The original SDBE 1.2 was ported to Oracle at SDE 2.0, fundamentally restructured at SDE 3.0 (adding shapelib and the Projection Engine), then ported to databases not named Oracle (DB2, Informix, Sybase, MS SQL Server) in subsequent snapshots.
Versioning was added to ArcSDE by Esri at 8.0 (which also discontinued Sybase support; PostgreSQL was added later).
I'm not sure who started the concept of multiversion state trees. It probably wasn't original to Esri, but the details of the Esri implementation certainly were.
Thanks Vince. I know I have read an article about the original work someplace... but my google skills are failing me.
What about the geometric network? Do you know when that was released?
No idea. I was trained on 1.2, a certified instructor at 2.0, and one of the class developers at 3.0, but Ed Services moved the training back to full-time instructors at 8.0, so various enhancements after that ran outside my radar. All the "What's New" docs are likely still online somewhere (well, at least 9.0 forward), so that seems a likely source for that history.
I "think" Geometric Networks were released with ArcInfo Workstation 7 but it had simple edges vs. the complex edge seen with the release of ArcInfo Workstation 8. My Google skills found this that makes me think this was close to the release date - Developing an ArcInfo SDE Geodatabase for Hydraulic Modeling (esri.com)