A little over a century ago, British and Indian archeologists began excavating the remains of what they soon realized was a previously unknown civilization in the Indus Valley. Straddling parts of Pakistan and India and reaching into Afghanistan, the culture these explorers unearthed had existed at the same time as those of ancient Egypt and Mesopotamia, and covered a much larger area. It was also astonishingly advanced: sophisticated and complex, boasting large, carefully laid out cities, a relatively affluent population, writing, plumbing and baths, wide trade connections, and even standardized weights and measures. What kind of a society was the...