There was a discussion recently on a LinkedIn group on "What makes us human?". This focused on human behaviour traits, good and bad.
I'm not usually very literal-minded, but I had trouble with this one. I observe and display all sorts of human behaviour traits and I'm quite comfortable discussing human nature, but "What makes us human?" is either a question about mechanics to which the answer is "the instructions in our DNA, or rather a very tiny fraction of those instructions, as most of them could also make us chicken" - or it's inviting a comparison with something.
So what are we comparing ourselves with? Sometimes, as in "we're only human", we seem to be comparing ourselves with God, but as knowledge of God is highly disputable, that doesn't take us far except to emphasise our fallibility.
We know that we evolved bit by bit from a common ancestor with Chimpanzees, so our humanness, however defined, developed gradually and some of our quite marked traits are still basically common ape traits. In particular, studies of Chimpanzees show that some behaviours and perceptions we think of as very human are shared with them - for example, taking care of the dead and appearing to grieve over them, or something remarkably like ambitious people's office politics.
It appears that whales and porpoises are highly intelligent and dolphins probably even more so, but we are so different that we've understood little of the nature of their intelligence (we've also found it convenient not to probe into this while we kill them). The bodies of marine mammals are so different from ours that communication becomes fundamentally different and the marine environment, as opposed to land, almost certainly will lead to thinking differently. Besides, our concept of intelligence is so culturally determined that measures like I.Q. and learned discussions fail to recognise certain types of intelligence even among humans, so maybe if we knew far more about dolphins we wouldn't recognise - or wouldn't value - some of their intelligence.
If we had met even one extraterrestrial intelligent life-form (and survived) we could begin to answer the question "What makes us human?" because then we'd have a real comparator, a very different way of thinking, socialising and so on. We would find some things were common, so not distinctive of us, and others distinguished us from the other life-form (I'll leave machine civilisations out of it for the moment). Of course, just one comparator would not be ideal (imagine deciding what made us human by comparison with just one other earthly species, hippopotamus, say, or sea kale) and numerous comparators would be preferable provided none of them wiped us out.
Lastly, the mention of machine civilisations (much discussed in science fiction and somewhat in speculative non-fiction science) reminds me that when we ask what makes us human, we may be comparing ourselves with computers or robots. Now since some scientists see us as machines anyway, that's a difficult one, the more so because we sometimes build machines deliberately to mimic human thought processes and behaviour. Maybe I'll return to that - comments welcome.
Some answers would involve the soul, but while I'm comfortable with this idea insofar as I understand it, I'm not wedded to the Christian orthodoxy that humans only have something special.
I can't resist widening the extraterrestrial theme to say that our entire understanding of life and of how it arose on earth is constrained by a complete lack of comparators. If only we find something on Mars or on those moons of Saturn...in my lifetime, please...