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Human Evolution 3
Year 13 Science
Ruawai College 2013
Bipedalism
Humans are habitually bipedal. It is likely that it was a development of bipedalism somewhere about 5 -4
mya that was the driving force in human biological evolution.
Selective advantages of bipedalism:
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freed hands – to carry food, tools, weapons, babies, to pick fruit from trees, to wield offensive and
defensive weapons
greater height - to look above the grass to see predators, find food, locate sites for shelter and to
appear intimidating
energy efficiency – for walking long distances – experiments show walking on two legs uses less
energy than knuckle-walking
keeping cool in the hot savannah – increased air flow around body, less body surface in direct
sunlight
Disadvantages of bipedalism:
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climbing not as easy
less stable on two feet than on four
smaller pelvis – females giving birth
more visible to predators
Exercise:
1.
Write a paragraph to describe the possible advantages of bipedalism for early hominins.
Free hands to carry food, tools, weapons, babies
Free hands to pick fruit from trees
Free hands to wield offensive and defensive weapons
Greater height to see predators, food, sites for shelter
Greater height to appear intimidating
Energy efficiency, for walking longer distances at slower pace
Keeping cool due to increased air flow around body
Decreased surface area in direct sunlight
Comparison of bipedal and quadrupedal skeletons.
Comparing the skeleton of a modern human (bipedal) with a living ape like a gorilla or chimpanzee
(quadreupedal) reveals the changes associated with bipedalism.
Skull
The human skull balances on axis vertebra at the
top of the spine – few muscles are needed to hold it
in place, little energy is needed to support the skull.
Humans have a foramen magnum underneath the
skull, towards the centre.
In apes, the foramen magnum (hole for spinal cord)
is near the back of the skull.
2.
What is the foramen magnum?
The hole for the spinal cord
3.
Compare the placement of the foramen magnum in humans and apes.
The foramen magnum is near the back of the skull in apes, and towards the centre of the skull in humans
Diet-related skull comparisons
Apes have:
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a sagittal crest to support large jaw muscles (top of skull)
nuchal crest to support muscles that hold the head up (side of skull)
large brow ridges to support the skull during mechanical stress of chewing (front of skull)
thicker zygomatic arch (cheek bone)
stronger mandible (lower jaw)
zygomatic arch and mandible support a strong jaw muscles for chewing tough plant material
Humans :
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do not have these bony areas for muscle attachment
have a large rounded cranium
have an omnivorous diet
have a skull that sits on the top of the spine
thin zygomatic arch
small mandible
4.
What is the function of the sagittal crest?
Support large jaw mucles
5.
Do humans have a sagittal crest?
no
6.
What is the function of the nuchal crest?
Support muscles that hold the head up
7.
Do humans have a nuchal crest?
no
8.
What is the function of large brow ridges?
Support the skull during stress of chewing
9.
Do humans have large brow ridges?
no
10.
What two features support jaw muscles for chewing?
zygomatic arch and mandible support a strong jaw muscles for chewing
Cranial Capacity:
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greater in humans (adult average 1 4000 cm3)
smaller in apes (around 450 cm3)
specialised areas of brain reflected in human cranial shape
Humans / Homo:
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spine is S shaped to act as a shock absorber
keeps body weight above the hip for upright movement
rib cage is flattened front to back
body weight is close to the spine and over the centre of gravity
Apes / Pan:
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11.
spine is C shaped
slight curve counterbalances the downward force of organs and chest
rib cage is flattened side to side
funnel shape of rib cage accommodates a large herbivore gut
What shape is the spine in humans?
S shaped
12.
What shape is the spine in apes?
C shaped
13.
In which direction is the rib cage flattened in humans?
Flattened from front to back
14.
In which direction is the rib cage flattened in apes?
Flattened from side to side
Humans:
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pelvis is short and wide – bowl shaped
reduces stress to upper body weight on hips
supports internal organs
strong bones support muscles that move the leg during walking
wide pelvis allows babies with large skulls to be born
femur angled inwards from the hip (valgus angle – knock kneed)
body weight is brought in under the pelvis when walking
lower end of femur has buttresses of bone on the knee joint to prevent sideways tilting movement
of lower leg – needed for upright walking
can fully extend legs
Apes:
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pelvis is long and narrow
pelvis provides a large area for leg muscle attachment
leg bones are shorter and there are no buttresses
femur hangs vertically from hip to allow good swinging motion through trees
cannot fully extend legs
15.
What shape is the human pelvis?
Bowl shaped
16.
What shape is the ape pelvis?
Long and narrow
17.
What length is the human femur?
long
18.
What length is the ape femur?
Short
19.
How is the human femur attached to the hip?
