I see that I will have to expand on my brief observation, but to get us on the same page, I will need to know what you mean by the premise "humans == conscious".
If this is to be taken as a statement of identity, I would regard it as a category error, but I will not expand on that here, as I doubt it is what you intended.
If it is to be taken as the claim that only humans could be conscious, I would regard it as both lacking any justification and begging the question.
I think you mean that people generally assume everyone else is conscious in much the same way as they themselves seem to be, which is essentially saying they hold a theory of mind. If so, then I agree with you, but where do we get it from?
I know of no argument that we are born holding this theory, and it seems implausible that we are, as we are born without sufficient language to know what it means. False-belief tasks suggest that we begin to develop it at about 15 months (they also suggest that some other animals have it to some extent.) At that age it is, of course, tacit (rather than propositional) knowledge.
It would be absurd to suggest that toddlers come to deduce this from some self-evident axioms. What does that leave? I don't think there are any suggestions other than the obvious one: we arrive at it intuitively from our observations of the world around us, and particularly other people.
Ergo, those of us who make use of a theory of mind came by it from observation of what you call "a fuzzy indirect set of second-order human behaviors", and no one, as far as I know, has come up with a better justification for believing it.