Star Trek: 10 Things You Didn't Know About The Borg
7. On The Origin Of Cybernetic Species By Means Of Mechanical Selection
In one form or another, the Borg have appeared (so far) in 52 episodes and one film of Star Trek. We have, in theory, seen the end of the OG Collective in The Last Generation, but beyond their Delta Quadrant debuts and a smattering of Vaadwaur history, we still know little to nothing from canon about the Borg's beginnings. If you'll kindly hook up a tubule to the data node, we'll have to assimilate some beta canon instead.
There's nothing natural in selection by the Borg, but they must have mechanically engineered their way up from something. One of the most recurrent origin stories for the Borg is tied to V'ger. As noted in Star Trek Chronology, Gene Roddenberry himself, after the production of Q Who, even "half-jokingly speculated that the planet encountered by Voyager [in Star Trek: The Motion Picture] might have been the Borg homeworld." The V'ger link is then used directly as the genesis of the Borg in the video game Star Trek: Legacy and in the Shatnerverse novels.
As we touched on in our Borg Queen article, the Star Trek: Destiny book trilogy gives another explanation for the origins of the cybernetic superstars. In those, it is the catastrophic interaction between the (usually friendly and technologically advanced) Caeliar and members of the crew of the NX-02 (the bloody MACOs!), as well as a bit of time travel, that leads to the birth of the Borg.