10,000 years of war increased the size of human groups from hundreds to millions of people

The Battle of Chacabuco, during the Chilean War of Independence (1817). Public Domain.

How did human groups grow from a few hundred people to large societies like the USA with more than 318 million people? Seshat overall coordinator Peter Turchin explains how warfare and intense competition between groups over millennia resulted in this tremendous evolutionary transition. Larger groups typically beat smaller groups, he explains, resulting in an evolutionary pressure for societies to grow in size and complexity. Early large-scale societies, however, quickly fell apart. To hold such large groups together, a number of cultural devices emerged, including elaborate bureaucracies, complex and productive economies, and new ideologies, rituals, and religions.

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To explore relevant publications, see Turchin et al. 2013 and Turchin 2007.


Peter Turchin: Ten thousand years ago, all humans lived in small-scale societies–band of foragers, villages of farmers. Today, we live in huge societies of tens, sometimes more, millions of people.

Take the United States, over 300 million people. How did this major evolutionary transition occur? That’s a big and unresolved question. The theory that we are working on is called cultural group selection. Cultural group selection suggests that the major driver of cultural and social evolution is competition between human groups and societies. So, until recently, the major form this competition took was warfare. So how does it work? Why did warfare result in the evolution of large-scale, complex societies. In warfare, if you want to win you want to bring many fighters to the battle. This means that larger groups would typically win over smaller groups. Eventually, they replaced all small groups and they had to contend with other big groups, which forced them to become even bigger. So, this set up a spiral of societies becoming larger, contending with larger societies, and eventually becoming really huge.

Now, that’s only part of the answer because it is very difficult to keep a large-scale society cohesive and functional. Its much easier to cooperate in a village or a small town than in a huge society like the United States, for example. So, cultural evolution had to find many different ways/devices/cultural traits that would allow large-scale societies to function reasonably well and without splitting apart. In fact, the first large-scale societies were extremely fragile and kept collapsing all the time. So, what happened over the last 10,000 years was that war between societies was the major evolutionary driver that caused them to become larger and more complex and it resulted in the evolution of such cultural traits as elaborate governance structures, like bureaucracies, for example, complex and productive economies, and ideologies, rituals, and religions which keep large-scale societies functional and allow them to become cohesive enough not to collapse.


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