Evolution of Bipedalism

Question 1

What is bipedalism considered to be?

Answer 1

Bipedalism is considered to be one of the defining characteristics of humans.

Question 2

Define arboreal.

Answer 2

Living in trees.

Question 3

How did original primates traverse the environment? Were they bipedal?

Answer 3

Original primates were arboreal and had many adaptations for an arboreal lifestyle.

Question 4

Why did original primates choose to inhabit trees and were arboreal?

Answer 4

Trees provide food and safety from many predators.

Question 5

What way do chimps and gorillas walk?

Answer 5

They have knuckle walking, but humans possess may traits associated with bipedality. Most of these are associated with the skeletal system.

Question 6

What determines how an animal will walk?

Answer 6

the skeletal system

Question 7

Why is the location of the foramen magnum different between humans and chimpanzees?

Answer 7

The location of the foramen magnum is different due to the way we walk and stand.

Question 8

Where is the foramen magnum located in chimpanzees?

Answer 8

A chimpanzee has the foramen magnum at the rear of the braincase since it knuckle walks

Question 9

Where is the foramen magnum located in humans?

Answer 9

A human stands up right and walks upright so the foramen magnum is underneath the braincase.

Question 10

How does the position of the foramen magnum in humans place the skull? How does this help?

Answer 10

This places the skull vertically (balances) on top of the vertebral column and positions the eyes for forward vision when standing upright.

The location allows for optimum function.

Neck muscles are now able to reduce in size as they have a less of a role in holding the head upright.

Question 11

How has the position of the foramen magnum changed from chimps to humans?

Answer 11

shifted from rear of the braincase to under the braincase.

Question 12

How has the position of the foramen magnum changed throughout human evolution?

Answer 12

Gradually came towards under the braincase with each species over time.

Question 13

Whats one of the most critical issues which must be solved for bipedal walking?

Answer 13

Maintaining balance is one of the most critical issues that must be solved in order to walk on two legs.

Question 14

How does a bipedal walker maintain balance in order to walk bipedally?

Answer 14

A bipedal walker must balance on one leg while lifting the other foot off the ground and swinging it forward during the walking cycle.

In humans, the thigh bone slopes inward from the hip to the knee (valgus angle) – this places the feet directly under the centre of gravity.

Question 15

How does a chimp walk and why?

Answer 15

The lack of a valgus angle places chimps’ feet wider apart – they must shift the body from side to side to place their weight over the feet.

Question 16

How do human muscles prevent humans from falling over when walking bipedally?

Answer 16

Well-developed muscles (gluteal abductors) on the side of the hips – in humans they prevent the body falling to one side when all the weight is on one foot in mid-stride.

Question 17

What is the strength of gluteal abductors in humans?

Answer 17

Well-developed.

Question 18

What is the strength of gluteal abductors in gorillas?

Answer 18

Chimps have weak gluteal muscles

Question 19

Describe the pelvis and limbs of humans compared to chimps.

Answer 19

Lengthened lower limbs and shorter, broader, more bowl shaped pelvis – these increase the stride length.

Enlarged joint surfaces - related to the different pattern of weight transmission through the pelvis.

Question 20

How many lumbar vertebrae do humans have?

Answer 20

Humans have 5 lumbar vertebrae.

Question 21

How many lumbar vertebrae do apes have?

Answer 21

Most apes have 4.

Question 22

Compare the lumbar vertebrae of humans and apes.

Answer 22

Apes have lumbar vertebrae which are relatively smaller than humans.

Question 23

The greater number and size of the vertebrae…

Answer 23

…allows for a more flexible back.

Question 24

Why do humans have a more flecible back then apes?

Answer 24

Humans have a greater number and size of the vertebrae which allows for a more flexible back

Question 25

What shape is the spine curvature in humans?

Answer 25

Lower/upper spine curvature in humans is S-shaped.

Question 26

Why is it an advantage for humans to have Lower/upper spine curvature that is S-shaped?

Answer 26

this places the centre of gravity and weight directly over legs/feet which provide the supporting platform, and allows the hips and trunk to swivel forward when walking. Improves stability and balance.

