It's probably not just a matter of "critical mass". The generation of phenomenal (self-aware experienced) consciousness is something a lot more sophisticated than that, a process (or a whole collection of related and nested processes) that the brain performs. Even simple animals have some rudimentary awareness of their environment. They react to environmental conditions, to food, to enemies and to the opposite sex (assuming that their species has sex). As nervous systems evolved and grew more complicated, animals didn't just react to their environment any longer. They acquired the ability to learn, to remember, even to imagine things that don't exist and hence to plan future activities. So now animals are picturing situations where one of the actors is "me", their own biological organism. They start to develop a sense of their own self. And what's more, that new "me" isn't just aware of things that are happening outside its physical body. It's also aware of memories, future plans and evaluative emotions that are being generated within the neural network as well. So at some point, an animal is going to get an idea that its "me" isn't simply its biological body at all, but something much more mysterious... something that rides around inside its head or someplace and is aware of its physical body just as it's aware of rocks and trees outside. The idea arises that the inner subjective "me" is some kind of transcendental 'spiritual' principle and the idea of a supernatural soul and the beginnings of philosophical idealism arise. My guess is that the explicit development of that idea was probably associated with the appearance of conceptual language robust enough to model the new philosophical concepts. But the sense of "me", even a vague sense of 'me' being something mysterious and transcendental, has probably been there for longer than humans have been human.