Kubrick originally wanted to do Childhood’s End but somebody else already had the film rights. Perhaps intelligent species evolve into something non-physical.
Richard Dawkins entertained this idea in "The Blind Watchmaker," to illustrate why we humans don't see any evidence of life simpler than today's life which is based on DNA/RNA, which are such complex molecules that something must have preceded it, but is no longer around.
He envisions us building thinking computers, that replace us, and thousands of years in the future, there's no evidence left of us (the intermediate form leading to them), and they wonder where they came from.