Final answer:
The unexpected bipedal trait of Ardipithecus is its opposable big toe implying both bipedalism and arboreal abilities. Ardipithecus showcases early bipedal adaptation, with traits like smaller canines and humanlike feet, reflecting a significant evolutionary transition.
Step-by-step explanation:
The unexpected bipedal trait displayed by Ardipithecus is the existence of an opposable big toe. This trait suggests that while Ardipithecus was bipedal on the ground, it could have also been quadrupedal in trees, displaying a locomotor behavior unusual among hominids. The opposable big toe, alongside the evidence of smaller canines and thin dental enamel, indicates that Ardipithecus had adapted for bipedality while still maintaining some arboreal capabilities. The reduced canines indicate social evolution towards reduced aggression and potential female preference for less aggressive males. The discovery by Tim White in Ethiopia of the Ardipithecus ramidus species, dated to about 4.4 million years ago (MYA), significantly contributed to our understanding of the locomotion and social dynamics of early hominids, which was a major transition in the human evolutionary lineage.
Compared to subsequent hominid species like Australopithecus afarensis, Ardipithecus ramidus showcased more primitive bipedal traits, which played a vital role in the eventual evolution of fully bipedal, terrestrial hominins. The presence of small jaws and humanlike feet with arches in Ardipithecus demonstrates this evolutionary progress.