Chapter 20 - Evolutionary trends in hominins

Question 1

Define Hominid.

Answer 1

a primate of a family ( Hominidae ) which includes humans and their fossil ancestors.

Question 2

Define Tribe.

Answer 2

The name given to a relatively new level of classification between subfamily and genus.

Question 3

How do Hominins differ from apes in their appearance, structure and behaviour?

Answer 3

Most noticeably, hominins are relatively hairless compared with apes, and the structure of their upper and lower limbs allows for a fully bipedal way of walking.

Question 4

What does walking with Bipedal Locomotion mean?

Answer 4

Walking on two legs with the body upright.

• Humans stand and walk with an erect posture and a striding gait that is unique.
• It is not found anywhere else in the animal kingdom.

Question 5

What is the difference between Genus and Species?

Answer 5

1. A genus is the lowest taxonomic rank, and the most basic unit or category of biological classification.
2. A Species is an individual belonging to a group of organisms (or the entire group itself) having common characteristics and (usually) are capable of mating with one another.

Question 6

Define Australopithecine.

Answer 6

A general term used to refer to any species in the genus Australopithecus.

Question 7

Define Australopithecus.

Answer 7

A genus of fossil ape-like primates found in rock layers dating to 3.6 million years before present.

Question 8

Define Homo.

Answer 8

The genus of hominin to which humans belong.

Question 9

Define Homo erectus.

Answer 9

A species of the genus Homo; lived between 300 000 and 1.8 million years before present.

Question 10

Define Homo neanderthalensis.

Answer 10

A regional population that existed in Europe during the last of the ice ages; recent evidence suggests that the Neadnderthals may date back as far as 230 000 years ago.

Question 11

Define Adaption.

Answer 11

An adaption is any characteristic that helps an organism survive and reproduce in its natural environment.

Question 12

What is the Foramen Magnum?

Answer 12

• It is a hole in the skull where the brain joins on to the spinal cord.
• In humans this hole is located centrally underneath the skull, but in quadrupeds the hole is towards the back of the skull.

Question 13

Explain how the Foramen Magnum have been an adaption for an erect posture?

Answer 13

During the evolution of modern humans from an ape-like ancestor, the foramen magnum has gradually moved forward until the skull is able to balance on top of the vertebral column.

• An ape-like gorilla needs large neck muscles to hold the head in position.
• In humans, the weight of the skull is borne by the vertebral column so that large neck muscles are not required.

Question 14

Why do gorillas need stronger neck muscles than humans?

Answer 14

This is because in humans, the skull is positioned to balance on the vertebral column; in gorillas, most of the weight of the skull is in front of the column.

Question 15

How are the Lumbar region (the small of the back) different in Gorillas and Humans?

Answer 15

• The vertebrae in the lower, or lumbar, region are wedge-shaped from front to back, thus forming a forward-jutting curve.
• This lumbar curve of the vertebral column improves body balance in the upright position.
• It enables the head to balance on top of the neck.
• In addition, the cervical curve in the neck brings the vertebral column directly under the centre of gravity of the skull.

Question 16

How have the jaws in humans adapt for an erect posture?

Answer 16

• Apes have protruding jaw, whereas in humans the facial profile is much flatter.
• During evolution from an ape-like ancestor, the size and protrusion of the human jaw has gradually been reduced.
• This change has been important in allowing the skull to balance on the top of the spine because the weight in front of the foramen magnum is approximately equal to the weight behind.
• Balance is thus achieved with a minimum of muscular effort.

Question 17

How have the pelvis in humans adapt for an erect posture?

Answer 17

• At its lower end, the vertebral column articulates with the pelvis.
• The pelvis in humans is broader, and shorter from top to bottom, than in apes, and bowl-shaped.
• The broad hip bones provide space for attachment of the large buttock muscles, which move the legs and keep the upper body erect.

Question 18

How is the Pelvis different in females.

Answer 18

• The bowl shape pelvis supports the abdominal organs when standing erect, and in the female it supports the developing foetus during pregnancy.
• The female pelvis tends to be slightly broader than that of the male to allow for the passage of the infant at birth.

Question 19

What is the carrying Angle?

Answer 19

1. The head of the femur, or thigh bone, is large and fits into the acetabulum (hip socket) of the pelvis.
2. Because the pelvis is broad, the hip sockets are wide apart, but the femurs tend to coverage towards the knees.
3. This arrangement of the femurs forms an angle to the vertical, termed the carrying angle, which ensures that weight distributions remains close to the central axis of the body when walking.

Question 20

What enables humans to have a striding gait instead of swaying from side to side as do gorillas?

Answer 20

The arrangement of the carrying angle which allows for greater stability when walking as it enables the body to be rotated about the lower leg and foot, and each footstep to follow a more or less straight line.

Question 21

How does the knee joint look like?

Answer 21

It is a two-part hinge joint, with one ‘hinge’ on either side of the ligaments in the middle of the joint.

Question 22

Why is the outer ‘hinge’ larger and stronger than the inner one ?

Answer 22

This is because the weight is transmitted to the outer ‘hinge’.

Question 23

What results in a force that tried to bend the knee backward but is resisted by the ligaments making up the knee joint?

Answer 23

The centre of gravity of the body which tends to fall through a line just in front of the knees despite the weight of the body being transmitted down each leg.

Question 24

Why is the human foot one of the most distinctive adaptions for bipedal locomotion?

