3. Selection for Bipedalism
& Nakedness
Success Criteria
Your learning has been successful if you can do the following:
I can describe the change from forest to savannah.
I can explain how this change in environment meant that arboreal lifestyle was no longer efficient, and selected for bipedalism.
I can discuss how this change in environment has influenced the evolution of skeletal features allowing for successful bipedalism.
I can discuss the adaptive advantage of bipedal movement.
I can describe some disadvantages linked to the evolution of bipedalism.
Vocabulary
Learn these so you can communicate this concept well.
Larynx (voice box): Helps to create different sounds when we speak.
Do Now:
Do Now in your books:
Which species are Hominids and which are Hominins?
Australopithecus africanus, Gorilla, Homo ergaster, Chimpanzee, Homo sapiens
Selection for Bipedalism
One of the possible theory for the transition to bipedalism has centered on drastic environmental changes that swept Africa more than five million years ago. Africa (and the Earth) had become significantly cooler and drier.
As it did, grasslands in sub-Sahara Africa expanded and rain forests contracted. This means that the landscape turned from near-continuous forest into an open grassland with smaller clumps of trees with no continuous forest.
Due to these environmental changes, different selection pressures acted on the common ancesters of hominids and chimpanzees.
According to the hypothesis, at least one type of these primates responded to the environmental crisis by venturing more and more into the open grasslands, looking for food, but retreating to nearby trees to escape predators and sleep at night.
Advantages of Bipedalism
Around 5-6 million years ago some primates began standing up and walking on two legs. Their success presumably improved their chances of surviving and passing on genes favoring this unusual stance and gait, leading eventually to bipedal hominids.
More efficient locomotion - bipedalism provided an energy efficient method that favours low speed and long distance
Seeing over the tall grass may have helped to spot predators or locate carcasses and distant food (but easier to be spotted be predators) carrying food away from kill site to a safe place or back to other members
Thermoregulation – keeping cool smaller surface area exposed to the sun (60% less) greater air flow across the body when it is higher off the ground air cooler above the ground (further from the hot ground)
Freeing the hands - Bipedalism frees up the hands for other functions such as using tools, carrying offspring, and carrying food.
Disadvantages of Bipedalism
Narrowed birth canal – painful birth. Birth canal size reduced because of bipedalism and the fact that babies have bigger heads can cause childbirth problems.
Large pelvic inlet negatively affects locomotion - Humans have bigger brains (than other primates) and thus there is a selection pressure for increased size of pelvic inlet to allow for bigger brains and higher intelligence......BUT a larger pelvic inlet affects locomotion negatively so there is also selection pressure for decreased size of pelvic inlet as it allows for more efficient locomotion.So there is a trade-off between having as large a possible pelvic inlet to allow birth of larger brained infants and keeping the pelvic inlet small enough for efficient locomotion.Due to changes in the pelvis, childbirth became much more difficult and painful.
Slipped discs in spines – S-shaped spine makes humans more prone to back pain and back problems– load carried by 2 instead of 4 limbs.
Hard work returning blood to the heart – varicose veins can result
Hernias – intestine may bulge through weakened abdominal wall as gut is no longer hung from spine by broad ligament as in quadrupeds
Flat feet – feet sometimes suffer strain because body rests on just 2 limbs. Arches of feet collapse producing flat footedness.
A human child takes about a year to walk. Quadrupedal mammals often walk within a few hours of birth.
M3.6 C4. Biological Evolution Flashcards - Hands-On.xlsxSelection for Nakedness
Human skin differs from the apes in two ways
Humans have just as many hairs per square cm of skin as a chimpanzee. Ours is simply finer and shorter (except for the scalp).
Humans do have many more sweat glands. We have 3 million sweat glands- more than any other mammal and can produce 12 liters of sweat per hour when working flat out.
This is a great cooling capacity enabling us to maintain high activity levels in the heat of the midday sun.
These two factors are related in that it is the evaporation of sweat that causes cooling and the free circulation of air enables this over the skin, which has very fine hairs.
Common hypothesis – shorter and finer hair (not hair loss) in early hominin allowed greater heat loss by increased radiation from skin surface and well developed sweat glands allows greater heat loss
Also parasite control easier.
