The earliest imprints of human activities in India go back to the Paleolithic Age, roughly between 400,000 and 200,000 B.C. Stone implements and cave paintings from this period have been discovered in many parts of the South Asia. Evidence of domestication of animals, the adoption of agriculture, permanent village settlements, and wheel-turned pottery dating from the middle of the sixth millennium B.C. has been found in the foothills of Sindh and Baluchistan (or Balochistan in current Pakistani usage), both in present-day Pakistan. One of the first great civilizations -- with a writing system, urban centers, and a diversified social and economic system -- appeared around 3,000 B.C. along the Indus River valley in Punjab and Sindh. It covered more than 800,000 square kilometers, from the borders of Baluchistan to the deserts of Rajasthan, from the Himalayan foothills to the southern tip of Gujarat. The remnants of two major cities -- Mohenjo-daro and Harappa--reveal remarkable engineering feats of uniform urban planning and carefully executed layout, water supply, and drainage. Excavations at these sites and later archaeological digs at about seventy other locations in India and Pakistan provide a composite picture of what is now generally known as Harappan culture (2500-1600 B.C.).
The major cities contained a few large buildings including a citadel, a large bath -- perhaps for personal and communal ablution -- differentiated living quarters, flat-roofed brick houses, and fortified administrative or religious centers enclosing meeting halls and granaries. Essentially a city culture, Harappan life was supported by extensive agricultural production and by commerce, which included trade with Sumer in southern Mesopotamia (modern Iraq). The people made tools and weapons from copper and bronze but not iron. Cotton was woven and dyed for clothing; wheat, rice, and a variety of vegetables and fruits were cultivated; and a number of animals, including the humped bull, were domesticated. Harappan culture was conservative and remained relatively unchanged for centuries; whenever cities were rebuilt after periodic flooding, the new level of construction closely followed the previous pattern. Although stability, regularity, and conservatism seem to have been the hallmarks of this people, it is unclear who wielded authority, whether an aristocratic, priestly, or commercial minority.
By far the most exquisite but most obscure Harappan artifacts unearthed to date are steatite seals found in abundance at Mohenjo-daro. These small, flat, and mostly square objects with human or animal motifs provide the most accurate picture there is of Harappan life. They also have inscriptions generally thought to be in the Harappan script, which has eluded scholarly attempts at deciphering it. Debate abounds as to whether the script represents numbers or an alphabet, and, if an alphabet, whether it is proto-Dravidian or proto-Sanskrit.
The possible reasons for the decline of Harappan civilization have long troubled scholars. Invaders from central and western Asia are considered by some historians to have been the "destroyers" of Harappan cities, but this view is open to reinterpretation. More plausible explanations are recurrent floods caused by tectonic earth movement, soil salinity, and desertification.