Inwards from hip
20.
How is the ape femur attached to the hip?
Hangs vertically
Human:
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toes are straight
foot has an arch which acts as a shock absorber during walking and running
at end of each step the foot pivots over the big toe
hands have a fully opposable thumb
fingers are straight to allow precision grip and dexterity
Apes:
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great toe spreads out from foot and is opposable ( able to grip)
other toes are curved
have flat feet
have short opposable thumb
curved phalanges (fingers) giving a power grip for brachiation
21.
What shape are human toes?
straight
22.
What shape are ape toes?
curved
23.
What shape are human fingers?
straight
24.
What shape are ape fingers?
curved
25.
Do humans have opposable toes?
no
26.
Do humans have opposable thumbs?
yes
27.
Do apes have opposable toes?
yes
28.
Do apes have opposable thumbs?
yes
The fossils of early hominins have skeletal features that link them to apes, indicating a common ancestry.
They also have skeletal features showing their links, through bipedalism, to modern humans.
29.
Complete the table to summarise the differences in skeletal structure between a modern human
and a modern ape.
Feature
Modern Human
Modern Ape
Skull
no crests or brow ridges – large rounded
cranium – central foramen magnum
nuchal and sagittal crests – brow ridges –
small cranium – foramen magnum at rear –
heavy mandible
Spine
S shaped
C shaped
Rib cage
Flattened from back to front
Flattened from side to side
Pelvis
Short and wide
Long and narrow
Femur
Long – angled inwards – buttresses of bone
at knee
Shorter – straight – no buttresses at knee
Feet
Straight toes – arched foot – big heel bone
Curved toes – opposable big toe – flat foot
Hands
Long opposable thumb – straight fingers
Short, partly opposable thumb, curved fingers
30.
Complete the table to describe the advantages of each feature to apes
Feature
Large brow ridges
Advantages to Apes
Attachment for muscles that support skull during stress of chewing
Short opposable thumb
and curved phalanges
Power grip for brachiating
C-shaped spine, slightly
curved
Slight curve counterbalances the downward force of organs and the chest
Tall, narrow pelvis
Provides a large surface area for leg muscle attachment
Femur hangs vertically
from hip
Allows good swinging motion through the trees
Big toe separate, facing
outwards from foot and
opposable
For gripping branches of trees
31.
Complete the table to describe the advantages of each feature to humans
Feature
Large rounded cranium
Advantages to Humans
To house and protect large brain
Fully opposable thumb and
straight phalanges
Allows precision grip for handling tools
S shaped spine
Acts as shock absorber and keeps body weight above hip joints for upright
movement
Short, wide pelvis
So body weight is close to spine and over the centre of gravity
Femur forms valgus angle
with hip
Body weight is brought in under the pelvis while walking
Arched foot
Acts as a shock absorber
32.
The change from quadrupedal walking to bipedal walking required major changes to the skeleton.
For each of the following features, describe the change and explain how it assisted bipedalism.
Skeletal
Feature
Skull
Change from quadrupedal to bipedal
How it assisted bipedalism
Foramen magnum became centralised in
the skull
skull balanced on the neck vertebrae, so
little muscular energy is needed to support
the skull
Rib cage
Flatter from front to back
Body weight concentrated as close to the
spine as possible
Less chance of toppling over / becoming
unbalanced
Spine
Spine S shaped
Weight of body carried directly above hip
joints / absorbs impact of walking
Less change of toppling over / becoming
unbalanced
Pelvis
Reduced height and pelvis bowl-shaped
Reduces stress on the hip joint – offers
increased surface area for muscle
attachment
33.
Write a paragraph to describe the possible advantages of bipedalism for early hominins.
Free hands to carry food, tools, weapons, babies
Free hands to pick fruit from trees
Free hands to wield offensive and defensive weapons
Greater height to see predators, food, sites for shelter
Greater height to appear intimidating
Energy efficiency, for walking longer distances at slower pace
Keeping cool due to increased air flow around body
Decreased surface area in direct sunlight
34.
The drawings compare the features of a female gorilla with those of Australopithecus africanus and
Homo sapiens. Use the details of the skull structure to explain the mode of locomotion of each of the
three species shown.
Evidence is provided from position of foramen magnum
and the development of nuchal crest. Gorillas are
habitually quadrupedal, this is shown by the rear
placement of the foramen magnum – head projects
forward and is not on top of the spine – and also the
nuchal crest – provides attachment for the large neck
muscles needed to support the forward placed head.
