I found this image on the web showing the evolution of the bass (F) clef but there's no date or source for each instance.
Can anyone provide the approximate date of these changes?
Guido de Arezzo invented the line system around 1025. He used a "c" or "f" to originally describe half steps. The drawing seems a little bit inaccurate, as in the beginning the top of "f" or "F" was positioned right at the second line, while later the two dots encompassed this line (which is the f.) Also - the "f" key was originally derived from the Bariton key, so actually the key we are using today would be a "Chiavette". (A Chiavette is a key that is positioned on a different line the usual.)
Willi Apel, a German born US Music Historian wrote a detailed book about the development of Notation ("Notation of Polyphonic Music, 900–1600 ") You can find it here: https://archive.org/details/notationofpolyph1953apel