Human societies have become incredibly big and complex over the past few thousand years, and they have done so primarily because of agriculture and warfare. That is the claim made by a large international collaboration that has spent more than a decade gathering data on the subject. But not everyone is convinced by these conclusions 鈥� or even by the starting assumptions.
The way we live today is very different from the way humans have lived for most of our聽 history. For instance, while our ancestors typically cooperated in groups of no more than a few thousand, some modern societies operate at the scale of 1 billion or more people and cover millions of square kilometres. But there is little agreement on why societies changed.
at the Complexity Science Hub Vienna, Austria, and his colleagues think the best way to explore the subject is to take a global approach. They have built a database 鈥� the 鈥� that tracks changes in societal structure across the world over the past 10,000 years. The database is informed by research from fields as diverse as archaeology, sociology, religious studies and economics.
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鈥淲e began in 2011. It鈥檚 been a huge amount of work,鈥� says Turchin. 鈥淲e wanted to have an informative data set with as much variability as possible because we are testing global theories, ideas that apply to all human societies.鈥�
The researchers have now built a computer model that uses the Seshat database to test those global ideas.
According to Turchin, almost all of the ideas assume that agriculture was a precondition for social complexity, partly because it allowed for food surpluses and the accumulation of wealth. 鈥淭hat鈥檚 so obvious,鈥� he says.
Beyond that, he says there are two basic schools of thought on what drove societies to become more complex. The first is that it was cooperation 鈥� the need to develop infrastructure and manage the risk of crop failures, for instance 鈥� and the second is that it was conflict, in the form of either class struggles internal to a society or external warfare with neighbouring groups.
Having run the competing ideas through the model, Turchin and his colleagues think they have good evidence that complexity was largely driven by just one of these additional variables: external warfare.
They say the adoption of new military technologies often preceded a step-change in the geographical size of societies by 300 to 400 years. For example, chariot warfare appeared and spread in Eurasia and Africa about 3700 years ago. A few centuries later, societies much larger than any seen before emerged 鈥� including the 1 million-square-kilometre Egyptian New Kingdom.
鈥淚 think the core of our ideas is very solid,鈥� says Turchin. But he doesn鈥檛 expect there will be universal acceptance of the conclusions. Early reactions to the work suggest he is correct.
鈥淧art of me loves the idea of this kind of work: trying to reduce history to a series of equations,鈥� says at New York University. But he sees problems with the work, not least with the starting assumption that complicated variables can be encoded unambiguously in a database.
鈥淔or instance, they measure hierarchical complexity in terms of military hierarchy, administrative hierarchy and settlement hierarchy. Imagine trying to come up with figures for those variables for the United States today 鈥� if you asked 10 different experts, you might get 10 different answers,鈥� he says. 鈥淭hen imagine trying to do the same for an ancient society where you don鈥檛 even have written records to help you.鈥�
at University College London thinks the study鈥檚 conclusions might be an artefact of the starting assumptions 鈥� particularly the idea that societies evolve from simple to complex in a way that can be measured objectively. Last year, Wengrow and his colleague, the late David Graeber, arguing against such a linear model for the development of human societies, and questioning whether all human societies were small and simple prior to agriculture.
But Campbell does see some positives. 鈥淚t鈥檚 raising questions about how we do history and understand the past, and looking for different methodologies,鈥� he says. 鈥淭hat鈥檚 what science is about.鈥�
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