A 2022 clone of phabricator.wikimedia.org/source/mediawiki.git gives first commits from 2003 by:
TODO when was wikipedia open sourced from Nupedia? The early days of Wikipedia are quite obscure due to its transition from Nupedia.

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Wikipedia is a free online encyclopedia that allows users to edit and contribute content. Its history can be traced back to a few key milestones: 1. **Founding**: Wikipedia was launched on January 15, 2001, by Jimmy Wales and Larry Sanger as a part of the non-profit organization Wikimedia Foundation. Wikipedia was initially conceived as a complementary project to Nupedia, a free encyclopedia that had a formal editorial process and was largely written by experts.