from diarrheal death — allowed poor countries to import in a generation what rich countries had spent a century inventing. The global average, about 32 years in 1900, stands near 73 today, per Our World in Data. Humanity as a whole now lives longer than the very richest countries did in 1950.

The map still carries scars. The countries at the bottom of today's ranking — Nigeria, Chad, the Central African Republic, Somalia — are held down by the old enemies, child mortality and infectious disease, compounded by war and weak states. Southern Africa's curve shows a horrifying notch: HIV cut more than a decade off life expectancy in countries like Botswana and Zimbabwe in the 1990s and 2000s before treatment bent the curve back up. And progress can reverse elsewhere too — American life expectancy actually fell in the late 2010s under the weight of opioids, and the whole world's dipped in 2020-21 with COVID.

Even so, the direction of a century is unmistakable, and it changes how the top of the map should be read. In countries like Japan and Spain, the old killers are so thoroughly beaten that most deaths now come from the diseases of long life — heart disease, cancer, dementia. The frontier itself keeps creeping: the record national life expectancy has risen by about two to three years every decade since 1840, a straight line that demographers keep predicting must bend, and which keeps not bending.

The deepest change, though, is invisible on any map: what a human life is expected to contain. A world where half of children died bred fatalism — you can hear it in old proverbs and see it in old family portraits, where children were often not named for weeks. A world where a newborn in even a poor country will most likely meet its grandchildren is, demographically speaking, a different civilization. We built it in about eight generations, and the map of life expectancy is best read not as a ranking but as a progress bar — one that shows exactly where the work remains unfinished.

Sources: Our World in Data, "Life Expectancy"; Angus Deaton, The Great Escape: Health, Wealth, and the Origins of Inequality; Steven Johnson, Extra Life: A Short History of Living Longer; Encyclopaedia Britannica, "Public health."

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