the environment limited some traits and allowed for others;
you (most likely) have three flavors of color cones in your eyes not four like birds(dinosaurs) and replies because the having more rods (B/W low light sensitive) was better suited to night survival. On the other hand our brains do not (usually) have a bone partition between the two halves because we did not need to sleep with one eye open underground.
Yes, apparently chickens (and other birds) really can sleep one half of their brain at a time. In chickens, the ones roosting on the edge of the flock keep their outside eye open and sleep the other side of the brain.
