Walking on two legs – bipedalism

Our bipedal body structure is unique amongst living apes. In fact, our ancestors started on the path to becoming human when they began walking on two legs.

Developing into a biped

About seven million years ago, our early ancestors climbed trees and walked on four legs when on the ground.

By five million years ago, our ancestors had developed the ability to walk on two legs but their gait was quite different from our own and their skeletons retained some features that helped them climb trees.

By 1.8 million years ago, our ancestors had developed long legs and an efficient striding gait that made it easier to travel longer distances. Our ancestors had also developed the ability to run. This new stride became possible when changes to the shoulders, chest and waist allowed the body to stay balanced during prolonged running.

Many changes were required to achieve this, including:

• re-shaping of the pelvis and lower limbs to bring the knees and feet directly under the body’s centre of gravity. This allowed the body to balance on one leg while the other leg was raised to take a step.
• modifications to the feet and toes so they developed into stable platforms with an energy efficient spring to push off the ground with each step.
• alterations to the skull and backbone which created an upright body in which the head was balanced directly above the backbone.

Comparing then to now

Seven million years ago

Seven million years ago, our early four-legged ancestors had a body structure comparatively similar to that found in modern chimpanzees.

Pelvis and legs

• pelvis was tall and narrow
• femora (thigh bones) were vertical (the distance between the knees was similar to the distance between the hips)
• femur and tibia (shin bone) joined in a straight line
• base of the femur (upper part of knee) had two similar-sized condyles (knobs) as both condyles equally supported the body’s weight
• knee joint could not lock to maintain a straight-legged stance

Feet and toes:

• foot bones were straight resulting in flat feet
• toe bones were long and curved for grasping tree branches when climbing
• big toe was opposable (could touch the other toes)

Skull and backbone:

• foramen magnum (hole through which the spinal cord passes) was located at the back of the skull base.
• backbone was attached at the back of the skull base so that the head was positioned in front of the body and needed substantial neck muscles for support.
• lower backbone was not strongly curved and the backbone was steeply sloping for a four-legged posture.

Now

Modern humans have evolved a unique bipedal body structure.

Pelvis and legs:

• pelvis is short and wide
• legs are longer than the arms
• femora (thigh bones) slant inwards (the hips are wide apart but the knees are close together)
• femur and tibia (shin bone) join at an angle
• medial and lateral condyles (knobs) at the knee-end of the femur are comparatively larger than those of quadripedal apes because they must act to bear more weight
• knee joint can lock, allowing the leg to fully straighten

Feet and toes:

• foot bones are curved, resulting in arched feet
• toe bones are short and straight
• big toe is aligned with the other toes and not opposable (cannot touch the other toes)

Skull and backbone:

• foramen magnum position is centrally located under the skull
• backbone attaches to the middle of the skull base
• lower backbone is strongly curved so that the upper body sits above the pelvis and the backbone is aligned for an upright posture