Bipedalism - on two feet.
•The single most distinctive feature of Hominids
•Hominid bipedalism is habitual and required

: knuckle walkers vs. bipedalists

Body Changes: knuckle walkers vs. bipedalists foramen magnum
 Circular hole at the bottom of the skull where the medulla oblongata (spinal cord) enters and exits the skull
 The foramen magnum is farther underneath the skull, so the head is more or less balanced on the spine.

Body Changes: knuckle walkers vs. bipedalists spine
The spine has two distinctive curves —a backward (thoracic) one and a forward (lumbar) one —that keep the body and weight centered above the pelvis.

Body Changes: knuckle walkers vs. bipedalists pelvis
The pelvis is shaped more in the form of a bowl to support internal organs.

Body Changes: knuckle walkers vs. bipedalists
Hip bones
The hip bones are shorter and broader, stabilizing weight transmission.
a) Homo sapiens.
(b) Early hominid (Australopithecus) from South Africa.
(c) Chimpanzee. Note especially the length and breath of the iliac blade and the line of weight transmission
(shown in red).

Body Changes: knuckle walkers vs. bipedalists legs
Lower limbs are longer.
In humans the thigh comprises 20% of body height, while in gorillas it comprises only 11%.

Body Changes: knuckle walkers vs. bipedalists
Femur and knee action
The femur is angled inward, keeping the legs more directly under the body.
Modified knee anatomy also permits full extension of this joint.

Body Changes: knuckle walkers vs. bipedalists feet
The big toe is enlarged and brought in line with the other toes
An arch forms, helping absorb shock and adding propulsive spring.

• Efficient bipedalism among primates is found only among hominids.
• All the major structural changes required for bipedalism are seen in early hominid fossils.
• Some researchers believe these early humans also spent considerable time in the trees.

Disadvantages of Bipedalism
Difficult childbirth
Hernias Varicose veins Lower back pain

Many theories to explain why bipedality evolved.
Probably a combination of factors occurring over a great deal of time.
• Carrying
• Long distance walking
• Hunting
• Gathering
• Visual surveillance
• Male provisioning
• Thermo-regulation
• Feeding from bushes

Theories:
Carrying upright posture freed the arms to carry various objects.

Theories:
Long-distance walking
Covering long distances is more energy effective for a biped than for a quadruped.

Theories:
Hunting carrying weapons and energy efficient long-distance walking made hunting more practical.

Theories:
Gathering feeding on seeds and nuts occurred standing upright.

Theories:
Feeding from bushes upright posture provided access to seeds, berries, etc., in lower branches

Theories:
Visual surveillance standing up provided better view of surrounding countryside (view of predators and other group members).

Theories:
Male provisioning males carried back resources to dependent females and children.

Theories:
Thermoregulation for body cooling. Vertical posture exposes less of the body to direct sun.

Theories:
Thermoregulation for body cooling. Vertical posture exposes less of the body to direct sun.