4Body Changes: knuckle walkers vs. bipedalists foramen magnumCircular hole at the bottom of the skull where the medulla oblongata (spinal cord) enters and exits the skullThe foramen magnum is farther underneath the skull, so the head is more or less balanced on the spine.
5Body Changes: knuckle walkers vs. bipedalists spineThe spine has two distinctive curves—a backward (thoracic) one and a forward (lumbar) one—that keep the body and weight centered above the pelvis.
6Body Changes: knuckle walkers vs. bipedalists pelvisThe pelvis is shaped more in the form of a bowl to support internal organs.
7Body Changes: knuckle walkers vs. bipedalists Hip bonesThe 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).
8Body Changes: knuckle walkers vs. bipedalists legsLower limbs are longer.In humans the thigh comprises 20% of body height, while in gorillas it comprises only 11%.
9Body Changes: knuckle walkers vs. bipedalists Femur and knee actionThe femur is angled inward, keeping the legs more directly under the body.Modified knee anatomy also permits full extension of this joint.
10Body Changes: knuckle walkers vs. bipedalists feetThe big toe is enlarged and brought in line with the other toesAn arch forms, helping absorb shock and adding propulsive spring.
11The 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.
12Disadvantages of Bipedalism Difficult childbirthHerniasVaricose veinsLower back pain
13Why Hominids Became Bipedal Many theories to explain why bipedality evolved.Probably a combination of factors occurring over a great deal of time.Theories:CarryingLong distance walkingHuntingGatheringVisual surveillanceMale provisioningThermo-regulationFeeding from bushes
14upright posture freed the arms to carry various objects. Theories:Carryingupright posture freed the arms to carry various objects.
15Long-distance walking Theories:Long-distance walkingCovering long distances is more energy effective for a biped than for a quadruped.
16Theories:Huntingcarrying weapons and energy efficient long-distance walking made hunting more practical.
17feeding on seeds and nuts occurred standing upright. Theories:Gatheringfeeding on seeds and nuts occurred standing upright.
18Theories: Feeding from bushes upright posture provided access to seeds, berries, etc., in lower branches
19Theories: Visual surveillance standing up provided better view of surrounding countryside (view of predators and other group members).
20males carried back resources to dependent females and children. Theories:Male provisioningmales carried back resources to dependent females and children.
21Theories: Thermoregulation for body cooling. Vertical posture exposes less of the body to direct sun.
22Theories: Thermoregulation for body cooling. Vertical posture exposes less of the body to direct sun.