Chapter 12 - Trends in Hominid Evolution

Question 1

Understand the classifications of humans within the primate order.

Answer 1

Review Table in Chapter 12.1

Question 2

List and Explain the Characteristics of PRIMATES

Answer 2

Most features evolved from arboreal or tree-like environment

1. Body- Not specialised for a particular environment
2. Limbs-
   - Generally unspecialised
   Hands/Feet
   - Pentadactyl – Five fingers or toes
   - Nails instead of claws
   - Grasping fingers and toes with friction ridges for gripping
   - First digit opposable
3. Eyes
   - Forward facing for 3D (stereoscopic) vision
   - Most are able to distinguish colour
4. Sense of Smell - Very poor
   - Smaller olfactory region in brain (decreasing in evolution)
5. Teeth
   - Four incisors in both the upper and lower jaw
6. Brain
   - Large and complex
   - Cerebrum size increases as primates become more highly evolved
7. Reproduction
   - Not restricted to a breeding season
   - Rhythmical sexual cycle
   - Usually only one offspring at a time
   - Long period of parental care for offspring

Question 3

What are the Characteristics of Hominidae?

Answer 3

Hominidae members are called hominids

Characteristics of Homininds

1. A larger, more complex brain
   - Increased cognitive ability as they can recognise themselves in a mirror
2. Five cusps in the molar teeth of the lower jaw
3. Arms can freely rotate at the shoulder
4. Wide, shallow chest cavity
5. No external tail
6. Appendix
7. Diurnal: Active during the day

Question 4

Variations that reflect chnages in DNA of Hominindiae

Answer 4

o Relative size of the cerebral cortex
o Mobility of the digits
o Locomotion quadrupedalism and bipedalism
o Prognathism and dentition

Question 5

Explain the Relative size of Cerebral Cortex in Hominidae Evolution.
Responsibility, How and Why it changed.

Answer 5

Cerebrum increased in size and complexity

How? Cerebral cortex increased in surface area; covering the brain

Responsible for

• visions, memory, reasoning, and manipulative ability
• Necessary to cope successfully with changes in the environment

Why? Large brain for body required due to selection pressures of the arboreal environment

1. More accurate visual and tactile perception
2. Better coordination between sensory stimulation and muscular response
3. Reliance on vision to move, locate and manipulate food generates complex sensory information that needs to be processed and stored by the cerebral cortex
4. Predetermined image of the completed tool only possible with highly developed brain

Question 6

Humans Vs Apes - Difference in Cerebrum and Cerebral Cortex

Answer 6

Greatest difference in cerebrum and cerebral cortex:

• Average human brain 900-2200cm3; averaging around 1350cm3
• Average apes average between 400-500cm3

Frontal lobe has the greatest enlargement in surface area

1. Higher functions of reasoning, planning and processing
2. Locate food, develop special skills and making tools
   - 47% of total conical surface
   - 33% of apes

Question 7

Explain Convolutions or folds increase surface area of the brain and cerebral cortex

Answer 7

• Convolutions or folds increase surface area of the brain and cerebral cortex

1. Larger and more convoluted
2. 50% increase in surface area of human brain with convolutions vs without
3. More behavioural responses to meet environmental problems
   - Mutual cleaning to reinforce relationships
   - Threats followed by fighting to maintain a hierarchy

• Cranium/large brain case used to house the brain
- Brains not fossilised but it fills with the whole cranium

• Brow tends to be vertical and lacks brow ridge
• Shortening of the snout
• Flatter face but nose still protrudes

Question 8

What is an Endocast?

Answer 8

Endocast: used to determine volume inside the cranium which determines brain size
the Cranial Capacity

Question 9

Explain the Mobility of the Digits (6 Reasons make it so)

Answer 9

1. Pentadactyl five digits on each limb
2. Highly mobile due to arboreal selective pressures
3. Prehensile: Able to grasp as digits were essential for climbing by wrapping long, curved digits around the branches of trees
   - Increasing ability to move digits independently of one another
   - Thumb and big toe are the most highly developed
4. Opposable: First digit can touch each of the other digits
   - Most primates show some opposability of the big toe
   - Lose when human foot became weight-bearing than a grasping appendage
   - Humans
   ### Longest thumb (and strong); can easily manipulate objects in comparison to other primates
   ### Short and broad hand
   ### Short, straight fingers
5. Precision grip = Amount of contact between index and thumb
   - Requires opposable thumb
   - Humans and Old World monkeys; Ground-living baboons, mandrills and macaques
   - Enables humans to handle small/delicate objects effectively
6. Power grip - Thumb and fingers apply force against the palm

Question 9

Explain the Mobility of the Digits (6 Reasons make it so)

