Sometimes, in the ancient writing samples found in the Indian subcontinent, we find that a mixture of Harappan and Brahmi features has been used. This definitely points towards a continuous evolutionary process that transformed the Harappan script into the later day Brahmi. This also explains why many of the Harappan signs seem to have been simply carried forward (even in actual form) in the Brahmi script.
Subhajit Ganguly, Call Of The Lost Ages - A Study Of The Indus Valley Script
brahmi, harappa, history, india, indus-valley, script