This post was published more than a few years ago (on 2004-08-15) and may contain inaccurate technical information, outmoded thoughts, or cringe takes. Proceed at your own risk.
Alan C. Kay's The Early History of Smalltalk, courtesy Thomas Gagné. As I understand it, extensions to the C language were based on Smalltalk-80, to create the Objective-C language I'm learning right now.