Winter.
All jokes aside, any "natural" fasting would probably be directly related to how much food is available, how perishable it is, and how long it keeps. Late winter and early spring were pretty hard times for folks living in places with 4 seasons (the time of Christian lent, in general). "natural" fasting was likely just a result of food availability.
And recommended and/or normal eating times have changed a lot through history, much as the foods we eat have. And so much of what we "know" is speculation: We can't realistically tell how often or what times any pre-history civilisation ate, for example.
"Winter" refers to an agricultural-age kind of shortage, not a ice-age-mammoth-hunters sort of situation. With most of the planet's humanity having less than 10k years of agriculture under their belt, it's perhaps been not quite as gene-shaping / evolutionary-impactful as the prior 100000-500000 years of mostly-hunting-and-scavenging (plus whatever little actually-edible non-poisonous plant/fungus stuff actual "gathering" outside cultivated-fields of bred-over-millenia varieties could in fact contribute — seasoning or snack-style most likely rather than the dominant/decisive nutriment/calory-package I'd wager).
But then, I'm not an authority on the matter, just yet-another-mildly-temporarily-curious-thread-partaker.
Certainly, hunting doesn't succeed every day or isn't done every day, so there's an assumption that "we evolved" under an alternation of feast-days (a handful) and fast-days (also a handful). I recall someone's napkin-calcs that one cow (not even mammoth), if fully (not just the fine cuts) utilized (and preservible so long), could nourish a single person for almost a year, and even a sheep for some ~3+ weeks. So as a small tribe you need not hunt or succeed every day — and when you really, really feast on fresh meat ad-lib one day, you'll easily be without (substantial) hunger at least for the next 1-3 days. I also heard once (not a scientist in that area either =) that animal predator carnivores don't nibble all the time, they strike and feast big every couple of days, nap it off, then just sorta roam and play.. outside of structured zoo/circus feeding schedules, that is. (Not that we're such a species as lions tigers or pumas, but we might have been closer to that M.O. for a long long time than today or back as tree-hopping primates.)