Writing became phonetic the moment people started drawing pictures for how words sound

The earliest writing systems, like Sumerian cuneiform and Egyptian hieroglyphs, began as pictures standing for objects. The breakthrough that let them capture any spoken word was the rebus principle: reusing a picture for its sound rather than its meaning, the way a drawing of a "bee" plus a "leaf" could spell "belief." That trick turned picture-writing into full writing.

— Geoffrey Sampson, Writing Systems: A Linguistic Introduction — Stanford University Press, 1985
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