The modern world we experience today is only a brief period in human history. Previously, for millions of years, humans lived in the "world of yesterday," which remains today in traditional societies. After discussing the rise of civilization in Guns, Germs & Steel, then its collapse in Collapse, Jared Diamond invites us to explore the lives of past and traditional societies to find lessons for the future. Examples presented include the !Kung of Africa, American Indians, Australian Aborigines, and various Papuan tribes that Jared Diamond became acquainted with during his decades of fieldwork there.

Traditional societies and their ways of life represent thousands of natural experiments designed to address various human problems. While modern society boasts advantages such as longer life expectancy and reduced violence, the results of these natural experiments can offer better ways to raise children, think clearly about danger, avoid modern diseases, and more.

The World until Yesterday provides a glimpse into what human society was like before we all entered the modern era, a past that is nearly lost but still felt even in today's civilization. The differences between the world today and the world of yesterday make us appreciate modern civilization more, while also showing us again the wise ways that traditional societies discovered but which we have forgotten.