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"n the early part of the first millennium B.C., Persians started constructing elaborate tunnel systems called qanats for extracting groundwater in the dry mountain basins of present-day Iran (see figure 1). Qanat tunnels were hand-dug, just large enough to fit the person doing the digging. Along the length of a qanat, which can be several kilometers, vertical shafts were sunk at intervals of 20 to 30 meters to remove excavated material and to provide ventilation and access for repairs. The main qanat tunnel sloped gently down from pre-mountainous alluvial fans to an outlet at a village. From there, canals would distribute water to fields for irrigation. These amazing structures allowed Persian farmers to succeed despite long dry periods when there was no surface water to be had. Many qanats are still in use stretching from China on the east to Morocco on the west, and even to the Americas."

"Before we'd even become Homo sapiens sapiens, humans lived everywhere from South Africa up to Britain and over to China. There were mountain people, coastal people, people who hunted woolly mammoths, and people who'd never seen a woolly mammoth in their lives. Just like we see with distinct groups of other animals, these differences of experience, adaptation, and expectation would have made for real cultural and even physical differences between populations. A few hundred thousand years later, as groups began to settle down and build cities they often enclosed them within massive walls. The ways different cultures interact across those walls could be seen as the central story of civilization. Top archaeologists from around the world have been exploring that story for the past week in public presentations and conversations at the 2015 Dialogue of Civilizations in Beijing."