Passage Summary: Scientists aren't sure why humans started walking on two legs; it could have been to see over grass, to reach food in trees, or to attract mates.

Reasoning: While bipedalism helps in grasslands, it also provides advantages in forests and might even have social or reproductive benefits.

Analysis: The passage presents several competing theories, which suggests that the 'grassland' theory is not the definitive or exclusive explanation for bipedalism. Because the trait offers advantages in multiple environments, the move to grasslands might not have been the sole catalyst for its evolution. When looking for the most supported statement, focus on an answer that reflects this uncertainty or the multi-functional nature of the trait.