evolution - How scientists can be confident that human embryo tail is really a tail?

From Wikipedia:

Human embryos have a tail that measures about one-sixth of the size of the embryo itself. As the embryo develops into a fetus, the tail is absorbed by the growing body.

How are we sure that it is a real tail and is not a superficial resemblance of something else?

I ask this because some creationist guy challenged me in this regard.