Question

Compare the facial prognathism and cranial capacity in apes and hominins. How do hominins differ from apes in their facial prognathism? Their cranial capacity?

How do apes and hominins compare with regard to their foramen magnum?

Why do apes have a larger nuchal area?

Why do apes have larger temporal muscles?

Why do apes have a larger zygomatic arch?

What is the function of the brow ridge?

What is the function of the sagittal crest?

How do the teeth and jaws of apes compare to the teeth and jaws of hominins?

What is a canine diastema? Do we see this in hominins? Explain your answer.

One structure appears to increase in size in hominins, what is it?

What structures (there are several!) decrease in size in hominins compared to apes?

Answer 1

Hominins have a larger cranial capacity (apes have a cranial capacity of approx 400cm3 compared with 1400cm3in humans). This is an indication of their brain size.

Prognathism: Apes have a pronounced muzzle, the teeth protrude out from their face. This is called a prognathism. This is common in early Hominins also, but later Hominins have a much flatter face

Foramen Magnum

This is the hole at the base of the skull where the spinal cord enters the brain. The foramen magnum is towards the back of the skull in apes, whose posture was not fully upright. In humans, the foramen magnum is at the fulcrum (balance point). The bones of the skull that balance on the vertebra / spine, called occipital condyles show the same trend

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.

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).

Teeth

Apes have much larger canines and incisors. While some apes do eat meat their diet is predominantly vegetarian and these teeth may have played a more significant role in defence. Apes also have a much larger grinding surface on their molars to process leafy and fibrous plant material.In humans the canines are roughly the same size in the two sexes.

canine diastema-a space or gap between the canine and adjacent teeth. It allows room for the point of the protruding opposite canine tooth and thereby permits the upper and lower teeth to bite together

One structure appears to increase in size in hominins-Brain

What structures (there are several!) decrease in size in hominins compared to apes-Canine tooth