Trying to remember title of book/story

Something I read probably 40+ years ago. A man and woman (spies or operatives of some kind) were captured and tortured by their enemy. They had been programmed with a keyword that if spoken aloud would wipe their minds of memories and sentient thought. When the enemy realized that they had done this he abandoned them alone on a planet. I recall in the ending they were naked, in a cave, watching the moon rise and hearing wild animal noises in the dark. It turned out to be an origin story about how human life got started on Earth.

Anyone remember this?