Modern brewers and vinters add yeast, but that is really recent and for control rather than absolute nescessity. Grapes and apples will happily ferment with just the yeasts on the skins. Beer will often ferment naturally, but historically the use of porous tools and transferring bits of the old ferment did the job more reliably.
I’ve never been sold on either coming first. If you store some berries together with some grains and they get wet you are likely to get an active ferment. The dregs of a really rudimentary beer are basically a terrible sourdough. Simply letting the vessel dry out near the fire gives you a sort of bread. Both bread and beer are likely to have arisen at the same time just because a hungry person who did all of the gathering work isn’t willing to throw out the food if it isn’t peak freshness.
