L11.2 Evolution: Skull, thorax, back

Question 1

Classification of primates

Answer 1

• Order family Genius species: Primates, Hominidae, Homo, Homo sapiens

Question 2

Evolution from primates

Answer 2

• Vertical clingers/leapers (Lemur)
  
  • Back upright
• Arboreal quadrupeds: New world monkeys (Lagothrix)
  
  • Arboreal quadrupeds
  • Semibrachiators (swinging from tree branches using their tails)
    
    • Allows back to be upright
• Quadrupeds
  
  • Arboreal (shorter tails) (Colobus)
  • Terrestrial (Baboons)
    
    • Digiti grade locomotion → weight on digits 2-5
• True brachiators (orang-utan)
  
  • UL to swing, long 4 limbs and no tail
• Terrestrial apes (gorillas)
  
  • Knuckle walking
  • Short LL, long UL → weight on kncules
• Humans - bipedal

Question 3

Evolutionary changes: Skull (5)

Answer 3

• Frankfurt horizontal (orbitale to porion)
  
  • Connects INF most orbit to Sup external ear opening → allows skull to be balanced
• Changes the Foramen magnum position (More ANT compared to chimpanzees; Base of skull in humans, back of skull for chimpanzees)
  
  • Allows weight to be directly transferred down
• Axis of foramen magnum
  
  • Apes: Oblique; Humans: Vertical → allows occipital condyles to be vertical
• Nuchal muscle adaptations:
  
  • Ape’s snout is more projected than humans → ↑ weight of skull in apes
  • Extra omo-cervocalis muscles
  • Rhomboids in apes also start at nuchal line, starts at T1-T5 in humans
• Basicranial flexion in humans (more in humans)
  
  • Flexion of basal cranium relation to SUIP cranium

Question 4

Evolutionary changes: thorax (2)

Answer 4

Thorax

• Deep & narrow in monkeys → good for quadrapedal locomotion
  
  • Allows scapula to (LAT) position at the side of ribcage → Glenoid cavity faces ANT
  • Vertical column situated on the outside edge of thorax
• Broad & wider in apes & humans → upright posture
  
  • Vertical column inside space of thorax → CoG balanced in centre
  • Scapula positioned at the back of ribcage → allows glenoid cavity to face outwards
• Shape
  
  • Chimps : funnel shaped ribcage → alllows support in ab & lumbar regions
    
    • Iliac blades much longer & projects POS
  • Humans: Barrel shaped ribcage
    
    • Pelvis blades projected LAT → lumbar region more flexible

Clavicle

• Clavicle: (length corresponds with brachiation)
  
  • Shorter in quadrupeds
  • Longer in humans
  • Even longer in brachiators

Question 5

Evolutionary changes: vertebral column (overview)

Answer 5

• S-shaped in humans with 2o curvatures → allows distribution of weight b/w the s-shape, weight falls off column
• C-shaped in apes with only 1o curvatures → allows quadrupedal locomotion
• Spine numbers: Apes (Humans)
  
  • C: 7 (7)
  • T: 13 (12)
  • L: 3-4 (5)
  • S: 6 (5)
  • Coc: 3-4 (4)

Question 6

Vertebral column: Cervical (2)

Answer 6

• Dens
  
  • Humans: In line with C1
  • Apes & monkeys: Dorsally angled (retroflexion)
• Spinous process:
  
  • Humans: Short& bifid
    
    • Bifid → allows greater attachment of nucheal ligaments → able to hold head in habitual position
  • Apes: Long & ending in knobs

Question 7

Vertebral column: Thoracic (1)

Answer 7

• 13 in apes (12 in humans); 10-11 in monkeys → narrow thorax
• Transverse processes:
  
  • Humans:
    
    • Long & POS positioned → take ribs with POS angulation
    • Wider sternum → wider ribcage

Question 8

Vertebral column: Lumbar (2)

Answer 8

• Size
  
  • Humans: Lumbar vert robust & progressively wider from L1-5
  • Chimps: Constant size
• Facet orientation:
  
  • Humans: transition from sagittal to coronal in humans
    
    • Allows slight rotation (5o) → enables bipedal swing
  • Apes: constant

Question 9

Vertebral column: sacrum & coccyx

Answer 9

• Length:
  
  • Apes & humans: Long sacrum & short coccyx
  • Primates with tail: Short sacrum & Long caudal region
• Lumbosacral angle:
  
  • Humans: ~60-65o
    
    • Allows weight bearing
    • Allows giving birth to child with larger brain
  • Apes: reduced

Question 10

Evolutionary changes: Back muscles (2)

Answer 10

• Erector spinae:
  
  • Humans: better dev → distinct bellies
• Iliocostalis, longissimus:
  
  • Humans: separate → slight rotation/lat flexion control