There's a comic about a jedi that joins the tuskens after falling in love with their culture and he's human to begin with , so maybe all the others are too or maybe creatures of all different races who follow the same religion . The theory I prefer though is that they are a native species of Tatooine and no-one knows what they look like underneath (except maybe Anakin who hack a few of their heads off and may have taken a peak!!!).
As stated by someone else, in the comic they look basically human with some ritualistic ornamentation, much like the tribes in some African countries. If you do the Tusken storyteller mission in KOTOR he tells you that they originally didn't have to wear the wraps and that Tattooine was originally lush and technologically advanced. A war broke out and Tattooine was reduced to the dustbowl it is now and the Tuskens have disbanded into different tribes.