It seems this is a better answer than many—there is rarely a ‘smoking gun’ single reason that a bodily design is selected for. There is usually a constellation of various bodily features that mutually reinforce a ‘successful’ species design. Also there are multiple reasons why a feature is selected — it could be that walking long distances, seeing further on the savannah, and using our hands are all contributors.

Humans are in several ways a constellation of unusual features that work together. We are odd in being relatively hairless mammals. It’s unusual we eat such a broadly omnivorous diet. Our heavy focus on intelligence is weird.

What seems to be the précis for our design is adaptability and sociability. We’re somewhat like orangutans (who also have comparatively long childhoods) in that we range over large distances, eat a lot foods, and have the intelligence to know how to adapt to these varying environments and foods. However, we’re more social than orangutans and range over wider areas.