Humans have been much more than opportunistic hunters for far longer than the last 12 000 years, or since the end of the last Ice Age and the beginnings of food production. By that time, most habitable land on Earth was already populated by us, largely on the spoils of big game hunting.
Sorry, something seems to have gotten cut out of there; what I intended to write was "we’re foragers/scroungers (like the other great apes) which have learned to hunt opportunistically, and have only enjoyed regular consumption of meat and animal products within the last twelve thousand years. You are correct that humans have been opportunistic hunters for a long time; indeed, longer than we’ve actually been human; and the availability of meat protein is thought to be one of the precursors to the development of intelligence and the more complex social interaction that the human species enjoys.
Stranger