As from the 6th century (post christum natum this time) a small letters type began to arise – the Half-unicial – with clearly visible upper and downer lengths.
Roman Capitalis (1. century b. Chr.)
Rustika (5. century)
It pointed the way to the Caroline minuscule, the small letters of which were fast and comfort to write. It was developped by monks, forced to write quickly.
This typeface was »commis-
sioned« by Charlemagne (himself, he could neither read nor write). He wanted a typeface that was written and read in his whole empire. As this typeface was fixed by law, it became the type of the occident.
Unicialis (6. century)
Halfunicialis (10. century)
So you see that our small letters didn't develop at the same time as the capital letters, but came out of them and also replaced the capital letters at times.
Humanistic Minuscule (15. century)
These were reintroduced only much later in order to achieve a better legibility.