Answer 9

1. Pentadactyl five digits on each limb
2. Highly mobile due to arboreal selective pressures
3. Prehensile: Able to grasp as digits were essential for climbing by wrapping long, curved digits around the branches of trees
   - Increasing ability to move digits independently of one another
   - Thumb and big toe are the most highly developed
4. Opposable: First digit can touch each of the other digits
   - Most primates show some opposability of the big toe
   - Lose when human foot became weight-bearing than a grasping appendage
   - Humans
   ### Longest thumb (and strong); can easily manipulate objects in comparison to other primates
   ### Short and broad hand
   ### Short, straight fingers
5. Precision grip = Amount of contact between index and thumb
   - Requires opposable thumb
   - Humans and Old World monkeys; Ground-living baboons, mandrills and macaques
   - Enables humans to handle small/delicate objects effectively
6. Power grip - Thumb and fingers apply force against the palm

Question 10

Locomotion – Adaptations to bipedalism and Quadrepedalism… WHY?

Answer 10

Quadrepedalism to bipedalism

BECAUSE Adaptation acted as a selective pressure during natural selection

Question 10

Locomotion – Adaptations to bipedalism and Quadrepedalism… WHY?

Answer 10

Quadrepedalism to bipedalism

BECAUSE Adaptation acted as a selective pressure during natural selection

Question 11

List the features in which lead to Locomotion; QUAD TO BIPEPEDAL (10)

Answer 11

1. Position of the Foramen Magnum
2. Curvature of the Spinal Column
3. The Jaw
4. The Pelvis
5. The Carrying Angle
6. The knee
7. The Foot
8. Centre of Gravity
9. Muscle Tone
10. Striding Gait

Question 12

Explain the Position of the Foramen Magnum

Answer 12

• Position of the Foramen magnum
o Hole in the skull where the brain joins the spinal cord

o Has gradually moved forwards to become more central

• Allows skull to balance on top of the vertebral column
• Weight borne by vertebral column so large neck muscles not required

o Ape like gorilla needs large neck muscles to hold the head in position

Question 13

Explain the Curvature of the Spine

Answer 13

o ‘C’-shaped curve in gorillas evolved to ‘S’-shaped curve in humans

o Improved body balance in upright position

• Enables head to balance
• Lumbar vertebrae wedge-shaped from front to back; forming a forward-jutting curve
• Cervical curve in neck bring vertebral column under the centre of gravity of the skull

Question 14

Explain the Jaw

Answer 14

o Prognathism; protruding jaw evolved to a flatter facial profile
- Gradually reduced size and protrusion

o Skull balances on top of the spine

• Weight in front and back of the foremen magnum is equal
• Achieved with minimum muscular effort

Question 15

Explain the Pelvis

Answer 15

o Vertebral column articulates with pelvis

o Pelvis in humans in broader and shorter from top to bottom than in aped

• Bowl shape supports abdominal organs when standing erect
• Greater stability for bipedal locomotion
• Supports developing foetus during pregnancy

o Female passage slightly broader than males to allow for passage of the infant at birth

o Broad hip bone space for attachment of large buttock muscles
- Helps move legs and keep upper body erect

Question 16

Explain the Carrying Angle

Answer 16

o Hip joint directly under trunk and head
 Body weight transferred from pelvis to legs

o Head of femur fits into acetabulum (hip socket) of the pelvis
 Sockets are wide apart but the femurs tend to converge at the knees
o Carrying Angle; Femurs forms an angle to the vertical
 Greater stability in an upright posture

o Weight distribution remains close to central axis of the body when walking
 In humans, weight tends to fall through the outside of the femur
 In apes, it falls through the inside

o Greater stability in upright posture

o When walking
 Body enabled to be rotated about the lower leg and foot, and each footstep follows a more-or-straight line
 Striding gait in humans
 Swaying side to side in gorillas or chimpanzees

Question 17

Explain the Knee

Answer 17

o Two-hinge joint with one hinge on either side of the ligaments in the middle

o Weight transmitted down the femur and outside the knee
- Outer hinge larger and stronger than inner one

o Centre of gravity falls through line in front of the knees

• Force that attempts to bend the knee backwards
• Resisted by ligaments in joint
• No energy to support the body when standing

Question 18

Explain the Foot

Answer 18

o Most of the body weight transmitted through tibia to ankle
 Tibia is larger and stronger

o From ankle, wight in transmitted to the talus
 Then tarsal bones, metatarsals and phalanges via foot arches

o Lost prehensility
 Big toes are large and aligned alongside other toes

o Metatarsals shaped so that two arches are formed
 Longitudinal arch; Running from front to back
 Transverse Arch: Running from side to side which is unique to humans and perfected bipedal locomotion

