Humanity – as applying to the human species – is something more than just the 7 billion individuals alive today or even the 110 billion (or so) members of homo sapiens who have ever lived. Humanity must include the 4 babies being born every second. It must include all those who have ever been, or will come to be, “human”. Any definition of humanity must transcend time. For any individual, to be “human” it is is not a case of just falling within a certain envelope of physical and cognitive characteristics, but must also include the individual’s behaviour.
Our genes determine our physical and cognitive capabilities – our potential. Nurture then determines how these are expressed in each individual case. While our genes may not determine specific, individual behaviour, they do define the envelope of all possible behaviours available to an individual. It is our genes which define the envelope of all possible human characteristics and also the envelope of all possible human behaviour.
The envelope of all possible human behaviour will – must – include all behaviour that many would consider “inhumane”. Hitler or Pol Pot or Breivik or ISIS fanatics are or were undoubtedly human, but their behaviour was or is “inhumane”. Their “inhumanity” does not remove them from the body of anydefinition of humanity. If not, it would mean that there could be some humans who – on account of their behaviour – were not part of humanity. That, I think, would be a contradiction in itself. Equally, if some entity exhibited behaviour which fell within the envelope of all possible human behaviour, but fell outside the envelope of physical and cognitive characteristics, then that entity would not be part of humanity.
The paradox is that what we call “crimes against humanity” are, in fact, part of humanity. Inhumane behaviour is an integral part of humanity.
If we wish to exclude certain types of behaviour from humanity, the inescapable conclusion is that we have to eliminate or modify the genes which allow the unwanted behaviour. But our physical characteristics and our behaviour are dependent upon the same genes. And if genetic engineering to change behaviour was possible, it would also mean that our physical and cognitive abilities would inevitably change.
Superman might pretend to be human but he could never be a part of any definition of humanity.