Can A single human voice is capable of singing harmonies?

Well, I don't know if a single human voice can is capable of singing harmonies, nor do I know if a single human voice is capable of singing harmonies, but a single human using only his vocal tract can, in fact, tap into harmonic overtones, effectively singing more than one note at once. This is "overtone singing," as in Tuvan tradition.

I don't know shite about music, though, and I'm just rattling off something I read.