Question 19

Explain the Centre of Gravity

Answer 19

o Humans; half of total height is in leg length
 Relatively long legs increase length of stride when walking
 Lowers centre of gravity where all the weight of the body is concentrated
 Contributes

o Apes; one-third of the total height is taken up in leg length
 Higher centre of gravity, at chest level
 Contributes to stability when moving bipedally or standing erect

Question 20

Explain Muscle Tone

Answer 20

o Partial contraction of muscles

o Sustained muscle tone evident in muscles that support the body when upright
 These muscles bring movement of spine, hip, knee and ankle and abdominal muscles

o Nervous system and sense organs maintain tone in muscles and equilibrium of the body

Question 21

Explain the Striding Gait

Answer 21

o Hominins perfected striding gait is walking upright where the hip and knee are fully straightened
 Apes have bent knees and bodies bent forward at hips

o When foot hits the ground weight is transmitted from the heel along the outside of the foot as far as the ball
 Ball crosses ball of the foot and is borne by the big toe
 Whole weight is then propelled by the big toe hence why it lost it opposability.
 Big toe is now a weight-bearing appendage

o Trunk rotates about pelvis
 Right arm naturally swings forward as the left leg is extended and vice versa
 Swinging of arms tend to keep shoulders at right angles to the direction of travel, reduced amount of energy expended
 If this didn’t occur energy would be wasted reversing the natural rotation

o Carrying angle ensure stability during walking
 Body can be rotated about lower leg and foot
 Each footstep a more-or-less straight line

o Chimpanzees lack a wide pelvis and carrying angle
 Sways side and side so body wight is over each leg in turn

Question 22

What are the Advantages to Bipedalsism?

Answer 22

• Energy efficient means of moving - greater stride, covering long distance
• Hands free to use tools, and carry items - offspring, food, tools
• Achieved greater height and ability to see further over the horizon/tall grass
• Less exposed to sunlight and increased exposure to breezes, increasing cooling mechanisms
• To free limbs to reach and grab onto trees

Question 23

What are the main hominin adaptations for erect posture?

Answer 23

1. Foramen Magnum Located centrally in the base of the cranium
2. Jaw Bone - Small and non-protruding, enabling the skull to balance on the vertebral column
3. Vertebral Column Lumbar vertebrae wedge-shaped, producing an ‘S’-shaped curve that brings the vertebral column under the centre of the skull
4. Pelvis- Broad, shallow from top to bottom.
   - Provides support for abdominal organs.
   - Attachment of femurs is wide apart, contributing to the carrying angle.
5. Femurs
   - Large head of the femur contributes to carrying angle
6. Knee Joint - Outer ‘hinge’ larger and stronger, to take the weight of the body.
7. Knee is able to be straightened.
8. Legs
   - Longer than arms, contributing to a low centre of gravity.
   - Carrying angle allows the weight of the body to be kept to the central axis.

9f. Foot
- Large heel bone and aligned big toe form a pedestal on which the body is supported.
- Foot has both longitudinal and transverse arches.

10. Muscle Tone
    - Partial contraction of muscles to support the spine, hip, knee and ankle.

Question 24

Explain Number and Shape of Teeth

Answer 24

Number and Shape of Teeth
• Dental Formula: Gives the number and type of each tooth in one quarter of a jaw

o Old World Monkeys, apes and humans
 32 teeth total; 2:1:2:3
 2 incisors, 1 canine, 2 premolars and 3 molars

Question 25

What caused the decrease in number of Teeth?

Answer 25

• Decrease in number of teeth over time
o Likely due to reduction in size of the face and jaw
 Allows skull to balance during bipedalism
 Presumed to have evolved from predominantly fruit diet of the apes

Question 26

Humans vs Apes Teeth

Answer 26

• Old World Monkeys and Apes
o Canines are large and sharply pointed
 Projects beyond the level of other teeth
o Large lower canine
 Diastema/Large gap between upper second incisor and upper canine
o Large upper canine
 Fits tightly against the crown of lower premolar is slanted back and has a sharp edge
 Sharpened via grinding
o Three cusps
o U shaped dental arcade
• Humans
o Four cusps on upper molars and five cusps on the lower ones
o Canines don’t project beyond the level of other teeth and interlock
 In size, like incisors
o Small canines and incisors take up less room
o Parabola shaped dental arcade

Question 27

Explain Prognathism and Brow Ridges

Answer 27

• Prognathism; forward-jutting jaw
• Distinct, bony brow ridge above the eye sockets
• Flattening of the face shifts weight centrally for bipedalism
• Development of a chin and prominent nose
• Larger frontal lobe increased, extending cranium forward
• Brain occupies a larger volume
• Reduced brow ridges

Question 28

What were the Hominin Trends in Evolution?

Answer 28

1. Reduced Prognathism and Brow Ridge
2. Shape and Number of Teeth
3. Locomotion
4. Complexity and Size of Brain
5. Mobility of Digits