3. Selection for Bipedalism
& Nakedness

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Which species are Hominids and which are Hominins?

Australopithecus africanus, Gorilla, Homo ergaster, Chimpanzee, Homo sapiens

How analysis of fossils provided evidence of how Ardipithecus ramadus transitioned into bipedalism in a changing environment.
Comparing the locomotion and skeletal features of quadrapedal chimpanzees, intermediate Australopithecus afarensis, and habitual bipedal modern human. No sound. 

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.

Disadvantages of Bipedalism

M3.6 C4. Biological Evolution Flashcards - Hands-On.xlsx

Selection 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

Scaffolded question on bipedalism vs brachiators (Concept 1 and Concept 3).

Bipedalism Q1 2018

Scaffolded question on skeletal changes linked to bipedalism (Concept 3).

Skeletal Changes High Scaffold

Concept 4: Support Notes

You may be required to compare trends in biological evolution of early bipedal hominins with living hominids. These trends involve:

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.


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. 


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

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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:

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.

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:

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:

In Homo sapiens there is: 

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:

Concept 6: Changes in the Manipulative Ability of the Hand

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Concept 6: Support Notes

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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:

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.

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.