Locomotion: Adaptations to Bipedalism and Quadrupedalism

Quadrupedalism

Walking on 4 limbs

Bipedalism

Walking on 2 limbs

Changes from Quadrupedalism to Bipedalism

Human spines and muscles evolved/adapted for walking bipedally with a striding gait, ensured survival for our ancestors and acted as selective pressure during natural selection.

Used in fossil classification

Foramen Magnum Position

Where brain joins spinal cord, hole in skull.

Ape: Neck muscles to hold skull in position.

Human Adaptation: Central, balance skull on vertebral column, doesn’t need neck muscles.

Spinal Cord Curvature

Human Adaptation: Upright posture, lumbar vertebrae wedge-shaped, producing s-shaped curve brings vertebral column directly under centre of skull improves body and skull balance.

The Jaw

Human Adaptation: Small, non-protruding, enabling skull balance on vertebral column with minimal muscular effort

The Pelvis

Human Adaptation: Broad; shallow from top to bottom, supports abdominal organs. Attachment of femurs wide apart, contributes to carrying angle. Supports female in pregnancy carrying foetus.

Carrying Angle/Femur

Human Adaptation: Pelvis and femur structure positions hip joint under trunk and head, transfers body weight to legs.

Broad pelvis and wide hip sockets cause femurs to converge towards knees, forming a carrying angle.

Angle ensures weight distribution close to body's central axis, enhancing stability allowing straight, striding gait.

Apes:, weight falls thru inside of femur, causing swaying gait when walking upright.

The Knee

Human Adaptation: From femur to knee(two part hinge joint), outer hinge larger and stronger to take body weight, centre gravity falls line in front of knees → force tries to bend knee backward but natural resistance by ligaments is able to be straightened with no energy.

The Foot

Human Adaptation: Large heel bone, aligned big toe, highly specialised locomotory organ lost prehensibility.

From knee, body weight transmitted through tibia(larger and stronger of two lower leg bones) to ankle(talus) to other tarsal bones then metatarsals and phalanges via foot arches; transverse and longitudinal.

Centre of Gravity

Human Adaptation: Legs longer than arms, contribute to low centre of gravity, Carrying angle allows body weight kept close to central axis

Muscle Tone

Human Adaptation: Partial contraction of muscles to support spine hip knee and ankle (e.g. anterior and posterior neck muscles for skull)

Evolution: Changes Allowing Bipedalism in Humans

Central foramen magnum, S-shaped spine, non-protruding jaw, broad pelvis, carrying angle of femur, knee larger on outside, long legs, longitudinal and transverse arches on feet, muscle tone

Striding Gait

Walking upright with straight hips, knees unique to hominins.

Human Adaptation: Weight shifts from heel to big toe propelling body → evolution of human foot for weight-bearing.

Arm swinging balances trunk rotation, conserving energy. Broad pelvis and converging femurs ensures stable weight distribution and straight walking

Apes walk with bent knees, forward lean, sway side to side

Advantages of bipedalism

• energy efficient moving

• free hands to use tools, carry items

• Upright stance: greater height, less body exposure to sun, increases breeze exposure for cooling mechanisms

Prognathism and Dentition

Teeth and jaw changed so humans don’t have diastema, have smaller canines and incisor and parabolic dental arcade.