Even if some or all of your main characters are not human, please avoid using the word "human" in your writing. It can be useful as a noun, sparingly (e.g. Only one of them was a human...) but when used as an adjective, it is often awkward and confusing.
Keep in mind that some or all of your readers will likely be humans themselves. As such, telling them that the display of such and such emotion is "human" tells them little or nothing at all. Phrases like "in the course of human events," in a story in which all of the characters are humans is approximately one word too long.
The word is also its own opposite, as shown in the sample set of definitions below, for when the word is an adjective:
- Having or showing those positive aspects of nature and character regarded as distinguishing humans from other animals: an act of human kindness.
- Subject to or indicative of the weaknesses, imperfections, and fragility associated with humans: a mistake that shows he's only human; human frailty.
If you feel you must show that your character is "only human," try using science instead. Humans are pretty easily distinguished from other life forms by any number of physical or mental characteristics. Try including a scene where your character uses a tool or language, exercises an opposable thumb, applies logic to something, or shaves a part of his or her own body.
Your readers will thank you.