First, we must discover the first time and place man began interacting with wheat and/or rye. Turkey and northern Syria are the likely locale, and I suspect the date is much older than is currently speculated and/or uncovered from current paleo-archaeological research.
Second, we must discover the common forms of fermentations occurring in that area at that time, both spontaneous and as they relate to man.
My hypothesis: Lb SF evolved from a set of milk-based fermentative LAB microflora (e.g., one in which man was involved) to a grain-based one through a rapid series of reductive evolutionary steps that resulted in massive gene-death (inactivation) and deletion (total decrease in genome size) as well as an efficient genomic design that allowed it to “exploit” (increase metabolic co-existence) several species, including rye and wheat grasses, humans, those mammals commonly associated with humans, insects, lactic-acid bacteria, yeasts, and bifidobacterium found in the colon, all at the time and place mentioned above.
Part of the above hypothesis would indicate several pre-conditions:
1. The lactic-acid bacteria involved in human-based milk fermentations have evolved to the human digestive tract, while also actively modifying the digestive tract’s own workings for a net gain for both host and symbiont by acting as both prebiotic and probiotic organisms at the same time. (More on this later.)
2. There is a common evolutionary pathway seen in LAB-human fermentations, with a certain set remaining ‘generalists’ and another ‘specialists,’ and yet they are all working toward the same, meta-biotic goal: increasing and maintaining co-existence (i.e., biodiversity).
3. Lb SF’s evolution resulted in its ability to play a prebiotic role for those LAB based in the human colon (and hence also rectum, feces, and so on). The likely scenario would have involved a human “noticing” spontaneous fermentation in wheat or rye grains from the human-based LAB mentioned in #1. “Backslopping” was likely already in existence as a method by early Homo sapiens as a method for milk-based fermentations. Conditions were not “wash-your-hands” sterile, obviously, when backslopping. Lb SF likely inherited, through lateral-gene transfer, the intracellular and intraspecies molecular-signalling genes (i.e., the ones specific to the human, and likely other higher, associated mammal, colon) already present in these “specialist” LAB present in these milk-based fermentations. Lb SF-based sourdough bread is abundant in prebiotic substrates for these colonically-based LAB to this day.