Answer 24

1. In becoming a highly specialised locomotory organ it has lost all of its grasping ability, or prehensility.
2. This is most noticeable with the big toe, which in humans is quite large and aligned alongside the other toes.

Question 25

What are metatarsals?

Answer 25

They are the bones of the foot between the toes and the ankle.

Question 26

How are the metatarsals shaped?

Answer 26

They are shaped in such a way that they form two arches:

1. A longitudinal arch running from front to back
2. Transverse arch running from side to side.

Question 27

How is the transverse arch unique to humans?

Answer 27

• These two arches have enabled humans to perfect bipedal locomotion.
• Humans walk bipedally using the striding gait - walking sin such a way that the hip and knee are fully extended.

Question 28

What do the relatively long legs do in humans?

Answer 28

• It increases the length of the stride when walking.

- Surprisingly, they also serve to lower the centre of gravity of the body.

Question 29

List all the extinct Hominins.

Answer 29

1. Homo neanderthalensis (Neanderthal man)
2. Homo erectus
3. Australopithecus

Question 30

Why are people classified as Hominins?

Answer 30

Because we can walk upright on two legs.

Question 31

Summaries the main adaptions for erect posture.

Answer 31

1. Foramen Magnum - Located centrally in the base of the cranium.
2. Jaw Bone - Small and non-protruding so that it enables 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 directly under centre of skull.
4. Pelvis - Broad; shallow from top to bottom. Provides support for abdominal organs. Attachment of femurs wide apart contributing to carrying angle.
5. Femurs - Large head to femur that contributes to the carrying angle.
6. Knee Joint - Outer ‘hinge’ larger and stronger to take weight of body. Knee able to be straightened.
7. Legs - Longer than arms, contributing to a low centre of gravity. Carrying angle allows the weight of the body to be kept close to the central axis.
8. Foot - Large heel bone and aligned bit toe form a pedestal on which the body is supported. Foot has both longitudinal and transverse arches.

Question 32

Why is Muscle tone so important?

Answer 32

• It is the partial contraction of skeletal muscles.
• To keep the head erect, for example, and stop it from slumping forward onto the chest, the muscles in the back of the neck are partially contracted; that is, they have tone.
• If someone falls asleep while sitting up, the decrease in tone is evident as the head nods until the chin is close to, or resting on, the chest.

Question 33

How does the striding gait work?

Answer 33

1. When the foot hits the ground, weight is transmitted from the heel along the outside of the food as far as the ball, crosses the ball of the foot (via the transverse arch) and is finally borne by the big toe.
2. At the final moment of striding, the whole weight of the body is propelled by the big toe.
   - It should now be evident just why the hominins lost the opposability of the big toe.
   - The human foot is now a weight-bearing appendage rather than a grasping one.

Question 34

Why do our arms swing when we are walking?

Answer 34

• The swinging of the arms tends to keep the shoulders at right angles to the direction of travel, and reduces the amount of energy expended.
• If the arms did not move as they do, energy would be wasted on reversing the rotation of the body after each stride.

Question 35

Although the human pelvis is broad and the hip sockets are wide apart, the femurs converge towards the knees. Explain why?

Answer 35

1. This arrangement of the femurs ensures that weight distribution remains close to the central axis of the body during walking.
2. The arrangement also allows for stability during walking, as the body can be rotated about the lower leg and foot, thus allowing each footstep to follow a more or less straight line.

Question 36

Why do Chimpanzees sway from side to side?

Answer 36

Because they lack a wide pelvis and carrying angle.

Question 37

What are features of the footprints that indicate a bipedal form of locomotion?

Answer 37

1. Deep impression showing the heel hitting the ground first.
2. The lateral transmission of weight from heel to the ball of the foot.
3. A well-developed longitudinal arch.
4. A big toe that was parallel to the other digits.
5. Deep impression where the toe pushed the foot forwards for the next stride.

Question 38

What is most of the difference in brain size associated with?

Answer 38

1. The cerebrum; the outer portion of the cerebrum is the cerebral cortex, and it is this portion of the human brain that shows the greatest degree of development.
2. Convolutions, which give a surface area 50% greater than a brain that has no convolutions.
3. The frontal lobe, has the greatest enlargement in surface area. (higher functions of thinking, reasoning, planning and processing take place.)

Question 39

Why do the human brow tend to be vertical and lack the prominent brow ridges possessed by the apes?

Answer 39

A large brain requires a large brain case, or cranium, and in humans more of the skull is used in housing the brain than in the apes.

Question 40

What features have given humans a flat face?

Answer 40

• Brow ridges

- Shortening of the snout

Question 41

Why do humans have a more prominent nose than any other primate?

Answer 41

For this reason humans have a far more prominent nose than any other primate.

Question 42

Define cranial capacity.

Answer 42

The volume of that part of the skull occupied by the brain.

Question 43

Define endocasts.

Answer 43

An impression of the inside of the brain case, made of rock or some other solid material.

-Endocasts may occur naturally, or they can be made using the skulls found.

Question 44

How are human dentition very distinctive compared to other primates?

Answer 44

1. In humans the canine teeth do not project beyond the level of the other teeth and interlock, as they do in the Old World monkeys and apes.
2. Human canines look more like incisors.
3. These small canine teeth and relatively small incisors take up less room in the jaw.
   - As a consequence, the shape of the tooth row, or dental arcade, has evolved into a different shape.
   - Instead of the U pattern of the apes, it has become parabolic in shape.