Terrific article! I know this may sound far-fetched to some, but perhaps China's long-standing civilization is due to their use of "night soil" or human waste as a fertilizer. When I read about ancient ruins, I find it odd that the existence of sewer pipes is considered a sign of an advanced society. Looked at another way, they are highly efficient devices for moving valuable nutrients into the ocean. Grain, vegetables, and meat are moved from rural areas into urban centers, where they are flushed into the nearest river and washed into the ocean. Over centuries, this will almost certainly reduce agricultural productivity, because ancient farmers couldn't economically borrow nutrients from distant places, as we have done by mining guano or producing fertilizers from natural gas. In contrast, China and Japan had efficient systems for moving feces and urine back into the fields. I heard a radio interview with William McDonough in which he described growing up in Japan in the 50s, hearing farm carts rolling into the city full of produce in the mornings and rolling out full of sewage in the evenings. I have also read that in 1880s Japan, people were paid for the contents of their privy barrels, and five or six people living in an apartment could pay the rent with the proceeds. Apparently farmers in China still build outhouses next to highways to collect night soil from passing truckers. I believe attaching this kind of value to human "waste" is a sign of a long-term agriculture that can sustain dense populations like those of China and Japan. Yes, I know they had periodic outbreaks of cholera, but there are other ways to cultivate rice without so much standing water (see Masanobu Fukuoka) and other ways to process the manure (see The Humanure Handbook).

All ancient civilizations depended almost completely on agriculture for their energy needs, and their EROEI was based mostly on fertility which determined how many people a single farmer could feed. Legionnaires, bureaucrats, miners, and craftsmen did not have time to farm, so they had to be fed on the agricultural surplus, and the gold may have served largely as a marker for the basic value unit of food. An influx of gold would then only cause inflation, and perhaps the more important aspect of conquest was obtaining fertile land, previously inhabited by "barbarians" too uncivilized to have removed the soil nutrients efficiently.