"Ratcheting" isn't actually just communicating knowledge to each other (although that is a prerequisite). Cultural ratcheting is when there is generational improvement on previous cultural knowledge or technological ability. Basically the next generation is taught what is known, and then attempts to improve it in some way to benefit the entire group. This has been displayed in no other species except our own.

The reason our intelligence was selected for is still a matter of intense debate among paleoanthropologists, however we do know that other key traits were selected for before larger brain size. This is key. For example, opposable thumbs and bipedalism were selected for millions of years before larger brain size. In this sense, in order for intelligence to be selected for, their must be necessary pre-adaptations. Once our ancestors were fully bipedal and able to manipulate objects with our opposable thumbs, it then became more beneficial to possess a larger brain with which to help us manipulate our environment in increasingly complex ways. Also, it is likely that we developed the ability to cook our food around the same time we experienced a surge in overall brain size. This would explain where we got our "extra energy" to fuel such an expensive organ.

If you want to learn more about this check out "Catching Fire" by Richard Wrangham:

http://www.amazon.com/Catching-Fire-Cooking-Made-Human/dp/04...