As jazz stems from the practices and traditions of enslaved people, the historic lack of appreciation for and therefore documentation of their cultures makes it difficult to trace back its roots. As Takiyah Nur Amin in Part 2: Jazz Dance History points out, Africa houses an immense variety of ethnicities with their unique dances, and the information about those of them that were brought to America is not detailed at all.
Another issue is the ever-changing nature of jazz. Chapters 7 and 8 mention different stages of the development of jazz, but today it is perfectly possible to find people dancing Charleston as well as people doing jazz that incorporates a lot of the elements associated with ballet (e.g. Jazz classes at Bryn Mawr). Moreover, other dance styles like tap or hip-hop that have an enormous following of their own can also be historically categorized as jazz, which further complicates what we mean by jazz and therefore how broad of a jazz history should we study.