Concept 4: Skeletal Changes Linked to Bipedalism
Concept 4: Support Notes
You may be required to compare trends in biological evolution of early bipedal hominins with living hominids. These trends involve:
Skeletal changes linked to bipedalism
Changes in skull and endocranial features
Changes in the manipulative ability of the hand.
Though closely related to apes such as gorillas and chimpanzees., there are a number of features that set hominins apart from apes. The main differences between apes and humans stem from human being bipedal.
In hominins, the body is concentrated into a narrow column above the feet. This means in humans, body weight is taken mostly by bone rather than by muscular energy as in apes.
Comparing the Skull / Cranium
Foramen Magnum
In apes, the foramen magnum (hole through which spinal column passes) lies at the REAR of the skull whereas humans have it under the skull nearer the centre.
This means that apes need strong neck muscles, which are attached to nuchal crest on the skull, to hold up the heavy skull from an angle and prevent the head dropping forward.
In humans because foramen magnum lies more or less in centre of skull, the skull is balanced on vertebra, so little muscular energy is used to support the skull directly over the Centre of Gravity (COG).
Nuchal Area
This is the area where the neck muscles attach onto the back of the skull, to keep it balanced on the spinal column / looking forward. Apes have very large nuchal areas and associated neck muscles because greater muscle strength is required keep the skull looking forward when the spine is attached further to the rear of the skull.
Comparing the Backbone / Spine & Chest
Backbone / Spine
In apes, the back bone is a single gentle curve (C shape). All the vertebrae are of a similar size as the spine carries the load equally.
In humans, the backbone is S shaped, this enables the weight of the body to be carried directly above the hip joints directly over the COG. This forward curvature coupled with the backward curvature in the middle of the spinal column allows the backbone to function as a spring, facilitating movement and taking the shock of jumping down or striding out. The vertebrae get larger towards the bottom as the load is heavier at the bottom in a vertical position.
Chest
In apes the chest cavity protrudes forward and is deeper – this accommodates arm use for locomotion particularly brachiation.
In humans, the chest is flattened from front to back so that the body weight in human is concentrated as close to the spine as possible. The barrel-shaped rib cage of bipeds permits effective use of the arms for non-locomotory functions.
The scapula (shoulder blade) in humans is shorter and broader with little ridges. This further ensures the centre of gravity is over the pelvis. Less ridges in humans on scapula as less need for attachment of muscles. The scapula shape in apes supports knuckle walking and brachiation.
Comparing the Pelvis / Hips
In apes, the pelvis is long and narrow. The femur attachment point is vertical.
To accommodate the hip joints and muscles necessary for bipedalism, the pelvis of bipedal humans is lower and broader (bowl shape) than that of knuckle-walking apes.
Change in shape of pelvis in hominins means that spine and internal organs are directly above leg bones which puts the weight over the COG. Change in the shape of the pelvis means that muscle attachment point for main leg muscle - gluteus maximus is at the back in humans rather than on the side of the pelvis. This results in more efficient locomotion in that the stride is straight not side to side as in the ape.
The bowl shaped pelvis in humans results in the femur pointing inwards forming the valgus angle (humans are knock-kneed) and a change in the carrying angle for the femur. This results in more efficient locomotion as increases stability by making it easier to stand on one leg when walking, as movement is no longer “side to side” as in apes.
Altered Limbs
Lower Limbs
Bipedal humans not only have longer lower limbs than quadrupeds, the valgus angle (the angle that the femur makes with the midline of the body) is also different.
Longer lower limbs shift the center of mass towards the lower body. Angling the femurs inward moves the center of mass closer to the midline of the body. The altered center of mass allows stable bipedal locomotion and conservation of energy when standing.
Restructured Foot
In the apes, the big toe (hallux) diverges which facilitates tree climbing ability. Apes toes are longer and curved to facilitate better grip on the branch.
In humans, the big toe is elongated and faces forward in humans. This gives forward thrust necessary when walking on 2 legs and increases efficiency of bipedal walking allowing humans to walk considerable distances easily.
