Our transition to upright posture required significant adaptations of our muscles and bones. Most primates can sit and stand, with some able to walk upright for short periods of time. What allows humans to sustain these acts is the primary and secondary curves running through our whole body— most significantly those in the lumbar spine or lower back. Chimpanzees, our closest ancestors among the primates, have a flat lumbar spine and as a result can’t sustain upright posture. It is our lumbar spine’s lordotic or anterior curve that enables our upper body and feet to bear and transfer weight.
There are other important differences between the human skeleton and that of the chimpanzees. Our knuckle-dragging cousins use their hands to help them move forward, and although they can walk on two legs for short distances, their walk doesn’t much resemble ours.
One reason for this is that our thigh bones slope inward from the hip to the knee, allowing the human foot to fall directly under our center of gravity. This led us to develop powerful pelvic muscles called gluteal abductors which stabilize our bodies while in mid-stride. Chimps’ thigh bones slope outwards causing them to stand and walk with their feet wide apart. What’s more, their pelvic muscles are much weaker than ours, so that they have to move their entire body from side to side with each step, just to keep their center of gravity over whichever leg is bearing weight. Most importantly, weight is not placed their across the whole chimp foot. Rather, they ground exclusively to the outside of the foot.
Human evolution followed many different paths. Our uprightness led to increased acuity of vision and the development of larger brains, which in turn required a wider-ranging diet including more high protein foods. To accommodate these advances, we needed to make our way down from the trees in order to forage over greater distances. In time, we began to do this exclusively on two legs.
Our descent from the treetops brought changes to the structure of our feet and the job that is required of them. The chimp foot requires an opposable “thumb” for grasping tree branches. In the human foot, the big toe has moved towards the midline and points in the same direction as our other toes. This seismic shift saw the big toe go from being a grasping digit to one which helps us move through space. In fact, when we are walking properly, every step ends with the entire weight of the body on the big toe.
Another evolutionary change in the foot is the move towards weight bearing responsibilities and the formation of the longitudinal arch. While many primates stand largely on their toes or on the ball of the foot, human beings are plantigrade animals and stand on the whole foot. The human foot is a weight-bearing platform, with spring arches that act as shock absorbers.
These transformations were necessary steps towards increased efficiency. As we evolved from quadruped to biped, our new foot became solely responsible for supporting us and moving us forward through space.