It’s been too long since I did one of these—five whole months! You’ve probably been terrified that you won’t get the information you need but don’t worry. It’s here.
First, some review. In English, we sue the Latin alphabet cultivated by the Romans. For the most part, they adapted the alphabet of the Etruscan region, a place with a unique language now losteven if we still use the symbols. The Etruscans actually took their alphabet from the Greeks, who adapted the alphabet of the Phoenicians for their own use with one major difference: the Phoenicians did not use vowels. Their alphabet was developed from the more pictographic Proto-Sinaitic, one of the first alphabets, created using Egyptian hieroglyphs as symbols for consonant sounds.
Anyway, pretty much all throughout history, versions of N are just M with a leg missing(except for the Greek lowercase nu, which for some reason is just ν). But although M always looked like N, its appearance still evolved over time. In Etruscan it looked like a y or a lowercase n with long legs. If you go all the way back to Phoenician, it looks even more like a y, or even just like a hook. Apparently this is because in the Proto-Sinaitic language, they chose the symbol for “nahas” to signify the n sound. Oh, and nahas? It means snake : ).
TL;DR: it wasn’t enough for M and N to sound alike. They had to look alike too.