The inklings are made of ink. Think of it as an Intelligent Nanoswarm Kollective :) For a long time Ive compared them to squids that evolved humanoid traits but I think those are just the forms they like to take as human society is a massive influence on their culture.
ever hear of "grey goo"? its a theory that says a group of nanomachines that self replicate could eventually destroy the planet due to its ceaseless replication with using whatever matter is around it. Why? because theyre stupid...theyre no different than an old clock in prague. it knows how to tick and knows how to tock but thats the end of it's abilities
But what if they were a lot brighter? what if the grouping came together and had hive intelligence? what if the nanomachines were able to choose a form at will? what if those formations developed individual wills, personalities, thoughts, and memories?
the splatoon world seems very plausible from that standpoint. our civilization is gone. somewhere in a lab was an experimental intelligent nanomachine colony and a cryogenically frozen cat. the rest as they say...is history, even if it is in the distant future
Personally, I've developed the idea that the inklings and the ink were two different organisms that evolved a symbiotic relationship and have now started to merge with one another like a Portuguese Man-O-War. It would be somewhat similar to a theory I heard about the evolution of chloroplasts or mitochondria or something on the cell level- at one point, a predatory single-celled organism consumed a smaller, more self-sufficient one, but did not digest it. Eventually the two evolved to effectively become one organism.
My conception of Inkling evolution is similar. a primitive ancestor of the inkling encountered this ink, or "grey goo" and somehow, instead of consuming or being digested by it, the proto-inkling became a permanent host with specialized ink sacs for mass-producing the ink. the ink, in turn, evolved to be less destructive, targeting primarily enemy ink and inkling flesh (which itself probably consists significantly of ink at this point). As mentioned earlier in the thread, the ink would identify enemy ink via some sort of identifier protein, and the inklings would have to be able to change the type of proteins they are using.
Also, once again, the ink creates a pocket dimension which allows things to sink into a puddle of ink that should be at most an inch deep.
Was that anything like what I said earlier in this thread? Probably not. It's been so long... Anyway, I have no support for this idea. I don't even remember when I first thought about this. All the same, I figured I'd mention it, because why not?
Edit: Actually, this idea does explain something. The splatoon cannon seems to state quite clearly that inklings evolved from real squids. While this could be a misconception on the inkling's part, it would explain some of their qualities. The idea that the ink (or, as I could call it, amazing technicolor goo) was a human genetic experiment actually fits quite well with this idea, although it doesn't explain why the Inklings evolved to resemble humans... maybe humans tampered with the squids before kicking the bucket as well?
Forgive me if I'm rambling here.