You can find my lengthy discussion of the topic here, mostly in response to Ivan van Seritma's They Came Before Columbus. The book is a mishmash of effects before causes, archaic scholarly sources, regressive notions of race, homogenized American and African cultures, and quotes from authors so abhorrently sampled that the authors literally prove the quoted rhetorical statements wrong on the next page. The book doesn't cover all the theories out there though, so I'm always interested in answering follow-up questions!