There are many versions of the development and early history of Unix, and many are inaccurate. Peter Salus' book A Quarter Century of UNIX has the authorative description of Unix's early history, and goes up to 1994. You should also consult Life With UNIX by Don Libes and Sandy Ressler for a similar, but shorter, description. This only goes up to 1989.
A timeline of Unix development taken from `Life With Unix' is available, with some of the dates altered to reflect the details in `A Quarter Century of Unix'. If any other dates are inaccurate, please let me know. The timeline covers 1969 to 1989: we need another diagram from 1989 onwards.
There are some on-line documents which describe Unix's history.