Question 27

Compare the knees of humans and chimps.

Answer 27

Abducted knees – humans can straighten the knee joint to extend the leg (chimps cannot do this). Allows the leg to swing through, enabling a heel to toe strike action of the foot. Also allows the knee joint to be locked, which allows standing straight for long periods. (chimps stand bent-legged, which is tiring).

Question 28

Describe the feet and toes of humans compared to chimps.

Answer 28

In humans, the hallux (big toe) is in line with other metatarsals and digits – this has resulted in the loss of opposability. Feet are more flattened (except the tarsometatarsal arch) - specialized as a weight-bearing platform with an arch that acts as a shock absorber. The ability to grasp with the foot is lost.

Question 29

Whats a cost of the humans s-shaped spine?

Answer 29

- Back pain, particularly in the lumbar region; slipped discs. - Hernia resulting from the fact that the gut is no longer hanging, as in the quadrupeds.

Question 30

What is an advantage of humans pelvis shape?

Answer 30

- Greater stability for upright stance - The weight of the body is transmitted directly to the legs.

Question 31

What is a cost of the humans pelvis?

Answer 31

A reduced birth canal leading to problems in child birth. This problem is compounded by the increase in brain size associated with human evolution.

Question 32

How has the femur adapted?

Answer 32

The femur is angled inwards, keeping the knee and foot directly under the centre of gravity. The outer condyle of the knee (where the femur meets the knee) is increased in size to cope with this angle change and stop sideways deflection of the muscles. The lower limbs have lengthened, enabling a faster, stronger stride.

Question 33

What's a disdadvantage / cost of human femur adaptations?

Answer 33

- Increased risk of facture of neck of the femur - Varicose veins resulting from the increase difficulties in return flow of blood from the lower limbs brought about by upright stance.

Question 34

Whats an advantage of the feet adaptations in humans?

Answer 34

- The aligned big toe provides the ability to walk with a forward thrust. - The arch offers shock absorption.

Question 35

Whats a disadvantage of human feet adaptations?

Answer 35

- The weight of the body is resting on the feet and can cause fallen arches, distorted bones and pain on walking.

Question 36

The evolution of modern humans from our hominin ancestor is commonly considered to have involved three major steps:

Answer 36

- Terrestriality - Bipedalism - A large brain (encephalization)

Question 37

What arguements are there about the steps of human evolution?

Answer 37

Most acknowledge these steps, but put them in a different sequence during human evolution In general, anthropologists seem to be in agreement that the transition to a terrestrial lifestyle took place very early in human evolution (7 – 4.4My ago).

Question 38

What occurred 7 Mya?

Answer 38

Around 7 Mya there were great climatic changes in the eastern Rift valley area of Africa.

Question 39

When did the transition from arboreal to terrestrial life occur?

Answer 39

7-4.4 Mya due to great climatic changes in the eastern Rift valley area of Africa.

Question 40

What happened to rain-forests as a result of the climatic changes and what problem did this cause for our ancestors?

Answer 40

Dense rain-forest was being replaced by savannah, so our ancestors were faced with the problem of how to move from tree to tree across dangerous open grassland.

Question 41

Who was at an advantage because of the spaced out trees and drier conditions?

Answer 41

Terrestrial species would be at an advantage when trees are widely spaced so partially terrestrial species may have been advantaged by the drier conditions..

Question 42

What was originally thought to be the reason why hominids became terrestrial?

Answer 42

It was originally thought early hominids may have been forced to become terrestrial as forests became savannah (now not so sure because of evidence from ‘Ardi’).

Question 43

What kind of walking do only humans have?

Answer 43

Despite all hominoid apes being able to walk on two limbs, only humans have habitual, erect bipedal locomotion that frees the forelimbs from an obligatory function of support and locomotion.

Question 44

What is the defining feature of the hominin line that led to Homo?

Answer 44

Current thinking then is that bipedalism is the defining feature of the hominin line that led to Homo.

