The CBB

I wonder about the reasoning for that orthographic choice. Normally I find it quite jarring, given that <c> typically represents the exact opposite type of sound (/c/, /k~s/, /ts/, etc.). I imagine it may have come from an old Semitic glyph for /ʕ/, perhaps related in some way to Arabic 'ayn <ﻉ>.

My assumption is that it's based directly on <ﻉ>, since the smaller, upper portion resembles <c>.

I thought it resembled the IPA sign, which in turn resembles 'ayn.

I think it comes from ʿ, which is used in DIN 31635 for ʿayn.

<ﻉ> <ʿ> <c>

Yes.

It seems the letter ayin already had a C-like shape at least here and there throughout history, e.g. https://en.wikipedia.org/wiki/File:Lati ... al_Ain.svg , and the Aramaic form resembling a slightly turned U. The Somali shape could be a direct borrowing from Egyptology since it seems fair that there would be some connection between the people transliterating hieroglyphs and the people deriving the new Somali alphabet. There may have been a small influence from Turkish, the only other alphabet using it for a voiced sound (so far as I know), whose creation was fairly recent at the time.

I'm a little confused, though, since its history seems to be inseparable from that of alif, which I'd always thought was a completely separate letter going all the way back to Egyptian.

Personally I've long suspected the symbols for the glottal stop and [ʕ] in old romanizations,
namely ’ and ‘,
or ´ and `,
or IPA ʔ and ʕ,
ultimately have their origin in the Greek smooth and rough breathing marks, standing respectively for a word-initial vowel and a vowel preceded by [h]... or am I wrong? I've never come across a trustworthy source explaining the history of these symbols. Or those of Egyptology.