"Cows have to go through quite a lot to make it happen, with a multistage digestion process, multiple stomachs, regurgitation and additional chewing of food, the right microbiome. In short it's not an easy trait to just «evolve»."
The complexity of digestive apparatus found in Bovidae family and other ruminant mammals is just an optimization. Herbivore monogastric mammals like horses consume cellulose too. There even isn't that necessary for the mammal to be (specialized) herbivore in order have the cellulose digesting ability, as pigs clearly demonstrate. Some extensions in our menu limits are always welcomed. Digesting lactose as adults was nice, but digesting cellulose, even if not that pleasant or efficient, would have been even nicer, as when things went historically bad, it would had came in handy if we could literally just run to the forest for more than to just hide.
The complexity of digestive apparatus found in Bovidae family and other ruminant mammals is just an optimization. Herbivore monogastric mammals like horses consume cellulose too. There even isn't that necessary for the mammal to be (specialized) herbivore in order have the cellulose digesting ability, as pigs clearly demonstrate. Some extensions in our menu limits are always welcomed. Digesting lactose as adults was nice, but digesting cellulose, even if not that pleasant or efficient, would have been even nicer, as when things went historically bad, it would had came in handy if we could literally just run to the forest for more than to just hide.