The human foot has two arches. The transverse arch of the foot also acts like a spring to take the shock of impact so walking long distances or endurance is possible, this helps to protect joints from damage when landing and striding out. Flat footed people find it painful to walk long distances.
The longitudinal arch stores forward momentum / energy and is used when pushing off to increase efficiency.
The calcaneus or heel bone is enlarged to take the impact of a “heel first” stride used in bipedalism.
Enlarged Joint Surfaces
Not only does the knee need to be restructured to accommodate the changed valgus angle, but joint surfaces must also be enlarged. This enlargement increases the contact area, helping the knee and other joints withstand the stress of standing or walking upright.
Shoulder
Homo erectus has a different shoulder to us. Instead of placing the shoulder joint on the side of the rib cage, parallel to the neck, Homo erectus had a more forward-facing scapula with a forward-placed arm. This meant the transition from a high-shrug-like chimp shoulder to our modern shoulder was not a simple one-step process. See diagram below:
The forward-facing arm of Homo erectus was limited compared to our us. Homo erectus could not pull its arm back. Most detailed tool manufacturing and foraging does not require pulling the arm back very far, but there are a few tasks that can not be done without pulling your arm behind your body. Homo erectus could not throw well, and could not run well. Throwing and running both involve swinging your arm way behind your body.
Concept 5: Changes in Skull and Endocranial Features
Success Criteria & Vocabulary
Click this drop-down menu to see the Success Criteria.
I can describe the trends in cranial capacity observed in selected fossils
I can explain how dietary, behavioural and climatic changes may have led to observable trends in cranial capacity
I can describe the trends in dentition observed in selected fossils
I can explain how dietary, behavioural and climatic changes may have led to observable trends in dentition.
Click this drop-down menu to see the list of Vocabulary.
Concept 5: Support Notes
There are other changes in the skulls of humans as compared to apes.
Cranial Capacity
Apes have small cranial capacity (around 450cc).
In the hominins, there is general increase in brain size and therefore cranial capacity in hominin lineage. Modern Homo sapiens have bigger cranium (1450cc) to accommodate increased brain development.
The human brain has a much larger cranial capacity compared to other primates (Human 1330 cm3 vs Chimpanzee 500 cm3). This is because:
Humans have far more white matter in the brain, as there are more connections between the different parts of the brain.
However, there are other mammals with larger brains than our own. It is not simply a matter of how large the brain is (H. floresiensis), but how it is organized. An important factor is how large the brain is compared to body size.
Humans have a more folded brain. Increasing the surface area and therefore processing power.
Humans have a larger:
Frontal lobe (problem solving, judgement, impulse control, social behaviour)
Cerebellum (regulates motor movements - coordinates voluntary movements such as posture, balance, coordination, and speech, resulting in smooth and balanced muscular activity)
Wernicke's (speech comprehension)
Broca's areas (speech production)
Together, the Broca's and Wernicke's areas are areas of the brain specialized for production and comprehension, of language.
Changes in the Skull and Dentition due to Diet
The modern human brain makes up 2% of our body weight but demands about 20% of our metabolic energy at rest - this makes the brain a very expensive (in terms of energy) organ to maintain. The selection pressures for increased brain size must have been considerable for additional energy to be made available - this could be provided during the course of human evolution by the acquisition of high energy food such as meat.
Brains are expensive organs to run - what allowed them to get bigger?
One theory is that: Bipedalism 🡪 free hands 🡪 increasing use of tools 🡪 increasing range of food sources (especially meat - high in calories) 🡪 increase energy intake and decrease energy output 🡪 more energy for brain to increase in size.
Selection pressure for brain expansion may have included:
Selection pressure to learn complicated skills such as tool making and language
Selection pressure to develop new brain structures to provide new functions such as language ability
Selection pressure to have an ability to imagine and connect ideas in new ways that enable new thought processes with survival advantage (e.g. understanding / predicting the rhythm of seasons / producing artifacts - tools from mental templates / understanding fracturing behaviour of stones)
Temporal muscles
These are the muscles that pull up the jaw (bite). The temporal area is where these muscles attach onto the skull. In apes these are both much larger. This probably corresponded to a much more primitive diet that included a lot more fibrous plant material that needed to be ground down.
