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33 3 Innovation within an Ecological Limit All ancient societies had to live within the limits of an important ecological constraint: the capacity of nitrogen-fixing bacteria to resupply agricultural soil with nitrogen. Even the great river valley civilizations, blessed by floods that routinely delivered nutrient-rich silt to agricultural soils, depended on the services of these single-celled creatures. The fertile alluvial soil along the Nile River, for example, depended upon the activity of nitrogen-fixing bacteria scattered throughout the entire 3.2 million square kilometers drained by the river, one-tenth of the entire African continent. Ancient societies did not greatly alter the quantity of nitrogen moving through regional ecosystems. For one thing, rivers collected and deposited silt whether people downstream made use of that material or not. For another, the Earth was home to a lot fewer people then. Today, about 120 million people live in the Nile River basin, nearly 80 million in Egypt alone. In contrast, 3,500 years ago, at the height of ancient Egypt’s power, the Nile River valley and delta, one of the most densely populated areas of the ancient world, supported 4 to 5 million people. At the time, the population of the entire world stood at about 50 million people, less than 1 percent of the global population today.1 The largest effect that ancient societies had on flows of nitrogen was due to landscape-scale changes in support of agriculture. These changes, such as the clearing of forests, the construction of irrigation systems and levees, and the elimination of wetlands, altered patterns associated with denitrification and the distribution of nitrogen-rich silt. Even where humans were bumping up the production of fixed nitrogen by cultivating plants that worked symbiotically with nitrogen-fixing bacteria, the landscape-scale changes mattered more. For example, 3,000 years ago, as the ancient Zhou dynasty emerged along the Huang He in northern China, many peasants in the watershed 34 LE A R NING TO BY PA SS A N ECOLOGIC A L LIMI T grew soybeans, an important nitrogen-fixing host. The land supported up to fifty people to the square kilometer, and most of the harvest was consumed locally.2 Even if we assume that all of the protein in a peasant’s diet came from what he grew and that no nitrogen-rich waste was recycled, the amount of fixed nitrogen that the cultivation of soybeans added to the Huang He would have amounted to something less than ten kilograms per hectare each year.3 The amount is on the same order of magnitude as that which escapes from minimally disturbed landscapes.4 In terms of regional environmental changes, physical alteration of the land, especially actions that increased erosion or eliminated wetlands, mattered more. On the other hand, the cultivation of crops that worked symbiotically with nitrogen-fixing bacteria mattered greatly to the peasants of the Huang He basin. The nitrogen secured by growing a legume such as soybeans ensured that a steady supply of protein could be obtained from the land; and 13 to 19 percent of protein consists of nitrogen, which must be available while plants are growing. If nitrogen compounds are unavailable to growing plants, their ability to produce green matter is also compromised, reducing their ability to capture the sun’s energy and hence to grow at all. Higher yields made possible by the cultivation of legumes had consequences that went far beyond simply putting more protein on the table. All ancient societies hovered near subsistence, and only a relatively small amount of nutrition was available to keep people who were not engaged in agriculture alive. As a result, relatively small increases in agricultural productivity could translate into an opportunity for substantially more people to devote their labor to something other than agriculture. And over the long term, the amount of labor that ancient societies could devote to something other than agriculture mattered greatly. N and the Large Consequences of Small Increases No ancient society could maintain a population of soldiers, miners, artisans, judges, accountants, and administrators unless peasants were able to produce enough food to feed them. And generating that surplus presented a signi ficant challenge. A rough rule of thumb is that, in a traditional agricultural society, about 90 percent of the population was needed to produce, process, and transport enough food to feed the entire society. In other words, it took about nine people engaged in agriculture to support one nonagricultural worker...

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