It's also the origin of the integral sign! (Mentioned on the Wikipedia article.) There's a nice accidental parallel between sigma notation (the discrete, "angular" summation) and integral notation (the continuous, "smooth" summation).

Another s fact is that Greek has lower case σ and additionally the variant ς that only appears at the ends of words, which is very Latin-s-like. https://en.wikipedia.org/wiki/Sigma