I know somebody who wrote a book on alphabets and said the Egyptian alphabet was based on some ancient Indian alphabet, which had its roots in some ancient astrological symbols.
There’s a lot of prehistory that is not well understood.
It's not clear the Harappan symbols were an alphabet (the hieroglyphs certainly weren't) or even written language, though they're clearly symbolic. Nobody has any idea whether the Harappan symbols somehow relate to astrology. My best guess is that they're similar to heraldry, cattle brands, or graffiti tags: ideograms representing personal names or lines of descent. But I'm no expert in the field.
The main thing is that there's a lot of prehistory. The Stone Age was 3 million years long; people have been cooking their food for 1.75 million years; anatomically modern humans are 250,000 years old; behavioral modernity (elaborate graves, fishing, artifact diversity, common figurative art) is from 50,000 years ago, or perhaps as long as 80,000; the first proto-cities like Çatal Höyük date from 10,000 years ago; the city of Eridu is from 7500 years ago.
In places, "history" (in the sense of written records) is only 500 years old; in most of Europe it's less than 2000 years old. Imhotep built the first Egyptian pyramid only 4700 years ago, but in his day, people had already had brains pretty much the same as our own for ten times that long. 90% of the time of people painting pictures and burying their dead happened before Imhotep; half of the time of people living in cities happened before Imhotep. And 99% of the time of people with brains pretty much the same as our own was before Columbus.
That is, the founding of proto-cities like Çatal Höyük was as long ago for Imhotep as Imhotep is for us. But Imhotep didn't have writings from Çatal Höyük to read or archaeologists to investigate it.
The Egyptian hieroglyphics might be derived from cuneiform, but the more popular hypothesis is that they were invented by someone who didn't know how to read or write.
