Human life expectancy stayed roughly flat for most of history, then doubled fast

Johnson opens with a striking baseline: for tens of thousands of years, including through agriculture, empire-building, and the rise of great civilizations, average global life expectancy hovered around 35 years or lower, driven down heavily by extremely high childhood mortality rather than by adults simply dying young across the board. This number barely moved through the rise and fall of Rome, the Renaissance, and most of recorded history.

Starting roughly in the 1800s, and then accelerating through the 20th century, life expectancy climbed dramatically to more than double its historic level in many parts of the world. Johnson frames this as arguably the most important change of the modern era precisely because it happened gradually enough that no single generation experienced it as a discrete event, even though its cumulative scale dwarfs almost any other measure of human progress.

Takeaway: the most transformative change of the last two centuries wasn't a war or an invention — it was simply how long an ordinary life got to last.