Human dignity is not just one concept among many others, it is central to humanism.
As I see it the whole edifice of humanism crumbles if it is not based on human dignity.
But can human dignity be based on something else?
Can it be justified? And how?

Here are three ways to justify human dignity:

1. Humans were created by God, and God created men and women in his image, they are endowed with a divine spirit which makes them essentially superior to the rest of God's creation.
   (This is the religious justification).

2. Human beings are not like other animals, they (we) are unique and special. We are persons, we have self-awareness, we are conscious not only of the moment but also of our past and we can project our selves into the future, make plans or promises; we can invent and inhabit fictitious worlds and communicate about them; only we have science that allows us to know the secrets of our existence, and so on ...
   (This justification is based on human faculties)

3. Humans have a special dignity because we are humans, and we ascribe dignity to ourselves because we like and admire ourselves. We are just great (not unlike a French patriot who deems France and its culture to be the best and most noble country and culture of the world, just because he is a proud Frenchman).
   (This kind of justification can be called 'speciesism', analogous to 'racism' or nationalism. I would call it collective narcissism)

So here are two questions: Do you think that human dignity is in need of justification?
If the answer is No: why not? because it is self-evident? (like "happiness is better than pain and sadness" is self-evident)
If the answer is Yes: what kind of justification do you prefer?