Question 45

What are some problems with understanding the origins of bipedalism and its exclusivity to the hominin line that led to Homo?

Answer 45

- Bipedalism did not appear suddenly, and the earliest changes are not yet defined - It is possible that bipedalism is not entirely unique to the hominins (Oreopithecus – swamp ape - 8M years old – is not a hominin (but might have been bipedal) – the aquatic ape hypothesis - Bipedalism might have evolved more than once within the hominins

Question 46

Did Sahelanthropus tchadensis walk on two legs?

Answer 46

Some features (foramen magnum?) suggest an upright stance but no bones below the skull have yet been found to confirm this. The suggestion that Sahelanthropus was bipedal (or even a hominid) is contested.

Question 47

Did Orrorin tugenensis walk bipedally?

Answer 47

The angle of the femur head and length of the femur neck suggest bipedalism- but the humerus of the arm and phalanges (fingers) suggest arboreal activity.

Question 48

Was Australopithecus afarensis (Lucy) bipedal?

Answer 48

- Based on skeletal features Lucy was bipedal. - - Also, fossilised footprints found at Laetoli (Tanzania), dated 3.6 - 3.8 Mya and presumed to be A. afarensis have some similarities to modern humans

Question 49

What do fossil footprints of A. afarensis show?

Answer 49

Deep impressions showing pronounced heel strike A well-developed medial longitudinal arch Adducted big toe, in front of the ball of the foot and parallel to the other digits A deep impression for the big toe consistent with pushing off with the toes

Question 50

What do wrist bone fossils of A. afarensis (Lucy) lack?

Answer 50

A. afarensis fossils from Maka in Ethiopia (3.4 Mya) includes several wrist bones. These specifically lack any knuckle-walking adaptation (e.g. a more rigid wrist).

Question 51

WHat do skeletal features and fossil footprints of A. afarensis suggest about the species being bipedal?

Answer 51

Skeletal features, and the fossilized footprints at Laetoli, suggest that A. afarensis was an accomplished, if slightly clumsy, biped that may still have spent some time in trees.

Question 52

What does the recent find "Kadanuumuu" suggest about A. afarensis being bipedal?

Answer 52

Additional features from the recent find of ‘Kadanuumuu’ (found in 2005), a 3.58 Million year old male specimen of A afarensis, however, suggest that A. afarensis was an obligate biped.

Question 53

Describe the wrist bones of Adipithecus ramidus.

Answer 53

The wrist bones are more flexible than modern apes and allow palmigrade movement (not possible in apes)

Question 54

What features did Ardi lack?

Answer 54

Ardi lacked any features typical of the suspensory locomotion, vertical climbing, or knuckle-walking of modern apes.

Question 55

What did Ardis hands, arms, feet, pelvis, and legs indicate?

Answer 55

Its hands (finger curvature), arms, feet, pelvis, and legs indicate that ‘Ardi, moved capably in the trees, but was supported on its feet and palms (palmigrade clambering)

Question 56

Describe the feet of Ardi.

Answer 56

The four toes are modified for upright walking but, unlike later hominids, it still had an opposable big toe.

Question 57

Compare Ardis ability to walk upright to Lucys ability to walk upright.

Answer 57

Its ability to walk upright was therefore comparatively primitive compared to Australopithecus (Lucy).

Question 58

Based on Ardi's foot, did Ardipithecus ramidus walk bipedally or not?

Answer 58

the Ardipithecus foot was consequently a combination that worked for both upright walking and climbing in trees.

Question 59

Describe Ardi's pelvis?

Answer 59

Ardi’s pelvis is very different from a chimpanzee but is still much more primitive (narrow and long) than that of Australopithecus.

Question 60

Describe the gluteal muscles of Ardi.

Answer 60

The gluteal muscles are repositioned so that A. ramidus could walk without shifting its centre of mass from side to side. Indicated by the shape of the ilium (hip bone) and by the presence of a special growth site (the anterior inferior iliac spine) unique to hominins among all primates.

Question 61

Describe ilia of hominins.

Answer 61

hominins have short, broad ilia.

