Harappan beads can be identified as to location of manufacture (that’s someone’s idea of a dream job, right there), and have been used to track trade routes all over Asia. They got around. The Sumerians’ own origin tales involve arrival by sea (in what is today the Persian Gulf or to some Shatt al Arab); the names they used for most of their great cities and the rivers are not Sumerian names, meaning they were borrowed from whomever lived there before the Sumerians took over. They had seagoing trade networks, and I would probably not be the first to suggest they came from the Indus Valley.
“...I would probably not be the first to suggest they came from the Indus Valley.”
Interesting notion; yes, that would explain a lot and certainly make the Harappans even greater than they have seemed as the result of initial scrutinay.