Complex sentence: Despite its title, the book does not claim that sharks never get cancer, only that they rarely do so, a fact which has been known since the first malignancy was found in a shark specimen in 1908.
Split into simpler sentences: Despite its title, the book does not claim that sharks never get cancer, only that they rarely do so. Contrary to the claim in the book's title, however, sharks do get cancer, a fact which has been known since the first malignancy was found in a shark specimen in 1908.