Question

one theory behind human bipedalism speculates that it originated in a mostly ground - based ancestor that practiced four - legged “knuckle - walking,” like chimpanzees and gorillas do today, and eventually evolved into moving upright on two legs. but recently, researchers observed orangutans, another relative of humans, standing on two legs on tree branches and using their arms for balance while they reached for fruits. these observations may suggest that ______
9 □ mark for review
which choice most logically completes the text?
a bipedalism evolved because it was advantageous to a tree - dwelling ancestor of humans.
b bipedalism must have evolved simultaneously with knuckle - walking and tree - climbing.
c moving between the ground and the trees would have been difficult without bipedalism.
d a knuckle - walking human ancestor could have easily moved bipedally in trees.

Explanation:

The original theory links bipedalism to a ground - based, knuckle - walking ancestor. But the new observation is about orangutans (human relatives) using bipedalism in trees to reach fruits. Option A logically follows as it suggests bipedalism could have evolved because it was beneficial for a tree - dwelling human ancestor, as the orangutan observation shows bipedalism's use in a tree - based context. Option B is incorrect as there's no info to support simultaneous evolution. Option C is not supported as the text doesn't discuss the difficulty of moving between ground and trees. Option D is incorrect as the text doesn't imply knuckle - walking ancestors could easily move bipedally in trees.

Answer:

A. bipedalism evolved because it was advantageous to a tree - dwelling ancestor of humans.