What is the essence of humanity?. That is to say, what is it about humans which makes them fundamentally different from other species?
When asked this question most people will give strange and misguided answers - often 'It's the ability to cooperate in a society' or 'the ability to show compassion'. These are patently untrue.
Tiny brainless fish have the ability to swim in schools, and survive quite well in their own society. They follow the direction of the collective - if one fish panics, and swims frantically in a particular direction, they all follow.
Many animals show apparent compassion. Certainly they make huge sacrifices for their own offspring, sometimes dying to protect them. While some would argue that this may have more to do with genetic survival than compassion, it is hardly a convincing characteristic of humanity.
In fact, the essence of humanity is rationality.
Only man has the ability to reason and to act on that reason for long-term benefit - to ignore the emotional programming, and the emotional desires which dictate actions to irrational animals.
Choosing something which may require a great deal of emotional and physical suffering in the short term, but will create a long term benefit can only be done by a human. Only a man can willfully improve himself, to make his life better, to shape his environment.
For rationality to exist, there must be incentive. Rationality does not exist in a vacuum. To deny a human the benefits from his rationality is to deny him the purpose of his rationality, and hence to deny his humanity.