I was reading National Geographic in a doctor’s office recently, an article about Jane Goodall. Her work rocked 1960’s scientists because she saw chimps using tools. At the time there were several criteria thought to separate humans from animals, and tool using was one of them.
Since then many of the others have been put to rest too. Perhaps there is still a definition, other than the species definition, that works, but I have to wonder what is the point? Why do we need to feel that something sets us apart? We are much more like animals than we pretend we are, and animals are much more clever, intelligent, creative, and emotional than we pretend they are.
The most you can say is that we are one end of a continuum. There is no denying that however stupid and destructive we can be, we are the most intelligent, we have the greatest capacity for construction and destruction of any creature on the planet. If you place humans at the extreme right end of the continuum, the next creature to the left of us isn’t so far away at all.
I think you could with confidence come up with criteria that set humans apart from sponges, nematodes, or fruit flies. But I don’t recommend trying for much higher than that. Our rationality, our creativity, our emotions evolved not so long ago (evolutionarily speaking). It would take longer than that to put more distance between us and the chimps, elephants and dolphins, and by then there would be something else in between.