What's great about the Theory of Evolution is it provides us a paradigm with which we can really explore the features of living organisms. The old "Godidit" hypothesis always works, but it's not very satisfying intellectually. It's pretty much a dead end.
So why would humans develop the capacity for tickling? This is just a recent curiosity of mine. A little google research provides some interesting insight. The theory is that laughter creates social cohesion, indicating to other members of the species that you aren't there for threatening purposes, improving social bonds, etc. Apparently tickling is a mere extension of that.
This may be true and it may be false. Searching google I find that creationists like to mock such explanations. In truth they are speculative. But they do conform to an overall paradigm that we know is true and they offer testable features. What do the creationists offer? Nothing but the untestable and unfalsifiable assertion that God is responsible complex things. There's not so much to mock on that view because there's basically almost nothing there to mock.