Final answer:
Ardipithecus ramidus' classification as a hominin is debated because of certain anatomical features that suggest both bipedal and arboreal locomotion, which casts doubt on a direct evolutionary path to Homo sapiens.
Step-by-step explanation:
The question 'Why may Ardipithecus ramidus NOT be hominin?' addresses the uncertainty surrounding the classification of Ardipithecus ramidus as one of the early ancestors of modern humans. Ardipithecus ramidus, which lived about 4.4 million years ago, has been inferred to be bipedal based on the forward position of the foramen magnum. Despite this, certain traits like the opposable big toe and the potential for quadrupedal movement in trees suggest a mode of locomotion that was not exclusively bipedal, raising questions about its assignment as a hominin.
The fossil evidence indicating reduced canine size and thin dental enamel suggests a divergence from typical African ape characteristics, supporting the argument for Ardipithecus ramidus as a hominin. However, the lack of clarity on whether the various morphological traits represent a direct evolutionary path to Homo sapiens is why the classification of Ardipithecus ramidus within the hominin lineage remains uncertain. Subsequent discoveries of earlier Ardipithecus species like A. kadabba contribute to the complexity of tracing human ancestry.