We really don't have any idea.

Something my Animal Physiology professor said recently:

"The sense of taste is a really complex thing, and we really have no idea how it works. We don't even actually know how proteins work. I mean, there used to be this whole lock and key mechanism to describe how proteins can be made to change shape, but now there's this thing with electron fields and activation, and even that may not be true. Protein channels, we don't know how those work either. I mean, why does a potassium channel only let potassium through when sodium is about the same size and the same charge? Why can't clorine get through the same channel, since its so much smaller? We really don't know. The models we have, they're just kinda made up, we say, this looks like what it should be, so thats how we're gonna model it."

We really do know very little about the universe. Our understanding is far from complete.