Sequence diagrams existed prior to UML. They were extensively used in telecom: “message sequence charts.”

They were part of a telecom language called SDL. There were tools to “compile” to various languages such as C. If you followed their rules, you were supposed to be able to go back and forth between C and SDL.

I only remember this because the biggest tool provider at that time was a French company called Verilog. They preceded the HDL by a few years, I guess.

I used to work for Motorola's telecom division and we used IBM Rational Suite to generate C++ classes from the SDL. Merges (and we used very long-lived release branches) were pure hell...

IIRC sequence diagrams were invented by one of Carl Hewitt’s PhD students during Hewitt’s earlyish work on the Actor model?