Zygomatic Arch
These are a bony arch just behind the cheeks. They provide a gap for the temporal muscles to pass through (see above). They are much larger in apes to accommodate much larger temporal muscles.
Brow Ridge
This is a bony ridge located above the eye sockets. Its purpose is to reinforce the weaker bones of the face (reduces vertical stress). In Apes they are much larger due to the tremendous strain put on the cranium by their temporal (jaw) muscles. Without this reinforcement the eye sockets would collapse. The brow ridge was one of the last traits to be lost in the path to modern humans
Sagittal Crest
The Sagittal Crest is a ridge of bone running lengthwise along the midline of the top of the skull. The presence of this ridge of bone indicates that there are exceptionally strong jaw muscles. The sagittal crest serves primarily for attachment of the temporalis muscle, which is one of the main chewing muscles. It is absent or greatly reduced in most Hominins (with the notable exception of the Paranthropus genus).
In summary:
Apes have need large jaw muscles for herbivorous, tough fibrous diet. Thus, they have:
a well-developed sagittal crest,
large zygomatic arches,
large jaws,
bigger teeth arranged in U shape
well pronounced brow ridges (redirects pressure from the jaw)
small cranial capacity as a vegetarian diet cannot support larger brain.
In Homo sapiens there is:
no sagittal crest,
small zygomatic arches,
small jaw, teeth are smaller and arranged in a parabolic arch
reduced brow ridges.
This is due to a more refined diet, no need for really strong chewing muscles, muscle attachment points, or teeth. As jaw has become smaller (with less herbivorous diet) and forehead larger (to accommodate larger brain) angle of face has become more vertical.
Changes in teeth, from apes to modern humans are:
Reduction in teeth size
Canines become less sexually dimorphic (meaning less difference in canines between males and females).
Concept 6: Changes in the Manipulative Ability of the Hand
Success Criteria & Vocabulary
Click this drop-down menu to see the Success Criteria.
I can discuss the changes/trends to the hominin hand.
I can explain the difference between power and precision grip.
I can discuss how bipedalism led to changes in the manipulative ability of the human hand.
Click this drop-down menu to see the list of Vocabulary.
Concept 6: Support Notes
Success Criteria & Vocabulary
Click this drop-down menu to see the Success Criteria.
I can discuss the changes/trends to the hominin hand.
I can explain the difference between power and precision grip.
I can discuss how bipedalism led to changes in the manipulative ability of the human hand.
Click this drop-down menu to see the list of Vocabulary.
Concept 6: Support Notes
The hand is the essential prerequisite for the manufacture of tools. The human hand differs from the apes in four ways:
The thumb is much more elongated and muscular.
The first metacarpal, which is the hand bone at the base of the thumb is connected to the wrist. This enables the thumb to touch the tip of every finger with ease.
The muscle that flexes the last joint of the thumb is separate to any other tendons allowing us to flex the last joint of the thumb independently.
The wrist is much more flexible, which improves our fine motor skills.
These adaptations allow a power grip, stone throwing, and precision grip, and writing.
Ape fingers are much longer and more curved which helps them with brachiating BUT reduce their fine motor skills.
Human have much better manipulative ability in the hand than apes. Both apes and humans are capable of grasping objects (power grip) but humans can use the precision grip as well.
Power grip: Fingers and thumbs wrap around the object.
Precision grip: Fingers and thumb hold the object.
In apes – the fingers bones (phalanges) are more curved to allow for brachiation. The thumb is less mobile and is shorter. Apes do not have precision grip and are not able to accurately manipulate small objects. In humans the phalanges are straight
In humans - the thumb is considerably longer. Humans are capable of precision grip because of the fully opposable thumb. The thumb is able to touch the finger tips due to the saddle joint and this allows for the precision grip.
The base of the thumb is joined to the wrist by a “saddle joint” – this enables the thumb to be brought acrosss the hand so that it can touch the tip of all four other fingers. The full opposability of the thumb with other fingers is much more developed in humans than in other primates
Humans have finer motor control and sensitive fingertips which in combination with precision grip enables them to manipulate small objects.
