In human prehistory, were tribal feasts a mechanism for spreading advantageous gut bacteria?
Suppose neighboring tribes, genetically similar in their human DNA, but heterogeneous in their microbial DNA, are subjected to selection pressure. Every year, each tribe has its ups and downs; some do well and some do poorly.
At the end of each year, the best, most winningest tribe puts on a feast and invites all the other tribes to dine: your basic banquet. In the Pacific Northwest it is called the potlatch, but it is probably a human universal, because, prior to chest freezers and grain silos, the best place to store your caloric surplus was in the bellies of your neighbours.
What if extra calories weren’t the only thing being stored in the bellies of neighbours?
If microbial diversity contributed significantly to selection outcomes — in other words, if the “down and out” tribes did poorly because they were depressed, or weren’t able to elicit maximal nutrition from their food due to absent symbionts, or some other factor traceable to gut composition—then maybe the feast, prepared and served by the winners, liberally sprinkled with winner bacteria, also served as a way to introduce better bugs to the worse tribes.
A spit roast, with actual spit.
The main dish, the Petri dish.