The original Latin alphabet contained only 20 letters. Most people are familiar with the fact that J, U and W were fairly late additions, coming during the Middle Ages or later.
Some time earlier, perhaps during the Empire or possibly the Republic, the letters K, Y and Z were borrowed from Greek. Is it possible to pinpoint with any kind of accuracy exactly when these were borrowed? I realize that narrowing this down to any closer than about a century or so will probably be impossible, but can we at least get it that close?
The sign that they’ve been adopted is probably when someone writes the alphabet and includes them. Or perhaps a pangram. I once saw a Latin pangram (text that contains all the letters) used as a penmanship exercise for students and it only had 20 different letters. I don’t know what period this pangram dated to, though.
I remember reading somewhere (no cite comes to mind, sorry) that K was a letter in the earliest written forms of Latin, in the alphabet derived from Etruscan, but was later phased out in favor of C. Originally, the unvoiced velar stop was written with one of three letters, depending on the following vowel: C with E and I, K with A, Q with O and U.
This may have been to signify somewhat different places of articulation with each of the vowels: slightly to the front for E and I, in the middle for A, and slightly to the back for O and U. Then in Classical Latin, C replaced all instances of this phoneme, except for Q before U before another vowel (as in quorum), and a few survivors of K like kalendae.
The form of the letter C is descended from gamma, originally pronounced /g/ – but the Etruscan language did not distinguish the voiced and sounds of /g/ and /k/, so the Romans learned it as an unvoiced k sound. Later, the letter G was invented for the voiced velar stop by adding an extrz stroke to C.
As for Y and Z, they were imported from Greek in the 1st century BCE, since Rome had been conquering Greece, the Greek language was studied by educated Romans, and plenty of Greek vocabulary was being borrowed.