Driving in Greece back in 1996 I mouthed-out the Greek-alphabetised road sign for Thessaloniki, and was surprised to realise that the symbol for Theta (at the start of the signposting word) resembles a round mouth with the tongue horizontally across (& between the teeth) as one's making the "th" sound. Is this actually the origin for the Theta symbol?
I looked up the Wiki article: https://en.wikipedia.org/wiki/History_of_the_Greek_alphabet which says theta derives from the Phoenician teth, which is a circle with a diagonal cross inscribed in it. So that doesn't seem to fit your theory, unfortunately. I was curious to see that the Phoenician alphabet only specified consonants, and the comments that this is more suited to Semitic languages. I recall from my time in Dubai that modern Arabic is like that too - only the consonants are written, I think, the vowels sounds being interpolated from the context. But I wonder if our forum member @wegs may know more about this - though she is of Persian rather than Arab extraction.
