Aa Bb Dd Ee Gg Hh Ii Mm Nn, etc.
Why are they different characters all together?
Here's a GIF detailing the evolution of uppercase letters.
As for lowercase letters, some of them that aren't replicas of their capital counterparts are based on Greek. For example, δ (lowercase delta) became d, and α (lowercase alpha) became a.
Also, some changes came once Charlemagne ascended to the throne of the Holy Roman Emperor in 771. Upon traveling to Parma, Charlemagne met an English scholar named Alcuin, who Charlemagne challenged to organize the educational system in the HRE. Alcuin did so by standardizing calligraphy used to copy manuscripts, which became known as Carolingian minuscule. (source)
hi! you may find something of interest in these threads (which include a couple of links to a post in /r/linguistics)