It's all incremental. Here's some ancient history; some dates might be wrong.
1997: a commercial game, Total Annihilation, is released (by Cavedog Entertainment, led by Chris Taylor). It introduces a lot of concepts such as physically simulated weapons and streaming economy. One of the reasons for its popularity is that it is highly moddable.
1998: UberHack is created by BraveSirRobin. It is a simple mod for TA with some design changes (like anti-air only targetting planes) and fixes, and becomes a popular base for other modders' work. A lot of standalone units start being created and distributed over the TA community, usually in various unit packs.
1999: TA Demo Recorder is created by a clan called Swedish Yankspankers. It is basically a mod that allows to record games and play them back (something the retail game lacked).
2001: Swedish Yankspankers figure out that TA has orthogonal camera and no ability to rotate it, which makes TA's 3D world look 2D. They decide to evolve the demo recorder into a 3D engine able to use TA content.
2001: Absolute Annihilation (AA) is created by Caydr. It is a decently popular TA unit pack mod based on UberHack.
2004: first public release of Swedish Yankspankers' engine, by this time called TASpring (which eventually became just Spring). There's a separate infrastructure from TA (including a lobby server that makes it easy to play different mods). AA is ported to Spring and becomes the #1 most popular mod there. People learn about Spring from TA communities.
2006(ish): Balanced Annihilation (BA) is forked from AA (by Noize), focusing on competitive 1v1 play. Also quickly becomes the #1 mod because the author of AA left.
2007:
Saktoth is amazed after some noob pwns him in 1v1 BA with 1 Dart by driving around his factory. "Wow, you can do that!?". Decides to make a mod where physical unit interactions are important, forking BA to create Complete Annihilation (CA), and gathers a bunch of enthusiastic players to help (these are just Spring players he met in the lobby, mostly).
[LCC]quantum[0K] helps with game design (they come up with the idea of unit roles and balancing guidelines to keep units unique);
Licho creates autohosts that ease keeping the mod played and
detrino designs an update system which automatically downloads game updates, which allows very quick development.
[LCC]jK creates the GUI framework and
CarRepairer creates GUI.
Histidine (aka KingRaptor) contributes in all areas. CA is where ZK really got its identity.
2010:
GoogleFrog proposes a redesign of CA (merging CA's two factions into one and flattening the tech tree, alongside the needed factory changes). This becomes ZK.
Licho creates further infrastructure (website, later on the server) which lets ZK become a standalone game.
