Where did the first kombucha SCOBY come from?

I work at a large kombucha brewery and my wife is the head brewer(13 yrs combined experience).

As with vinegars and sourdoughs, the kombucha cultures(yeast and bacteria) are generally present in most environments. They can be created "from scratch" under the right conditions. It's much easier to start a vinegar from nothing, as the variety of yeasts and bacteria is smaller.

No one is quite sure of the exact origin of kombucha, but it is likely that it was created by accident or casual experiment involving sweetened tea and an existing vinegar ferment. The microbes that ended up thriving evolved or were selected for their ability to utilize the caffeine and other components present in the tea, and were then propagated forward.

Most folks find it easier to pitch kombucha from an existing set of microbes that to select gradually over time for the flavor and microbial content, as there are many wild yeasts and bacterias that create funky, not so fun flavors and smells, like "foot" or "body odor".