The more central placement of the foramen magnum in
bit, as the Australopithecus and reduction of the nuchal
crest suggest a habitually bipedal habit as the head is
more centrally placed on top of the spine – doesn’t need
a large neck muscles for support. Bipedalism is unlikely
to have been as effective as that of modern human. The
completely central placement of the foramen magnum in
Homo sapiens and no nuchal crest indicates the head is
now completely central on top of the spine, so
bipedalism is completely habitual – very effective form of
movement
35.
A number of features of the postcranial skeleton (all parts of the skeleton except the skull) are
linked to bipedalism in humans (and other hominins).
Chimpanzee
Human
Chimpanzee
Human
The foot of a chimpanzee is different from that of hominins.
a.
Describe one skeletal feature of the foot of the chimpanzee that differs from that of a hominin.
Big toe directed sideways in chimp (opposable) – in hominin toe is larger and points forward – is in line
with other toes – not opposable
Arch in hominin foot is not present in chimp foot
b.
Explain how this difference is lined to bipedalism.
Chimp’s big toe, which allows for prehensile gripping branches when climbing, would get in the way with
bipedal movement
Forward facing big toe in hominins provides a platform for pushing against the ground and allows thrust
for walking upright
Arch allows cushioning when walking long distances in the upright position and supports body weight
above it
Chimp’s weight supported by four limbs and uses forelimbs to walk on as well
c.
Describe two differences between the chimpanzee and human pelvis girdles that are related to
bipedalism.
Long and narrow in the chimp / wider and bowl shaped in hominin
Femur attachment point vertical in chimp / angled in hominin
d.
Explain the relationship between the shape of the pelvic girdle and the ability to walk bipedally.
Surface area larger / position on the bone for large muscle attachment changed to allow leg joint to
move backwards and forwards in bipedal walking, also more powerful movement
Carrying / valgus angle changed to prevent tilting / side to side movement when the opposite leg is off
the ground
The shorter bowl shaped hip bones support the upper body weight and hold internal organs in place for
upright walking
e.
The diagram shows the pelvis of a modern human,
with the pelvic inlet identified. Homo erectus had a
narrower pelvic inlet than Homo sapiens.
i.
Explain why the pelvic inlet is larger in Homo sapiens than in Homo erectus.
Pelvic inlet is related to size of infant’s head – size of brain had increased in Homo sapiens, so infants
were born with larger skulls
The shortening and widening of the pelvis lead to a corresponding opening of the pelvis inlet – coincides
with larger skulls of the Homo sapiens babies being born
ii.
The width of the pelvic inlet is related to locomotion. Homo erectus had a narrower pelvic inlet
than modern humans, and walked more efficiently. Explain how conflicting selection pressures have acted
on the evolution of the pelvic inlet in humans.
Bipedal walking would have been less efficient – the hip joints are further apart than is required for
optimum bipedal locomotion – but this decrease in adaptive advantage would have been more than
compensated for by increased survival of females and offspring during childbirth
Human
chimpanzee
f.
Homo and Australopithcus skeletons both have pronounced valgus angles, and broader and
shallower pelvic girdles, compared with the chimpanzee skeleton. In addition, the big toe of Homo is in line
with the rest of the toes. In Australopithcus afarensis there is some separation between the big toe and
the remaining toes, and in the chimpanzee the big toe is widely separated from the rest.
Explain the adaptive significance of each of these skeletal differences in relation to its impact on the
effectiveness of bipedalism.
H. sapiens and A. afarensis are both habitually bipedal in movement, though structural comparison
suggest that bipedalism is more effective in H. sapiens. Chiimpanzees are mainly knuckle walkers in
movement but are capable of bipedal gait – during bipedalism, they tend to roll from side to side /
swagger, which is inefficient.
Valgus angle – pronounced in both H. sapiens and A. afarensis, bringing the knees closer together /
knock kneed, and so under the centre of gravity so that the body is supported when one leg is raised off
the ground – knee joint is buttressed to prevent side slippage. Both these adaptations produce the
effective gait of these two species and not the ineffective swagger of chimps.
Pelvic girdle – short, rounder, wide, shallow, bowl shaped in both H. sapiens and A. afarensis – pelvis in
chimps is long and narrow. This gives effective weight transfer from spine to the legs, facilitating bipedal
gait – also helps to support internal organs in an upright position.
Big toe – this is forward facing in H. sapiens for effective bipedal gait, as the body pivots over the big toe
– giving a forward thrust – with each step. The big toe did not become completely forward facing in A.
afarensis suggesting bipedalism was perhaps not as effective as in modern humans – A. afarensis still
likely to have some degree of quadrupedalism. Chimps have an opposable big toe which reflects their
largely arboreal lifestyle, where the foot is used for moving along branches / holding on. Bipedalism in
chimps is limited, and not an effective way of moving for them .