At heart, the Industrial Revolution has been a revolution in energy conversion. (339)

Harari points out that for millennia before the Industrial Revolution people had employed “a large variety of energy sources. They burned wood in order to smelt iron, heat houses and bake cakes. Sailing ships harnessed wind power to move around, and watermills captured the flow of rivers to grind grain” (335). But to do so, they had to live near a supply of trees and accessible, fast flowing water. And winds had to blow with strength and regularity.

According him, the problem more crucial than access to these energy sources lay in the fact that

people didn’t know how to convert one type of energy into another. They could harness the movement of wind and water to sail ships and push millstones, but not to heat water or smelt iron. Conversely, they could not use the heat energy produced by burning wood to make a millstone move. Humans had only one machine capable of performing such energy conversion tricks: the body. In the natural process of metabolism, the bodies of humans and other animals burn organic fuels known as food and convert the released energy into the movement of muscles. Men, women and beasts could consume grain and meat, burn up their carbohydrates and fats, and use the energy to haul a rickshaw or pull a plough.

Human and animal bodies — “the only energy conversion device available — meant that almost all human activities relied on muscles of animals and human beings, which in turn relied upon food provides by plants, which depend upon the sun. “Almost everything people did throughout history was fuelled by solar energy that was captured by plants and converted into muscle power” (335). Human history, he concludes, “was consequently dominated by two main cycles: growth cycles of plants and the changing cycles of solar energy (day and night, summer and winter) (335).


Harari, Yuval Noah. Sapiens: A Brief History of Mankind. New York: Harper Perennial, 2018.

Last modified 26 September 2021