Some canines have evolved culturally insofar as they are adjuncts of humans, to the point where canines can now navigate busy streets for blind humans.
Do you regard the acquisition of this more sophisticated skill as an "advance" on being a wild dog, or a "devolution" to a lower state?
I don't know. Domesticated canines simply are domesticated canines. I am not Dr Doolittle.
If not culture, what other candidates are there for defining humanity?
Well, biology, for starters. Our chromosomes identify us as human, regardless of transient things like culture. Whether there are other criteria, we may well ask: consciousness would be another: mankind has unique kinds of that, it seems.
Artificial selection is often deliberate as for instance breeding cows that produce creamier milk. Humans are less often artificially selected for breeding purposes, and artificial selection by humans on humans is normally accidental.
I am not sure that human consciousness is better than that of a rainforest, or a sewer rat.
Certainly, as you yourself have noted, "culture" isn't stable across time, or even stable among human populations in the same time: so it's not a basis at all.
Cultures are not stable: culture is stable.
Species are not stable: biology is stable.
Nobody, including religious men, knows whether God is cultural artefact or ontic reality.
So you claim to know what everyone else can or cannot know, do you? I have to ask you for your credentials on that one: how do you know what everybody else knows?
There are mystics who for all I know , have experienced a deity.