Question 62

What does Lucy's pelvis indicate?

Answer 62

The pelvis, of A. afarensis (‘Lucy’), shows that her species had already evolved nearly all of the adaptations for effective bipedalism.

Question 63

What were later changes to the hominid pelvis related to after A. afarensis?

Answer 63

Later changes to the hominid pelvis were related to changes in the birth canal, not to walking.

Question 64

In what species did bipedalism become more refined?

Answer 64

became more refined with the Australopithecines.

Question 65

When was bipedalism developed in the hominin lineage?

Answer 65

By ~ 4.4 Mya, bipedalism was developed in the hominin lineage

Question 66

What came first, bipedalism or increase in brain size?

Answer 66

bipedality has been around a long time and considerably predates the increase in brain size.

Question 67

What do some scientists believe about bipedalism and terrestiality?

Answer 67

There are several scientists that believe that bipedalism started in the trees and terrestriality subsequently followed. They believe we "hit the ground walking".

Question 68

Could Ardipithecus ramidus walk effectively and run?

Answer 68

The current suggestion is that A. ramidus was an effective upright walker and could also run, but probably with less speed and efficiency than subsequent hominins.

Question 69

What were the two critical transition between early human ancestors and human bipedalism?

Answer 69

1 - Ardipithecus shows adaptations that produced a pelvis that was useful for both climbing and upright walking 2 - From Ardipithecus to A. afarensis, further modifications produced a pelvis and lower limb that were more effective for upright walking and running but that were no longer useful for climbing.

Question 70

Could early hominins such as Lucy climb trees?

Answer 70

Recent evidence suggests that early hominins may well have still climbed trees - possibly for food, protection at night, and/or to escape from day-time predators. Maybe Lucy (and A. sediba) was neither predominantly arboreal nor fully bipedal.

Question 71

What do Homo erectus footprint fossil reflect?

Answer 71

These prints reflect the height, weight, and walking style of modern humans

Question 72

What do Homo erectus footprint fossil indicate/suggest?

Answer 72

The footprints suggests that Homo erectus probably had our springy, efficient stride and could potentially range over long distances and move into more diverse habitats

Question 73

Why become bipedal?

Answer 73

- Great climate shift - Free up hands - for using tools, for carrying babies? - Improved predator avoidance (could see farther over open savannah if stand erect (but if it evolved in woodland?)). - Food gathering (80% of bipedal behaviour in chimps is related to stationary feeding, only 4% to direct locomotion.) - Transporting a valuable/unpredictable resource (Coula nuts) - Energy efficiency - Thermoregulation

Question 74

Why is the arguement that "bipedalism occured to free up the hands for stone tool use" refuted?

Answer 74

Lucy (3.0 - 3.9 Mya) was an efficient biped but stone tools do not appear in the archaeological record until about 2.5 Mya, which seems to refute the notion that bipedalism evolved to free up the hands for tool use

Question 75

Describe the study done by Carvalho on chimps carrying valuable and unpredictable resources.

Answer 75

Recent experimental results (Carvalho et al. 2012) showed that when transporting a valuable and unpredictable resource (Coula nuts): - There was a four-fold increase in the frequency of bipedal transport; - They carried twice as many items when walking bipedally. They suggest that selection for effective food carrying, perhaps resulting from unpredictable availability (i.e. ‘carry as much as you can’), led to the evolution of bipedalism

Question 76

Describe the energy efficiency advantage of bipedalism?

Answer 76

change in the distribution of food meant hominids had to travel farther to find food. Bipedal locomotion is more efficient at low speeds than quadrupedal locomotion.

Question 77

Describe thermoregulation advantage of bipedalism.

Answer 77

bipedal posture reduces the area of body surface exposed to the hot sun so minimises thermal stress. Also hair loss

Question 78

What is not questioned about bipedalism?

Answer 78

What is not questioned, is that it was among the most important adaptations of the early hominids and remains one of the defining features of humans.

Question 79

Is how and why bipedalism evolved an answered question?

Answer 79

How and why bipedalism evolved is an open question that has not been fully resolved.