Below is just an excerpt from my book Moen jo Daro: Metropolis of the Indus Civilization (2600-1900 BCE), for a detailed history and chronology of the seals you may refer to my book Indus Seals Beyond Geometry.
“Much before the discovery of Moen jo Daro, a seal depicting a humpless bull was unearthed and the story of the Indus seals typically begins with this discovery. The seal was rectangular in shape and engraved with a row of six signs or symbols above the image of the bull and ‘under its neck were two stars’ one of these is already faded. It was discovered in the last quarter of the nineteenth century from Harappa and its sketch and description was published by the Archaeological Survey of India in 1875. Alexander Cunningham the director general of the Survey while reporting the seal had also rejected it ‘They (symbols) are certainly not Indian letters; and as the bull which accompanies them is without a hump, I conclude that the seal is foreign to India.’ In 1877, however, he changed his opinion and suggested that the seal signs were possibly precursors of the Brahmi script inscribed on the pillars of King Asoka Maurya (304-232 BCE). This is yet another enthralling part of the seal story as it pushes the history of writing two thousand years beyond Brahmi, the oldest known script of India. The history of seals can even go beyond as occasionally seals keep appearing from fresh excavations, and some of these are dated to an earlier pre-Harappan phase. In fact, as early as in 1960s when Walter Fairservis scooped heaps of potsherds from Balochistan sites, he found among them a few button seals engraved with geometric patterns. Button seals are the earliest known seals in Indus civilization. A decade later more seals and some of them even older than Fairservis’ discovery were unearthed by Jean-Francois Jarrige from the site of Mehrgarh in Balochistan.”
parveentalpur 