Bipedalism Tied to Position of Hole Found at Base of Skull: A Study

Bipedalism, or the use of just two legs for walking, is directly tied to the position of a hole in the base of the skull that transmits the spinal cord, according to a study published in the Journal of Human Evolution.

The discovery represents a key revelation in a decades-old debate regarding whether this feature of the cranial base, known as the foramen magnum, can act as a direct link to bipedal fossil species.

Located under the braincase in humans, the foramen magnum is found further toward the back of the skull in chimpanzees and most other mammals.

According to Chris Kirk, associate professor of anthropology at the University of Texas and co-author of the study, the report not only offers new insight into human evolution, but validates using the position of the foramen magnum as a diagnostic tool in fossil research as well.

"Now that we know that a forward-shifted foramen magnum is characteristic of bipedal mammals generally, we can be more confident that fossil species showing this feature were also habitual bipeds," Kirk said. "Our methods can be applied to fossil material belonging to some of the earliest potential human ancestors."

The researchers examined the location of the foramen magnum in 71 species from three mammalian groups -- marsupials, rodents and primates -- in order to rule out other possible explanations for a forward-shifted foramen magnum, including differences in brain size.

The results showed that the foramen magnum is positioned toward the base of the skull not only in other bipedal mammals besides humans, such as kangaroos and jerboas, a small rodent that lives in the desert.

"As one of the few cranial features directly linked to locomotion, the position of the foramen magnum is an important feature for the study of human evolution," Russo said. "This is the case for early hominin species such as Sahelanthropus tchadensis, which shows a forward shift of the foramen magnum but has aroused some controversy as to whether it is more closely related to humans or African apes.