Humanness exists and means 'the quality, state, or condition of being human'. You might also consider humanhood, 'human nature or character'.

Is there such a word as humanness?

The condition or quality of being human.

What defines humanness?

1. humanness - the quality of being human; "he feared the speedy decline of all manhood" humanity, manhood. quality - an essential and distinguishing attribute of something or someone; "the quality of mercy is not strained"--Shakespeare.