Question

List five specific morphological features, other than their overall size or the size of the canines, that are different between the mandible of a great ape and the mandible of a human. Be sure to include which version of the feature is found in apes and which version of the feature is found in humans.

Answer 1

Cranial Capacity: Hominins have a larger cranial capacity (apes have a cranial capacity of approx 400cm3 compared with 1400cm3 in 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 (at the rear of the skull in apes, central in humans)

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

The difference between ape and human corpus dimensions are strikingly divergent, this hypertrophy could plausibly be associated with a functional response for reducing masticatory stresses. This explanation is undermined given that adductor force in humans is probably well below that utilized by Great Apes. That is, humans likely do not produce the forces necessary to create mandibular stresses equivalent to those in Great Apes given the amount of bone we utilize