If you haven't yet encountered it, check out Michael Levin's lab and work. Among other things, they are trying to figure out what "basal cognition" is; the idea being that even if we can't point to some dividing line in the end, we'll have a better understanding of what cognition is and where it shows up. And it shows up surprisingly far down!
The definition of "intelligence" that he works with comes from William James: the ability to achieve the same goal by different means. It's a useful definition, given the remarkable stuff coming out of his lab.