Theory number 4I have noticed some similarities in Indus Valley signs and Brahmi characters. Could it be possible that although evidence of the Indus Valley script disappears around 1700 B.C., some signs somehow made their way into Brahmi script which first appears in around the fourth century B.C.?
Check into this…For this theory to be true, people would have had to know about the Indus script in the years between 1700 B.C. and about 400 B.C. However, we have no evidence of the Indus script being used during this time. It seems unlikely that if the signs were known about we wouldn't have any evidence that they were used for over a thousand years after our latest evidence.
I can think of only one way that this theory could be true. If there were texts written in Indus script which survived over the period between 1700 B.C. and 500 B.C. but do not survive today it might be possible. For example, if they were written on a material which might not survive, like tree bark or parchment.This is certainly worth considering until we have evidence that definitely shows it is impossible.