The Indus Valley Civilization, in its mature phase, thrived for about 700 years, from around 2600 B.C. to 1900 B.C.

The script has remained undeciphered for more than a century. Carved stone seal from the Indus Valley civilization (ca. 2,500 B.C.E.). Photo by DEA/G. Nimatallah/De Agostini via Getty Images.

A stone seal inscribed with the Indus Valley script and depicting an Indian rhinoceros, found at Mohenjo-daro. DEA/A. DAGLI ORTI/De Agostini Editorial/Getty Images ...

An Indus Valley civilization excavation site in Pakistan. DeAgostini/Getty Images Mr. Parpola postulates that the signs found on the clay tablets were pictures that should be read as complete words.