Explanation:

In 1860s, when railway lines were being laid down for the first time in the Punjab,engineers stumbled upon the site of Harappa in present-day Pakistan. Then, in 1930s archaeologists found the site, and realised that this was one of the oldest cities in the subcontinent.As this was the first city to be discovered, all other sites from where similar buildings (and other things) were found were described as Harappan. These cities developed about 4700 years ago.