A large oval opening in the base of the skull through which the medulla oblongata passes, linking the spinal cord and brain. Its position is an indication of posture. If the foramen magnum is far forward on the skull base, it indicates an upright posture, like that of humans, with the head balanced on top of the spine. In four-footed animals, the head hangs from the end of the vertebral column, and the foramen magnum is placed posteriorly. In apes, with the assumption of semierect posture, the foramen had moved partially downward and forward. In human evolution, the foramen magnum has continued to move forward as an aspect of adaptation to walking on two legs, until the head became balanced vertically on top of the vertebral column.