Chapter 9 - The Evolution of Bipedalism

Mosaic evolution

pattern of evolution in which the rate of evolution in one functional system varies from that in other systems

3 major trends in Hominin (human) evolution

• bipedalism

• encephalization

• dental reduction (our teeth get smaller)

bipedalism was the first step in human evolution, big brains and small teeth evolved later

phases of huamn bipedalism

• begins with heel-strike

• knee is fully extended

• foot is dorsiflexed

• force is transmitted through the ground along its lateral borders (plantar flexors)

• finish with a final push off of the big toe (toe off)

basically

• falling forward with gravity then swinging our other leg to catch ourselves

• very energy efficient

chimp bipedalism

(bent-knee, bent-hip gait)

• the knee and hip remains bent during stance

• foot is less dorsiflexed at heel-strike

• awkward "shuffling movement with marked mediolateral swaying of the body from step to step

structural alterations of obligate bipedalis

1. foramen magnum orientation

2. narrower ribcage

3. spinal curvature

4. shrorter, broader pelvis

5. lengthened lower limbs and enlarged joints surface areas

6. an angled femur which is served by reorganized musculature. an extensible knee joint

7. a platform foot with enlarged big toe that is brought in line with the other toes

• toes decrease in relative length and degree of curvature

• bigger calcaneal tuber (heel)

multiple curves of the spine

• thoracic and sacral regions are concave anteriorly (thoracic/sacral kyphosis)

• cervical and lumbar regions are convex anteriorly (cervical/lumbar lordosis)

humans vs. apes in skeletons and how it relates to their body movement

• human spine curvature allows for bones to stack so they can stand upright against gravity, while non-human apes can’t

• pelvis goes out to the side, allowing space for muscles that can allow humans to also move side to side

  • non-human apes cannot really move side to side, only forward and backwards

why are humans’ long legs and shorter arms beneficial

• longer legs allows for longer strides

• arms are swinging acting as a counterweight to your legs to keep balance (opposite arm for opposite leg)

how are human hind limbs modified to bear weight under the body

• medial femoral condyle projects further distally than the lateral one

• Valgus condylar angle

• top of the tibia is asymmetrical to accomodate this

• Distil tibia is flat to transfer weight efficiently