The Bronze Trade Origins of Cities

Why Metals Explain The Rise of Civilization

One interesting fact about the origin of complex urban civilization (ie, defined cities characterized by hierarchies, elites, and often writing) is how long it took for them to arise after the beginnings of agriculture some twelve thousand years ago. While there were a few urban areas, like for instance Çatal Höyük, these were really just large villages in which the entire population was engaged with agriculture in the surrounding areas and packed cheek by jowl for defense purposes with few evident markers of social stratification.

In fact, it took about as much time between the origin of agriculture and the emergence of the first true cities, in the 4th millennium BC, as has elapsed since then. When cities did first arise, you saw the near-synchronous emergence of urban centers in the Bronze Age characterized by internal stratification and elites across Egypt, Mesopotamia, the Indus Valley, and Central Asia. Why did it take so long for these cities to form, and what explains the timing? Why did they start up so quickly all over the world?

It’s a foundational question at the heart of urban economics, politics, and trade, and a new paper by Matthias Flückiger, Mario Larch, Markus Ludwig, and Luigi Pascali provides a provocative answer: the key to the emergence of cities in the Bronze Age was, well, Bronze.

Tin and Copper

The key thing about bronze is, to make it, you need two minerals: tin and copper. Both metals are somewhat rare, though tin is considerably rarer. Bronze was sufficiently valuable in that era — for both agricultural production and to make bronze weapons — that it therefore makes sense to spend enormous effort to transport both copper and tin to agricultural areas to smelt into bronze for use in production and defense.

The origin of tin, in particular, is a source of huge scholarly debate in the historical community, and Flückiger et al. side with the current consensus: at least in the initial phase of the Bronze Age, tin was mined in the mountains around Uzbekistan and Afghanistan by the BMAC civilization. It might seem slightly odd that some of the best early signs of urban civilization come from Central Asia, but it makes much more sense if this was a terminus for an essential trade product. From there, tin travelled over mountains to the Indus Valley civilization, and from there by boat to Ur. Ur itself, the world’s greatest urban center at the time, was situated perfectly as a trade entrepot for transshipment of metals further upstream in Mesopotamia.

Cities like Ur grew by inserting themselves into this metal trade — they were basically stationary bandits who taxed trade at crucial nodes in the metal shipment network, which provided a surplus to sustain local elites. We then see the rise of local elites, new religions, and complex writing to manage this trade.

It’s a very neat theory, and the authors provide some compelling evidence. First, they can draw connecting lines from local agricultural populations to local sources of metal – note in particular the Central Asian tin sources.

Next, they draw a trade index — the lowest cost trips connecting agricultural land to the nearest copper and tin mines, producing a set of optimal trade lines. Areas higher in this index are essential more central to the metal trade network — and this is also where the cities generally line up.

This result shows up in a regression as well — ie, areas which are more dense in trade links are more likely to have a city, even controlling for other relevant factors. Interestingly, you see that local agricultural productivity, which you might imagine is important for local urbanization, actually enters in here negatively after controlling for transit.

A good example here, which the authors discuss in some detail, is Assur, the capital of the Old Assyrian empire, located at the confluence of the Tigris and two important tributary rivers. The land around Assur was fairly desert-like, even back then, but it commanded an important node in the trade network, and we have extensive records of Assyrian merchants trading all over this region, especially in Anatolia. The implication is that cities, and ultimately whole civilizations like the Assyrians, were founded on the surplus to be had from taxing and trading high value metals passing through their territory. Supporting the analysis further, they argue that the hold out power — something like the ability of merchants to skip a particular city and head elsewhere instead — should associate with taxation authority, and seems to line up with the size of the elite class. This general argument is also reminiscent of the paper by Bleakley and Lin – that cities are often found on the fall line, where portage is necessary up rivers. So centrality in trade networks (and probably in transhipment across modes of transport as well, ie unloading cargo) is plausibly a really important driver of cities across much of history.

Another piece of broadly confirming evidence here is the Uluburun shipwreck — a Late Bronze Age sunken ship with enormous troves of trade goods, including huge amounts of raw tin and copper, indicating the vast scope of trade that took place in this period, especially for those key metals.

The End of the Bronze Age

The origin of the tin in the Uluburun wreck is still being debated, but the general historical consensus seems to be that at least some tin in that time period was still being sourced from Central Asia, while some was starting to get sourced from Cornwall in Britain.

