154 Adaptations for Bipedalism
CL N AS OT SR F OO OR M US E
270
Key Idea: Important changes in the skeleton are associated with the move to bipedal locomotion in early hominins. The reconstruction of Lucy (Australopithecus afarensis), opposite, shows the skeletal features of an early bipedal hominin. Lucy still possessed ape-like features but she was a fully-bipedal hominin with all the adaptations associated
with bipedal locomotion. Although there is no doubt that Lucy was habitually bipedal, a number of skeletal features suggest that tree climbing was still an important part of this hominin's niche, perhaps associated with escape, security, or foraging. A. afarensis is an important link between the quadrupedal locomotion of apes and bipedalism in hominins.
Chimpanzee
Human
Broad, basin-like pelvis
PR E O V N IE LY W
Long narrow pelvis
Gluteus muscles prevent tilting when the opposite leg is off the ground.
Femoral head angled and strengthened
Foramen magnum (FM) toward the back of skull
The carrying (valgus) angle ensures the knee is brought under the body during walking.
Foramen magnum (FM) further forward so the skull balances on the spine S-shaped spine acts like a spring
Quadruped: Straight spine and rear-ward FM, femur is at right angles to knee so an upright stance is less stable.
Biped: S-shaped spine and forward FM. Femur (thigh) is angled out from knee (the carrying angle). A longer femur provides a longer, more efficient stride.
Chimpanzee
Human
End of femur at the knee joint
End of femur at the knee joint
Australopithecine
End of femur at the knee joint
Bony buttress
Lateral condyle
Inner (medial) condyle
Chimpanzee foot
Human foot
Lighter shading represents points of contact with the ground
Lighter shading represents points of contact with the ground
Australopithecine footprints
Direction of weight transmission during walking
Curved toe bones
153
A-1
The human foot is adapted as a weight bearing platform rather than a grasping structure. The toes are reduced relative to those of chimpanzees. The foot is arched so transmits weight from the heel, along the outside of the foot, across the ball and through the big toe. This weight transference conserves energy during locomotion.
Heel bone missing from fossil
Foot bones (OH8) from Bed I at Olduvai Gorge
N AS OT SR F OO OR M US E
The foot of a chimpanzee has relatively long, curved toes, with an opposable big toe adapted from grasping but ill-suited to upright walking. The foot transmits weight from the heel, along the outside of the foot, and then through the middle toes.
Big toe aligned with other toes (not opposable)
The australopithecine foot had an aligned big toe, as in humans, making it difficult if not impossible to grasp branches with the hindlimbs. The heel bones that have been found also indicate habitual bipedalism. Computer simulations suggest that A. afarensis could walk like humans but could not have walked like a chimpanzee.
CL
Big toe diverges (well separated from other toes
Large heel bears increased weight
©2021 BIOZONE International ISBN: 978-1-98-856637-5 Photocopying Prohibited