Do we really need two cameras for 3D vision?
First of all, our brain does not only rely on our stereo visionary system to estimate the depth. There are many cues in a image scence for depth estimation, of which stereo, vision belongs to a sub-type called Binocular cues. 

Technically there are many other methods of depth estimation, like Structure from motion, Perspective,etc. Just take a look at here and here.