Cranium Capacity / Brain Volume

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.

Brain Organisation

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. 

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 there are exceptionally strong jaw muscles. The sagittal crest serves primarily for attachment of the temporalis muscle, which is one of the main chewing (jaw) muscles. It is absent or greatly reduced in most Hominins. 

Humans do not have this because it is no longer needed for attachment for smaller jaws with less forces required, means less muscles needed to attach. This saves energy, which increases chance of survival resulting in energy that can be used elsewhere. 

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.

In humans, the majority of weight is now concentrated in the cranium. Smaller zygomatic arches because the temporal muscles have reduced in size due to diet changes (cooking of food). Smaller temporal muscles mean less muscle attachment sites required, so smaller mandible.

In apes, the majority of weight is concentrated in the jaw for attachment of large temporal muscles to chew through a tough diet. Larger zygomatic arches to allow for large temporal muscles to pass through. Large attachment sites required, so larger mandible. 

Jaw / Mandible

In humans, the mandible is small in size because human teeth size is small (due to eating a softer diet). Less prognathism (shorter muzzle). Humans have a chin, to provide structural support to the gracile mandible. 

In apes, the mandible is large in size because apes have large teeth (needed to eat the tough, fibrous diet). More prognathism. 

Brow Ridges

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. 

It is reduced in humans because they no longer have a large jaw or need muscles or much strength to move the jaw and so has led to a flattened forehead. This is important because it saves energy which increases chance of survival which can be invested elsewhere. 

The brow ridge was one of the last traits to be lost in the path to modern humans. 

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.

Pelvis

In apes the pelvis is long and narrow, and has a box-like shape. In humans it is short and broad and is bowl shaped.

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.