You are now challenging the statement about plants being the "default" food. (Maybe you are not the original responder; I can't tell on my phone UI)
I think you're going to have to back that up with a citation.
For most of history, ordinary people could afford only plant based food. Meat was occasional. Industrial meat production in the West changed the economics of that.
Vegans are not the only people who care about our unsustainable practices. We all should. We could start by properly accounting for the externalities, ecological footprint, worker exploitation, and yes, the built-in cruelty to animals. I think the OP's suggestion to label animal parts is a step in the right direction.
We didn't eat plants by default, we ate a mixture of plants and meats and other animal-derived products since we lived in caves.