We experience ourselves and our surroundings through our consciousness, and yet it is such a mysterious thing. Since we know consciousness only through our own experiences, we find it natural and easy to point out at things that make us special - like our ability to use language and our complex reasoning patterns - and then somehow extrapolate that these things that make us special are intimately connected to our ability of experiencing consciousness.
But, unless I am very mistaken, there is literally no evidence to support this hypothesis. It is a conjecture that we've made up because it is easy to believe it. We start from the position that we agree that we humans are conscious - and then other systems have to somehow prove their consciousness... Despite our inability of proving that humans experience consciousness!
Personally, I am of the view that the phenomenon of consciousness is a lot more widespread than we currently imagine. It find it hard to believe that that we are so special, and that it was necessary for humans to evolve until this magical phenomenon of "consciousness" began to take shape.