Who Wrote This Science Fiction Story?

And what’s its name?

I started reading this science fiction story about 15-20 years ago and never finished it. I don’t remember the title, author, or whether it was a novel or short story. Fortunately, I do remember something about the plotline:

Humans in FTL ships visit a planet in another solar system. The planet is mostly desert. They are astonished to find what appears to be a small tribe of humans living on this planet. They eavesdrop on these humans for several months to learn their language and behavior patterns. One of the crew is transformed to look like these natives and inserted onto the planet. This anthropologist approaches the tribe and introduces himself. He claims to be from a tribe far away, looking for trade opportunities. The chief of the tribe calls him a liar and says that there are no other people on this world. (The chief understands the concept of “world”.) The chief welcomes the stranger anyway. The tribe lives in a cave in a large rockpile. The chief takes the stranger down tunnels that get smoother and smoother the farther underground they go, until the chief brings the stranger to what is recognizably a manufactured door on manufactured hinges. The door is opened and … something happens. I don’t remember what. The tribe are human but are not colonists from Earth. Something like that.

I’ve been looking for this for the past decade, at least. Thanks.

Bueller … Bueller … Bueller …

Someone please answer I want to know what’s on the other side of the door!