This brings us to discussing the cataclysmic end of the Bronze Age, culminating in around the period 1177 BC in Greece and the Middle East, but actually ending before then in South Asia.

The Indus Valley has a number of unique characteristics relative to the other civilizational zones in this era. It rose and fell quite quickly — a period between 2600 BC and 1900 BC. It was also unusual in social organization; here’s what Patrick Wyman says:

What we don’t see in the Indus Valley Civilization is evidence of strong hierarchies. As far as we can tell, no kings, exclusive cliques of nobles, or other elites dominated Indus society. There are no grand royal tombs or palaces, no monuments to the power and ego of a powerful figure determined to make his or her mark on the built landscape. Cities were filled with large complexes of relatively uniform houses, with rooms for domestic and craft activities arranged around a central courtyard. These houses were then grouped into separate quarters or neighborhoods. No one individual or group seems to have stood above the rest. For that matter, there’s precious little evidence to suggest that even the great cities somehow exercised power or sway over their smaller neighbors.

This doesn’t mean that everybody was totally equal within Indus society; instead, it means that we don’t yet have the evidence or the conceptual tools to understand how Indus society was organized. It was certainly comprised of different groups, who presumably cooperated and competed with one another, but we don’t really have any idea on what basis those groups were organized: maybe kinship, maybe occupation, maybe ethnic identity or religious affiliation.

Shedding some potential light on these identities is a new paper arguing that the as-yet undeciphered Indus Script were licenses issued by guilds to enforce standardized taxation rules, and the emblems were signifiers for the local guilds themselves.

The argument I would make is that it would make a lot of sense of these guilds were really jatis – ie endogamous caste organizations which are an essential part of South Asia today. It’s possible this institution was present already in the Indus Valley Civilization, growing to organize the emerging trade networks. While pharaohs ruled in Egypt, priest-kings held sway in Mesopotamia, maybe India innovated a kin-based method of social organization to handle trade, which then ultimately provided so durable as to last to the present day. It will take more research to sort that out, but it would be fascinating if the legacy of the Bronze Age can be seen now the social organization of the Indian subcontinent.

Then you have the rapid collapse of the Indus Valley Civilization around 1900 BC, centuries before the ruptures that happened further west. Many accounts emphasize climate shocks, but it would make a lot of sense if the ultimate cause were trade related. Around this time, the sources of tin in the BMAC civilization were facing serious stresses, with the entry of steppe nomads from the north. Potentially the steppe nomads cut off the tin trade to the Indus Valley; maybe the tin started to move directly west overland through Iran rather than over the mountains to the Indus Valley (maybe being shipped directly to Mari); or potentially new tin in Cornwall was brought online and proved to be cheaper than the Central Asian tin.

Whatever the reason; the collapse of the eastern side of the Bronze Age world was followed by wholesale collapse of the western part. Clearly, the end of international trade routes that kept the system going was part of the process of complete system collapse. But why did the trade routes fall in the first place?

One theory is that this is when mass infantry tactics were developed, which turned out to overturn existing states. A related theory, which Flückiger et al. seem to favor (as do others), is that iron was invented around this time period and started to diffuse. Because iron can be found in many more places, it produces a more decentralized trading network. And so you have a potentially catalytic effect — the introduction. of some iron weapons and warfare starts to eliminate some Bronze Age states, which may feed the demand for investing in more iron weapons, and so the whole system collapses. It’s certainly a very attractive theory from an economics perspective, though I gather direct evidence for actual iron weapons used in the Bronze Age collapse is more mixed (though iron weapons themselves may degrade more over time), and so it’s hard to reject the theory that the Bronze Age collapse happened for other reasons and iron weapons were the endogenous technological response (which is probably the dominant historical view today).

There is much more in the paper — including detailed statistical and trade analysis to support all of this, historical analyses of specific episodes and really neat datasets, and all sorts of new questions such as what role do amber, jade, obsidian, and developments in the New World have to do with it. In the end, cities and complex political organization reformed themselves in the Iron Age — but this time, the basis of trade (for whatever reason) was now bulk trades in primary agricultural commodities. But this whole paper was one of the most thought-provoking articles I read, and I hope it spurs more research in these fascinating areas of human history.