Hominid Evolution
Adaptations to Bipedalism
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What is bipedalism?
Hominids include the great apes (orangutans, gorillas and chimpanzees) and humans, while Hominins include Humans and their fossil ancestors. Two main features that separate Hominins from the apes is that they are relatively hairless, and they are full bipedal (walk on two legs with their forelimbs free).
Bipedalism is the method of locomotion that sets Hominins apart from the other Hominids; humans are fully bipedal whereas apes are quadrupedal knuckle walkers. Hominins have many adaptations compared to apes that enable them to have bipedal locomotion, and these include adaptations to;
Skull
Vertebral column
Pelvis
Femure
Knee
Foot
Arm-to-leg ratio
Centre of gravity
Adaptations for Bipedalism: Skull
Hominid
Foramen magnum is found at the back of the base of the skull
More prognathic (jaw protruding more)
Heavy U-shaped jaw with large canines with diastema
Hominin
Foramen magnum at the centre of the base of the skull
Flatter face (less prognathic)
Lighter horeshoe shaped jaw with smaller canines with no diastema
Advantage to the Hominin
Allows the skull to balance on the vertebral column with little effort
This reduces the requirement for large neck muscles
Adaptions for Bipedalism: Vertebral Column
Hominid
Single curve (C shape)
Non-wedge shaped lumbar vertebrae
Large processes on neck vertebrae
Hominin
S-shaped curve
Wedge shaped lumbar vertebrae
Small processes on neck vertebrae
Advantage to the Hominin
Allows for centre of gravity to pass through the knee and foot; skull can balance on the column with little muscle support
Forms lumbar curve to help balance the body
Allows for increased neck movement
Adaptations for Bipedalism: Pelvis
Hominid
Long and narrow and titled forward
Hominin
Short, broad, bowl shaped and vertical
Advantage to the Hominin
Short and broad pelvis provides surface for attachment of leg muscles; this allows for the movement of the legs while keeping the upper body erect
Bowl shaped to support abdominal organs (support developing foetus during pregnancy
Vertical angle allows alignment with upright vertebral column
Adaptations for Bipedalism: Femur
Hominid
Femurs come straight down from pelvis
Shorter femur
Hominin
Modified acetabulum and longer ball shaft on femur
Femurs angle in from pelvis
Longer femur
Femur is more robust
Advantage to the Hominin
Creates a carrying angle and brings the knees closer together, thus making it easier to transfer weight from one foot to another when walking; this allows humans to have a striding gait instead of swaying from side to side
Allows for a longer stride
Adaptations for Bipedalism: Knee
Hominid
Single hinge
Hominin
Two-part robust hinge joint with ligaments
Advantage to the Hominin
Robust (load bearing)
Centre of gravity falls in front of the knee; this forces the knee backwards but is stopped by ligaments; little energy is required for the standing position
Adaptations for Bipedalism: Foot
Hominid
Longitudinal arch
Opposable big toe
Small calcaneus
Hominin
Longitudinal and transverse arches
Large big toe with it alongside others
Large robust calcaneus
Advantage to the Hominin
Produces a spring action and acts as a shock absorber when walking
Acts to give a push off (lever) when walking as energy is transferred through it
Bears impact of bodies weight (load bearing)
Adaptations for Bipedalism: Arm-to-leg ratio, Centre of gravity
Arm-to-leg ratio
Hominid
Arms longer than legs
Centre of gravity is higher within the body (chest level)
Hominin
Legs longer than arms
Centre of gravity is lower within the body (located in the pelvis)
Advantage to the Hominin
Allows long striding gait
Increases stability while bipedal and ability to stand upright
Centre of gravity
Hominid
Centre of gravity falls in front of the knee and feet
Centre of gravity is higher within the body (located above the pelvis)
Hominin
Centre of gravity falls through the front of the knee and centre of the feet
Centre of gravity is lower within the body (located in the pelvis)
Advantage to the Hominin
Increases stability while bipedal and ability to stand upright
Upright stance is achieved with minimal effort