Compared with the chimpanzee (left), the human lower spine (the lumbar region) has a curvature (a lordosis) that positions our center of mass (circle) above our hips when we stand. When we squat on the ground (the way people often sat for millions of years) or slouch when sitting in a chair with a backrest, we tend to rotate the pelvis backward and flatten the lower spine, reducing this lordosis.
