One suggestion is to read other fantasy featuring unique races, and see how those authors handled it. Start with Tolkien, whose work has largely defined the very stereotypes and tropes we now use to shortcut our own work - we don't always have to "explain" elves, dwarves ,etc. because he already did the hard work! (Not all elves are Tolkienesque, of course - assuming the audience will assume they are makes for lazy, boring writing.) Beyond that... Many characters across the Eternal Champion cycle of Michael Moorcock are non-human humanoids; Steven Erikson's/Ian C. Esselmont's Malazan books include a number of unique races/subraces; Ekaterina Sedia's The Alchemy of Stone uses gargoyles as an ancient race; China Mieville's Bas-Lag series has a virtual zoo of races; Philip Pullman's gallivespians in the His Dark Materials series; Martha Wells's Books of the Raksura have a lot as well. Etc. You're in good company creating a unique race.
A large part of being a writer is being a reader. Nothing beats experiencing how other authors have succeeded (or failed) in doing what you're trying to do. Read enough, and you'll internalize a lot of options and solutions.
Another suggestion is to consider whether those questions need to be answered in full, or at all. Lnloft has it spot on. Your job is to tell a compelling story. To accomplish that, your secondary world needs to be engaging and consistent, and to feel complete. That doesn't necessarily require that you explain evolutionary mechanisms, the complete lineage of family lines, whether the geography of your world is shaped by tectonics, etc. I mean, you can do that, but does that help tell the story, or just drag it down? Consider the LotR books (again). Tolkien created an encyclopedia for Middle Earth, with all sorts of mythology, backstory, etc., but didn't reveal it all in The Hobbit. Or Fellowship. Or The Two Towers. Etc. By the time The Silmarillion was published, we read it because we cared; Tolkien already had us invested in Middle Earth through the grand, epic adventures, even though we didn't have everything at hand.
If your readers don't feel your world or race makes sense, or find your story incoherent because of missing details or cultural motivations, you have a problem. But if your readers feel the story was sensical and dynamite, and now they have more questions about your world, you totally nailed it. Sounds like a series to me!
And again as lnloft points out, you might benefit from crafting your own Silmarillion for your eyes only. You need to know enough about your world to ensure you can write one that feels whole, even if it's not. If you don't know how your race got there, or their past history with the humans, you might be missing opportunities in your storytelling, or worse, some story elements won't congeal, and readers will catch on. Do you need a whole encyclopedia? Probably not. But, if you have some down time between projects, or need a break from crafting your narrative, or even if you're trying to figure out a character's deeper motivations and worldview, you might do well to answer some of your readers' common questions.
To address your last point - there's always a backstory. To everything. It may seem mundane, but it's there. Consider how many culture clashes there have been on Earth, how much history, so much of it lost - and without it, we wouldn't be who we are. Why are there so many races of humans? Why do some cultures create certain myths or monsters, and others not? Where did the word "salary" come from? Why does Ireland have no snakes? Why does clothing and style differ between peoples in the same part of the world? Why do Western businessmen wear ties? Why is a region in the southwest corner of Newfoundland called the Wreckhouse? The reasons can be absolutely fascinating, stories unto themselves. Backstory is something you should think about to one degree or other; if nothing else, it can inform details in your story that make the world seem rich and complex - even if you're just cheating!
My last suggestion is a more straightforward mechanical one. Many authors choose to start chapters/sections with epigraphs. These usually set the tone/theme of the section, but can also pass info to the reader outside the narrative. It's a bit of a cheat, can't be the only thing you do, and can come off as cliché; it's a scalpel, not a chainsaw, so tread carefully. The Malazan books exploit this fantastically. David Edison's The Waking Engine does so not to explain his world, but to deepen it - his epigraphs are quotes from dead people, but written after they've died (this makes sense in context). Perhaps your epigraphs include some oblique cultural references that give us a taste of the hidden history, or even extracts from "dry historical documents" that tell us something about how the races have interacted in the past.
Here's an absolutely terrible, awful example epigraph that you should not emulate:
Silly little human feet, weak and pudgy, can't tear meat;
Silly, pudgy, weak or not, they're tasty in the stewing pot!
- Syik nursery rhyme
Starting a relevant chapter with that would tell the reader that syiks aren't human, their feet must be very different, and that they may eat humans, but that syik culture is largely recognizable to us since they have nursery rhymes, eat stew, etc.
I think you can do better than that, though.