That's an instance of slippery slope fallacy at the end. Mammals share so much more evolutionary history with us than rocks that, yes, it justifies for example ascribing them an inner subjective world, even though we will never know how it is to be a cat from a cat's perspective. Sometimes quantitative accumulation does lead to qualitative jumps.

Also worth noting is that alongside the very human propensity to anthropomorphize, there's the equally human, but opposite tendency to deny animals those higher capacities we pride ourselves with. Basically a narcissistic impulse to set ourselves apart from our cousins we'd like to believe we've left completely behind. Witness the recurring surprise when we find yet another proof that things are not by far that cut-and-dry.