...that I hadn't considered before, is that humans aren't just descended from lower life forms: we're descended from particular lower life forms. Not this blue-green alga, but that blue green alga; not that burrowing mammal, but that other burrowing mammal. Makes the whole thing a lot more personal and slightly unsettling. There are no assurances that our ancestors were the best of their kind--just the best suited for that one brief moment.
And there are probably plenty of reasons to take issue with the term "lower life forms," but that's not really the argument I'm trying to have.