Final answer:
Ardipithecus ramidus is considered a hominin due to evidence of bipedalism, reduced canines, and an opposable big toe, along with small upper arm bones, all of which indicate a divergence from non-hominin apes.
Step-by-step explanation:
Ardipithecus ramidus may be considered a hominin for several reasons, including its dated presence in the Pliocene era, around 4.4 million years ago (MYA). It exhibits key traits such as bipedality, evidenced by the forward position of the foramen magnum indicating an upright walking posture, and reduced canines that differ from those of contemporary African apes. Moreover, its small upper arm bones suggest it did not use its arms for weight support during locomotion, which, along with the evidence of an opposable big toe, supports the hypothesis that Ardipithecus was bipedal on the ground and likely quadrupedal in trees. This combination of traits, such as thin dental enamel, points to a divergence from other non-hominin apes, solidifying its standing in the hominin lineage.