Why did our ancestors start walking upright?

Being four-legged has its perks. As a quadruped, your center of gravity is lower, there's less wind resistance when you're running, and, best of all, you can use your hind foot to scratch your ear.

All of this raises a big question: What were our apelike ancestors thinking when they started walking upright? 

A prevailing hypothesis is that they were prompted by climate change. As African forests declined due to temperature fluctuations some 2.5 million years ago, the hypothesis goes, our australopithecine ancestors descended from the trees and ventured out into the open savanna, an environment thought to be friendlier for those standing on two feet....