I am actually quite comfortable with the notion that a rock might be conscious proportional to the complexity of activity it performs when compared to a brain, That level would be incomprehensibly small so it would seem like nothing, but it would be like the width of an atom. A human cannot comprehend the width of an atom compared to the width of a bacteria. They are both too small that the only way think about them is an abstract level in terms of how much one is to the other. In absolute terms they appear to be nothing but we can accept that it is non zero. Why couldn't a rock be conscious at a level that is so small that it is insignificant, and entities, as they get more complex increase in consciousness. Suggesting that it doesn't do that suggests that either the brain needs something other than just activity or that if consciousness does come from activity alone, there is some threshold point of activity where it becomes non-zero. That seems like a significant phase change and we seem to have no evidence to suggest that such a change exists.

There's been exhaustive studies on this comparing our ability to think to the next smartest beings on the planet the Chimpanzees and the Bonobos. If Star Trek were real there'd be exhaustive studies on the difference between Humans and Vulkans too. The difference between us and other Hominids is very small from pure difference in DNA but obviously there's many things humans can do that other hominids can't. Anyway I think one thing for consciousness is the mirror test. Which many animals can pass. Even some ants apparently.