Passage Summary: A scientist thinks our ancestors stood up before they made cool tools because you need your hands free to build things, and standing up is what gives you free hands.

Conclusion: Human ancestors likely began standing upright before they developed the ability to make sophisticated tools.

Reasoning: Making advanced tools requires the use of free hands, and standing upright is the physical change that allows hands to be free.

Analysis: The scientist's argument relies on the assumption that standing upright is a necessary prerequisite for having free hands. To weaken this, look for an answer that suggests hands could be free for toolmaking through other means, such as sitting or leaning. If prehistoric ancestors could sit on a rock and use their hands to craft tools while remaining quadrupedal for movement, the scientist's chronological requirement collapses. The gap here is the leap from 'needs free hands' to 'must be standing upright.'