When you look up the noun "mortal" in the Concise Oxford Dictionary it says :

A being that is going to die. But why "especially human", what does that mean ? A rather clumsy attempt it would seem to exclude animals.

The Greek philosophers (as I read in André Comte-Sponville's delightful "The Little Book of Philosophy") have a far better and very precise definition of the word :

Here we have a clear distinction, all living beings die, but (presumably) only human beings actually know, are aware, throughout their lives, that they are going to die.
This of course is the defining characteristic of the human species. At the point in time at which he breaks out of the restricted awareness sphere of an animal, which thinks in terms of eating, mating, protecting his territory, (in other words his immediate needs), he becomes a true human being. For a human the dimension of time is extended to cover his entire life span, and expanding in conjunction with that are the dimensions of the wider world and universe around him.

It is precisely because of our wider awareness that we have started to observe, explore, discover, think, invent, create, to eventually reach the very point of development at which we are at this moment. All our achievements of the past have been possible because of our ever increasing awareness of the world, the universe and of ourselves.
This process represents in fact the evolution of our species. No wonder then that this process is primarily focusing on the further development of our awareness center, the brain, and is proceeding at a cracking pace, close to one and a half times faster than the average speed of evolution in general (according to the modern biologists of today).

But the birth and growth of this wider awareness has had, and still has, its growing pains. The wider awareness makes us at times better than animals, but at times too much worse than them. We see that in the news on TV and in papers on a daily basis. We have become human beings, but as yet have not reached the stage that we are truly humane (= "benevolent, compassionate, inflicting the minimum of pain").

Fear too has been a byproduct of our greater awareness. Fear (initially) of thunder, lightning, storms, and all the things we can not (or as yet do not) understand. So we created Gods to explain them (assigning them the same function as a patriarch, head of a family, but at an amplified scale), and who could punish or reward us according to our actions, behaviour.
Most of the fears have been resolved. The Gods too have reduced in number, but some are still there (although gradually receding in the Western world), holding us back from reaching our destiny : to become human in every sense of the word.