system
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human
ß = ſ + s

In other words, ß was originally a ligature, combining the old-style long s (ſ) with a regular s (s).
gpt
In other scripts, it was a ligature of long-s and z, hence its name Eszett.  
(I think both variants developed roughly at the same time, but a few centuries ago when printing letters were standardized, in roman fonts (like this one) the ss ligature was favored, while in gothic/fraktur fonts it was the sz-ligatur. Then German was almost exclusively written in gothic fonts for a few centuries, until the mid 20th (it was actually the Nazis who changed it back.))