The Peruvian songwriter Daniel Robles recorded this song in 1913, and copyrighted it in the United States in 1933 during his travels in America. When Simon recorded it with his added lyrics, he thought it was a traditional song, as that's what Los Incas told him. When Robles' son filed a lawsuit, Simon had to give Robles a composer credit on the song, with his estate getting those royalties.
In discussing the song, Simon always talks about it as being based on a traditional Peruvian song, and we've never heard him mention Robles. This wasn't the first time Simon got tangled over songwriting credits on traditional melodies: Simon & Garfunkel's Scarborough Fair / Canticle
was based on a folk song, but his arrangement came from a singer named Martin Carthy. Simon was always clear on his influences, but legal misunderstandings were a problem in these cases.