Final answer:
Ardipithecus ramidus is considered a hominin due to elements such as bipedality, a forward-placed foramen magnum, reduced canines, thin enamel, and limb proportions that are unlike those of quadrupedal apes.
Step-by-step explanation:
Ardipithecus ramidus is posited to be a hominin because of several anatomical features that align with the hominin clade. The species, found in Ethiopia and dated to about 4.4 million years ago (MYA), shows evidence of bipedality as deduced from the forward position of the foramen magnum. Additional characteristics supporting its classification as a hominin include traits such as thin dental enamel, a reduced canine indicative of less sexual dimorphism compared to African apes, and an opposable big toe. The implication of less pronounced canines suggests a shift in social behavior that favored less aggression, possibly with a female preference for males with milder temperaments. Ardipithecus ramidus's arm bones are smaller, which hints that they were not typically used to bear weight like those of quadrupedal primates, hence, further pointing towards bipedal locomotion. Nonetheless, the opposable big toe suggests a capacity for arboreal movement, emphasizing facultative bipedalism combined with quadrupedal climbing abilities.