1. Bipedalism

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

3. Body Changes: knuckle walkers vs. bipedalists

4. 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.

5. 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.

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

7. 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).

8. 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%.

9. 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.

10. 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.

11. The Bipedal Adaptation
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.

12. Disadvantages of Bipedalism
Difficult childbirth
Hernias
Varicose veins
Lower back pain

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

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

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

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

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

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

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

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

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

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