Skull: foramen magnum

In a biped, the skull is balanced on top of the vertebral column. The foramen magnum (the hole in the bottom of the skull where the spinal cord connects to the brain) is positioned centrally underneath the skull in bipeds (‘centrally’ positioned or more ‘anteriorly’ positioned). A centralized foramen magnum positions the skull over the long axis of the body and helps maintain the center of gravity in bipeds. 

In quadrupeds, the skull extends in front of the spinal column. The foramen magnum and neck muscles are positioned toward the back of the quadruped’s skull (‘posteriorly’ positioned).

As a result, the position of the foramen magnum is a clue for paleoanthropologists about the way in which an extinct animal may have stood and walked.

Spine: Spinal curvature

The spine, or vertebral column, is a series of bones in the neck (cervical vertebrae), thorax (thoracic vertebrae), lower back (lumbar vertebrae) and pelvis (sacrum and coccyx). The way the spine is curved allows for the center of gravity to be in the best position for the type of locomotion it engages in (e.g. quadruped vs. biped).

The quadruped has a C-shaped curve that makes the thoracic region of the spine (the middle of the back) slightly convex. This allows the center of gravity to fall in between all four legs on the ground. However, if you stood a quadruped up on its back legs, the C-shape of its spine would put the center of gravity in front of its feet, causing the animal to fall forward in front of the feet.

Bipeds have two secondary and opposing (concave) curvatures in the cervical and lumbar regions in addition to this main thoracic curve, which gives the spine a S-shape. These secondary curvatures bring the center of gravity closer to the hips, ultimately resting over the biped’s two feet, and stabilizing the body.

Spine: Size of vertebrae

In a biped, the amount of weight being supported increases as you move down the spine; the cervical vertebrae support just the head, but the thoracic vertebrae carry the weight of the head, neck, arms, back, and thorax. So the bodies of the vertebrae of a biped get increasingly large as you approach the lumbar region or lower back, with the lumbar vertebrae the largest in size. In contrast, weight-bearing doesn’t increase along the quadruped’s spine, and the vertebral bodies of different regions are of nearly equal size.

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