Lecture 22 - Skull Evolution

Question 1

Anapsid Skull

Answer 1

No post-orbital foramen

Question 2

Synapsid Skull

Answer 2

A single post-orbital foramen

*mammals are synapsids: post-orbital foramen is closed/incorporated as the zygomatic arch

Question 3

Diapsid Skull

Answer 3

Two post-orbital foramen

Question 4

What are the cranial effects of bipedality?

Answer 4

1. Foramen magnum position
2. Cranial base angle
3. Nuchal plane orientation

Question 5

Foramen magnum position

Answer 5

Foramen magnum in humans is anteriorly shifted compared to other apes

• caused by reorientation of the vertebral column under the skull
• present in all hominins

Question 6

Cranial Base Angle

Answer 6

• angulation of the sphenoid
• increased distance of epiglottis, vocal change: vocal tract proprtions affect the ability to produce discreet vowel sounds (quantal speech)
• brain expansion drives further angulation

Question 7

Neoteny

Answer 7

Humans retain a more juvenile form of skull shape

Question 8

Nuchal Plane Orientation

Answer 8

• an inferior foramen magnum results in a more balanced head
• as prognathism (snoutiness) is reduced, less need for large nuchal muscles
• this leads to a very small nuchal crest in modern humans

*chimps have less balanced head = less efficient nuchal muscles

Question 9

Teeth Evolution

Answer 9

-dentine shows up at the same time as jaws do

*enamel plates (scales) move into mouth to become teeth

Question 10

Ape vs. Human Teeth

Answer 10

• larger incisors and canines in apes
• larger post-canine teeth in humans

Question 11

Why the change in incisor size?

Answer 11

In hominins, incisors less important for food digestion?

• change in diet?
• does processing with stone tools replace the incisors?

Question 12

Why the change in canine size?

Answer 12

Social structure change = no aggressive display needed in hominins?

Question 13

Why the change in post-canine size?

Answer 13

• hominin molars and premolars small in apes and early homs
• very large in australopiths
• moderate in Homo

Question 14

Bite Force Mechanics

Answer 14

• facial shape response to dental loading = flaring zygomatic arches, huge post-canine teeth, large chewing muscles, sagittal crest, large mandible resists twisting
• reduced prognathism increases mechanical advantage during chewing
• reduced resistence lever arm, closer to TMJ (COR)

Question 15

Why has teeth size decreased in Homo?

Answer 15

Diet Change

• including meat in the diet = less mechanically challenging calories
• more stone tools processing = less chewing time

Question 16

Chimp diet vs. Industrial modern human diet

Answer 16

% of the day spent chewing

Chimps = 40%

Humans = 5%

*2.5 million fewer chews by humans each year

Question 17

Brain Size Evolution

Answer 17

• early human evolution = slight increase in brain size
• progressive increase in genus Homo

*slight decrease in modern humans

Question 18

Brain Size Development

Answer 18

• modern humans grow brains faster and longer than chimps
• growth rapid in the first year of life

Question 19

Is brain size correlated with intelligence or sociality?

Answer 19

• cerebral size is correlated with group size
• change in social dynamics in human evolution = increased brain size

*social dynamics more responsible than IQ for increase in brain growth and size

Question 20

What are modern humans defined by?

Answer 20

Our heads

• small browridge
• globular neurocranium
• small, vertical face
• chin = remnant of developmental history