Final answer:
Ardipithecus kadabba might not be a hominin due to uncertainties about its bipedality and other hominin traits, which are essential for classification. Comparisons with A. ramidus, which shares some traits with later hominins, show less certainty for A. kadabba. Incomplete fossil records and definitional criteria of the genus Homo further complicate its classification.
Step-by-step explanation:
Ardipithecus kadabba might not be classified as a hominin due to ongoing debates about its anatomical features and their implications for bipedalism and other hominin traits. While Ardipithecus ramidus, a closely related species, has been shown through various skeletal signs such as the position of the foramen magnum and reduced canines to likely be a hominin, A. kadabba's status is less certain. The bipedality of A. kadabba has been questioned, and without definitive evidence of this and other hominin-defining traits, its classification can be contentious.
The uncertainty is compounded by difficulties that arise when working with fragmentary fossil records. Traits like bipedality, canine size, and postcranial skeletal structures are important in hominin classification, but the incomplete nature of many early hominin fossils can make such classifications provisional and subject to change with new discoveries. Furthermore, traits that define the genus Homo, such as increased brain size, dexterity for tool use, and certain dental characteristics, are not all evident in Ardipithecus kadabba.