Mammals certainly have consciousness, since we know that their brains are very similar to ours. This, we know for sure.
Reptiles have it too, but their brains are less developed compared to mammalian brains .. so, their conscious experience may lack something that we have.
On the other end of the spectrum , we know that bacteria don't have a conscious experience : for there is no nervous system to account for it.
Insects lie somewhere in between : They may have a limited conscious experience, they may not, this we do not know. And if they have it, then we are commiting an immoral deed when killing them.
I don't ask about how the heart beats, because You and Me, can both look at a heart, and see its beating, and give a full account for it .
But we cannot look at the same conscious experience, you look at yours, I look at mine, and we cannot give a full account for it. Yes, there is a gap in our understanding.
And if there is no gap, then there is another gap that makes us think there is a gap. If consciousness is explainable and there is no gap, then the gap is in the fact that we think it is not explainable.
That is, answers seem to be intellectually unsatisfactory , either because the problem lies in our imagination, or in our language, or in both.