This article on the IBM Developer site has a history of shell evolution. It isn’t anything novel, but the nice family tree diagram and comparison of scripts in different shells are illuminating.
They don’t discuss zsh. The zsh FAQ outlines a brief history of that shell. In that FAQ, zsh is described as closest to the Korn shell (ksh). There are also some options to make zsh act like other shells like CSH_JUNKIE that makes it more compatible with csh. Sort of the best of both worlds at the expense of some addition knobs and complexity.