How did these keywords and concepts come to life? What were the forces and problems that made them appear? What was the first language to have them?
Actually, it's not just about public/private/protected, but rather the whole range of keywords that enforce some rules (abstract, final, internal).
But, please, do not assume things. Answer if you know at least part of the answer or answer if you lived those moments. References are greatly appreciated.
this sort of thing starts out with multiple language designers asking 'what's a simple, logical name for this concept'? then, over time certain names become popular (sometimes because they're good names, sometimes just because). add 20 years, and most people end up picking the same names, based on what they've seen.
a similar question, perhaps, to asking how new words get added to (say) the English language.
public, private and protected access modifiers come from C++. It seems that public and private already existed in "C with classes", short lived precursor of C++. This is probably detailed in The design and Evolution of C++.
I think abstract and final come from Java and internal from C#.
For C++, the origins of private and public protection come from before Stroustrup's experiments with C With Classes, but from an even older system - the Cambridge CAP computer. This is described in section 2.10 of "The Design & Evolution of C++".
As for protected, that has had a murkier past & I don't have a good reference for it.
Simula (1967), considered to be the first OO language, has modifiers called protected and hidden. I assume that public is the default, I can't remember. It also uses virtual.
And, with thanks to Pavel, Simula introduced the most important keywords (and concepts) of class, this, new, downcasting and reference types.
SmallTalk (1980), a later but much more fundamental OO language, gave us Methods responding to Messages. This basically is the same functionality as virtual functions. Messages and Classes were later imitated in C (non-OO) to give the Windows API polymorphic behavior. But still needing ugly switch-statements and function pointers to replace inheritance.
The first use of Properties was, as far as I know, in Delphi (Object Pascal, < 1994).