Bipedal Adaptations

Question 1

Overview

Answer 1

• Human knees aligned under the body’s center of gravity because femurs are angled inwards
• Human legs straighten completely when walking
• Human spine has additional curves to keep centers of mass of head and trunk aligned for bipedalism
• Big toe not opposable in humans, which allows for an archer foot
• Ratio legs:arms greater for humans than other apes
• Human pelvis broader

Question 2

Why bipedalism?

Answer 2

• Most energy efficient way to travel long distances
• Carrying capacity
• Reduction of overall heat stress by convection (by exposing body to air current)
• Allows for better vision in open environments and defensive action by freeing hands to throw objects

Question 3

Energy Efficiency

Answer 3

• Human walking is 75% less costly than quadrupedal and bipedal walking in chimpanzees
• Humans save more energy than quadrupeds when walking but not when running
• Human running is 75% less efficient than human walking

Question 4

Feet

Answer 4

• Enlarged heels to bear the increased weight of modern humans
• Platform to support the entire body, rather than a grasping structure
• Smaller toes than ancestors
• Non-opposable hallux, which is relocated in line with the other toes
• Humans have archer feet, rather than flat feet
• Very efficient transference of weight contributes to energy conservation during locomotion

Question 5

Pelvis

Answer 5

• Modern human hip joints are larger than in quadrupedal ancestral species to better support the greater body weight
• Shorter, broader shape brings vertebral column closer to the hip joint, providing a stable base for support of the trunk while walking upright
• Placement of vertebral column closer to hip joint allows humans to invest less muscular effort in balancing on unstable ball and socket joints.

Question 6

Knee

Answer 6

• Enlarged to support weight
• Degree of extension decreased
• Small extension peak called double knee action decreases energy lost by vertical movement of the center of gravity
• Humans walk with knees kept straight and thighs bent inward so knees are under the body, not out to the sides like in ancestral hominids
• This gait aids balance

Question 7

Vertebral Column

Answer 7

• Bends forward in the lumbar (lower) region and backwards in the thoracic (upper) region
• Lumbar curve means we do not always lean forward, which saves muscular effort in bipedal animals
• Forward bend means humans use less muscular effort to stand and walk upright
• Lumbar and thoracic curves bring the body’s center of gravity directly over the feet
• Degree of body erection is significantly smaller to conserve energy

Question 8

Skull

Answer 8

• Skull vertebral column
• The foramen magnum is located inferiorly under the skull, putting the weight of the head behind the spine
• Flat human face helps to maintain balance on the occipital condyles
• Erect position of the head is possible without the prominent supraorbital ridges and the strong muscular attachments found in apes
• Muscles in the forehead only used for facial expressions
• Increasing brain size significant in human evolution
• Increase began around 2.4 million ya but modern levels of brain size were not attained until after 500,000 ya

Question 9

Bipedal adaptation in Australopithecines

Answer 9

• Femoral neck long and slender
• Femur at a valgus angle at the knee joint to allow the midline of the body to pass between the feet
• Anatomy of the pelvis similar to humans
• Unlike nonhuman apes, the toe is in alignment with the other digits
• Foot-arch present
• The heel is much larger than in chimpanzees and serves as a shock absorber

Question 10

Homo sapiens

Answer 10

• Obligate biped
• Smaller, V-shaped jaw
• Chest more barrel-shaped
• S-curved spine
• Longer leg bones
• Enlarged heel bone (stable foot arch)

Question 11

Australopithecus

Answer 11

• Facultative biped
• Protruding jaw
• Longer arm bones
• Higher, narrow pelvis
• Longer femoral neck
• Longer toes (big toe splayed)

Question 12

Beginning of bipedalism

Answer 12

• Began in primates about four million years ago, or as early as seven million years ago with Sahelanthropus, has led to morphological alterations to the human skeleton.
• These changes include to arrangement and size of the bones of the foot, hip size and shape, knee size, leg length, and the shape and orientation of the vertebral column.

Question 13

Chimpanzees

Answer 13

• Skull attaches posteriorly
• Spine slightly curved
• Arms longer than legs and also used for walking
• Long, narrow pelvis
• Femur angled out

Question 14

Hominins (early austra)

Answer 14

• Skull attaches inferiorly
• Spine S-shaped
• Arms shorter than legs and not used for walking
• Bowl-shaped pelvis
• Femur angled in