How much could the world plausibly change if a new era appeared within a century or so? A review of the biggest past changes offers a weak basis for expectations about the magnitudes and types of future changes. If we go way back, the universe began, and then life arose. But those events happened billions of years ago and are poorly understood. Within the last few million years, however, the biggest changes were concentrated in three key transitions: the introduction of humans, farming, and industry. Humans foraged, that is, searched, for food from a few million to about 10 000 years ago. From then until a few hundred years ago, we farmed and herded. Since then we have developed and relied on industry. Social group sizes have steadily increased over this history. While most mammals live in groups of two to 15 individuals ( Kamilar et al. 2010 ), most human foragers lived in bands of roughly 20 to 50. Most farmers lived in village-based communities of roughly 500 to 2000 ( Kantner and Mahoney 2000 ). While larger empires often existed, they made little direct difference to most people’s lives. Today, most people live in metropolitan regions of roughly 100 000 to 10 million ( Giesen et al. 2010 ), and also in nations of roughly 1 million to 100 million. These sizes fit a simple if mysterious pattern: each era’s community sizes have been roughly the square of the previous era’s sizes; a band is roughly a group of groups, a village is roughly a band of bands, and a city is roughly a village of villages. These three human eras of foraging, farming, and industry have encompassed similar numbers of people. About 20 billion humans have been born since 1750, roughly 50 to a 100 billion were born between 10 000 years ago and 1750, and a similar number of near-humans were born in the million or so years before 10 000 years ago (Haub 2011). So of all the humans who have ever lived, only about 3–8% are alive today. These three eras also saw similar amounts of change, in the sense that they encompassed similar factors of total economic growth. During each era the human economy (i.e., the total economic capacity to produce valued things) doubled relatively steadily (i.e., via exponential growth) from seven to 10 times.
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