I used to think it was abstract reasoning. Humans have the ability to imagine themselves as an animal. Then Terrence McKenna spoke of how animal predators are successful by imagining themselves as their prey. Be that true or not, it leveled my previous thinking on the matter.
I used to think humans were exclusive because of our ability to evolve and expand language. Bumble bees will never evolve their language beyond the waggle dance. There are no new words available to wolf howls or whale song. But here’s a story of a chimp who learned sign language. Whether or not the chimp could create new words on his own remains unresolved. But the fact remains that he could learn new words.
Now I think humans are exclusive simply upon the merit of being capable of creating non material objects such as unicorns and believing they have an existence in reality. I don’t know if that’s to be considered as advanced or psychotic.