Scientists Uncover Link Between Skull and Two-Legged Walking in Mammals

What do your skull and the way you walk have in common? Quite a lot, according to anthropologists. It turns out that there's a direct link between upright two-legged walking and the position of the foramen magnum, a hole in the base of the skull that transmits the spinal cord.

In 1925, scientists uncovered the first known two-legged bipedal human ancestor. Known as Australopithecus africanus, this specimen has been examined by researchers for years. Yet physical anthropologists have continued to debate whether this feature of the cranial base can serve as a direct link to bipedal fossil species.

The foramen magnum in humans is centrally positioned under the braincase because the head sits atop the upright spine in bipedal postures. In chimpanzees and most other mammals, in contrast, the foramen magnum is located further toward the back of the skull.

In order to learn a bit more about this part of the skull, the researchers measured the position of the foramen magnum in 71 species from three mammalian groups: marsupials, rodents and primates. By comparing it across mammals, the researchers were able to rule out other potential explanations for a forward-shifted foramen magnum, such as differences in size.

So what did they find? It turns out that a foramen magnum positioned toward the base of the skull is found not only in humans, but also in other bipedal mammals as well. These included kangaroos, kangaroo rats and jerboas.

"Now 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," said Chris Kirk, one of the researchers, in a news release. "Our methods can be applied to fossil material belonging to some of the earliest potential human ancestors."

The findings are important for understanding evolutionary characteristics that could tell scientists a bit more about ancient fossils. In addition, it gives them further insight into how humans evolved in turn.

The findings are published in the Journal of Human Evolution.