The earliest excavations in the Indus valley were done at Harappa in the West Punjab and Mohenjodaro in Sind. Both places are now in Pakistan. The findings in these two cities brought to light a civilization. It was first called the 'The Indus Valley Civilization'. But this civilization was later named as the 'Indus Civilization' due to the discovery of more and more sites far away from the Indus valley. Also, it has come to be called the 'Harappan Civilization' after the name of its first discovered site.
Among the many other sites excavated, the most important are Kot Diji in Sind, Kalibangan in Rajasthan, Rupar in the Punjab, Banawali in Haryana, Lothal, Surkotada and Dholavira, all the three in Gujarat. The larger cities are approximately a hundred hectares in size. Mohenjodara is the largest of all the Indus cities and it is estimated to have spread over an area of 200 hectares.
The archaeological findings excavated for the last eight decades reveal the gradual development of the Harappan culture. There are four important stages or phases of evolution and they are named as pre-Harappan, early-Harappan, mature-Harappan and late Harappan.
The pre-Harappan stage is located in eastern Baluchistan. The excavations at Mehrgarh 150 miles to the northwest of Mohenjodaro reveal the existence of pre-Harappan culture. In this stage, the nomadic people began to lead a settled agricultural life.
In the early-Harappan stage, the people lived in large villages in the plains. There was a gradual growth of towns in the Indus valley. Also, the transition from rural to urban life took place during this period. The sites of Amri and Kot Diji remain the evidence for early-Harappan stage.
In the mature-Harappan stage, great cities emerged. The excavations at Kalibangan with its elaborate town planning and urban features prove this phase of evolution.
In the late-Harappan stage, the decline of the Indus culture started. The excavations at Lothal reveal this stage of evolution. Lothal with its port was founded much later. It was surrounded by a massive brick wall as flood protection. Lothal remained an emporium of trade between the Harappan civilization and the remaining part of India as well as Mesopotamia.
In 1931, Sir John Marshall estimated the duration of the occupation of Mohenjodaro between 3250 and 2750 B.C. Subsequently, as and when new sites were discovered, the dating of the Harappan culture is modified. The advent of the radiocarbon method paves way for fixing almost accurate dates. By 1956, Fairservis brought down the dating of the Harappan culture to between 2000 and 1500 B.C. on the basis of radiocarbon dates of his findings. In 1964, D.P. Agarwal came to the conclusion that the total span of this culture should be between 2300 and 1750 B.C. Yet, there is further scope of modification of these dates.