The unique human skull anatomy is, for the most part, determined by upright posture. One of the key advantages of upright posture is that it increased blood flow to the brain, which enabled the brain to become larger. It also led to an enhanced cooling system that further supported the larger size brain.
The disadvantage is that upright posture caused a bend in the base of the cranium. The bend in the base puts the brainstem in a precarious position over the large hole in the bottom of the cranium. This large hole is called the foramen magnum and through it the brainstem connects to the cord. The location puts the brainstem at risk for Chiari malformations and conditions which can further lead to chronic cerebospinal venuous insufficiency (CCSVI), as well as normal pressure hydrocephelus (NPH).
Due changes in cranial design, the head sits in near perfect balance on top of a relatively small and short pedestal (the cervical spine) beneath the center of the mass of the cerebrum. Balance is further achieved by a relatively small face that lacks a muzzle and protrudes beyond the face only slightly in some designs like European crania, or barely at all in others such as Asian designs. See racial skull design for further information on design differences.
As the frontal lobe grew in size it created a larger forehead that rose steeply up over the top of the eye sockets.
Further affecting skull anatomy, upright posture changed the nature of load bearing forces acting on the skull and spine and related connective tissues. In contrast to humans and bipedalism, in most four legged mammals the head is supported by powerful neck muscles attached to large anchor bones in the spine called spinous processes. While standing or moving about on four legs, the bones of the upper back are under compression loads from the weight of the head and the muscles are under constant tension. The tension from the muscles form large ridges of bone at the point of attachment on the rear of the cranium called nuchal lines.
In contrast to four legged animals, the human head is almost perfectly balanced on top of the spine which bears most of the compressive loads. This reduces the tension acting on the muscles of the neck considerably, which decreases the tension on the attachment points on the head and the size of their ridges. Smaller neck muscles also save energy during upright posture
Consequently, the human head is proportionately much larger while the neck muscles much smaller. The balanced postion of the head and smaller neck muscles saves energy. On the other hand, having a large ten pound ball on top of a small neck with small muscles, makes humans highly susceptible to head and neck trauma, especially whiplash type forces.
Some malformations in skull anatomy predispose humans to Chiari malformations also called cerebellum tonsillar ectopia or CTE. It likewise predisposes humans to venous drainage problems similar to the current theory regarding CCSVI in the cause of multiple sclerosis. Lastly, these conditions can in turn impair cerebrospinal fluid(CSF) flow. Obstruction to CSF flow can further lead to conditions similar to NPH.
For additional information on skull anatomy see skull diploe, neurovascular tunnels, posterior fossa and foramen magnum.