Death in childhood as the historical killer
Life expectancy of roughly 30-35 years in pre-industrial societies masks a harsh demographic reality: if you survived to age 20, your chances of reaching 60 were reasonable. The low average resulted from staggeringly high infant and child mortality. Dysentery, pneumonia, malaria, smallpox, and plague killed 30-50 percent of children before age five. Those deaths dragged the average life expectancy down, even as adults who survived childhood often lived into their 60s.
Modern life expectancy gains came from reducing exactly this bottleneck. Vaccination (smallpox, diphtheria, polio), antibiotics (penicillin from 1940s), clean water systems, and sanitation cut childhood death rates from 40 percent to under 3 percent in developed nations. With children surviving, population-level life expectancy doubled.
Sanitation and medicine as accelerators
Sewage systems and clean water reduced waterborne diseases in the 1800s before germ theory was fully accepted. London's cholera deaths plummeted after the Broad Street pump was disabled in 1854, decades before antibiotics existed. Nutrition improvements (bread, sugar, potato availability from colonial trade) strengthened immune resistance. Smallpox vaccination (introduced by Jenner in 1796) became widespread enough to interrupt transmission.
Antibiotics after 1940, vaccines against measles and polio in the 1950s-60s, and modern obstetrics cut maternal and infant mortality further. Life expectancy rose from 50 in 1920 to 80 in developed nations by 2020. Low-income nations where these interventions remain incomplete still see life expectancies below 65.