National Geographic

The Way You Walk is Tied to a Hole in Your Skull

On the bottom of your skull, there is a distinctive hole. The technical name for the opening is the foramen magnum – the “great hole” that the spinal cord and other critical soft tissues run through. This bone aperture has taken on great importance in the ongoing investigation into when the human lineage split from those of other apes. If the foramen magnum indicates the position of the spine in relation to the head, and therefore whether the creature was bipedal or moved about some other way, then the position of the opening might indicate when our ancestors developed the upright, bipedal posture so often taken to be the hallmark of humanity.

Animated skull showing the position of the foramen magnum beneath. By BodyParts 3D, distributed under a Creative Commons Attribution-Share Alike 2.1 Japan license.

Animated skull showing the position of the foramen magnum beneath. By BodyParts 3D, distributed under a Creative Commons Attribution-Share Alike 2.1 Japan license.

Not everyone has agreed on the importance of foramen magnum to inferring bipedal habits among hominids. Paleoanthropologist Raymond Dart used the orientation of the hole on the “Taung child” skull of Australopithecus africanus to argue that the fossil represented an early human, and, more recently, Michel Brunet has used the same argument to hypothesize an early human identity for a controversial fossil called Sahelanthropus tchadensis. But other researchers have cast doubt on whether this curious skull opening really is a clear indicator of striding about on two legs. A connection between the foramen magnum and bipedalism seems right, but there’s not much hard evidence to back up the link. And as researchers William Kimbel and Yoel Rak suggested in a 2010 study, the position of the skull opening may have more to do with the posture of the trunk than walking upright.

In a new Journal of Human Evolution study, however, anthropologists Gabrielle Russo and Christopher Kirk suggest a way around the impasse. While they point out that “Past attempts to link foramen magnum position with bipedalism specifically have also been complicated by the fact that H. sapiens is the only habitually bipedal living primate species,” Russo and Kirk note that habitual bipedalism has evolved independently among mammal groups other than primates. If bipedalism and foramen magnum position are linked in these beasts, then there’s likely to be a functional connection for humans, too.

The yellow-footed rock wallaby, a bipedal marsupial. Photo by  Peripitus, distributed under Multi-license with GFDL and Creative Commons CC-BY-SA-2.5 and older versions (2.0 and 1.0).

The yellow-footed rock wallaby, a bipedal marsupial. Photo by Peripitus, distributed under Multi-license with GFDL and Creative Commons CC-BY-SA-2.5 and older versions (2.0 and 1.0).

Russo and Kirk examined non-human primates, marsupials, and rodents in their study, comparing bipedal forms – such as kangaroos and wallabies in the case of marsupials – with quadrupedal relatives. While the rodents and marsupials showed some overlap in the position of the foramen magnum between bipedal and quadrupedal species, in general Russo and Kirk found that bipedal species in each group tended to have a more forward-positioned opening. That goes for us, too – the foramen magnum in the human skull is positioned in a way not seen in any other living ape.

Lemurs and their close relatives, too, throw anatomical support to an upright posture being tied to where the foramen magnum is situated. In their sample, Russo and Kirk compared strepsirrhine primates – lemurs, lorises, and galagos – that hold their bodies upright with those that typically keep their spines parallel to the ground. These primates are not bipeds like us or some of the other mammals in the study, but, nevertheless, those that hold their bodies upright have a foramen magnum positioned forward of species with different postures.

A Verreaux's sifaka, showing an upright posture. Photo by Jeff Gibbs, distributed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

A Verreaux’s sifaka, showing an upright posture. Photo by Jeff Gibbs, distributed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Our species isn’t unique in having a foramen magnum positioned on the undersides of our skulls. Russo and Kirk point out that a forward placement for this skull opening evolved at least four times among mammals with strikingly disparate anatomy. In fact, the strepsirrhine primates in the study indicate that a bipedal, striding posture isn’t even necessary to evolve such a foramen magnum placement. Holding the trunk upright, regardless of how the mammal moves, is tied to the forward shift of the skull landmark, too. Not that bipedalism and an upright posture always go together. The kangaroos and wallabies in the study have forward-placed foramina magna, but they tend to hold their bodies more horizontally.

By looking further outside extinct humans and our close ape relatives, Russo and Kirk have shown that the position of the foramen magnum is influenced by ways of moving as well as body posture. The two do not necessarily have to go together in the way we embody – remember the kangaroos and the lemurs. Nevertheless, applying the finding to our prehistoric kin, Russo and Kirk conclude that the position of the foramen magnum is a good indicator of bipedal locomotion. What that means for the posture of the spine, however, isn’t immediately as clear.

At least two fossil apes in contention for the title of “earliest known human” – Ardipithecus and Sahelanthropus – appear to have foramina magna more like ours than living, non-human apes. Both may have been bipedal, Russo and Kirk conclude. But does this mean that both were really part of the human lineage? There is a traditional assumption that bipedalism evolved only once near the base of our family tree. If a potential hominin looks bipedal, then it is welcomed into our family. But this need not be so.

Bipedalism isn’t a derived anatomical trait that ties close relatives together, but a way of moving that has evolved multiple times over evolutionary history and exists in a variety of forms. The way a kangaroo is bipedal is not the way a jereboa is bipedal is not the way we are bipedal. How Ardipithecus and Sahelanthropus embodied bipedalism was probably quite different from the way we do, and it’s still unknown whether these primates are our ancestors or early experiments whose lineages were snuffed out millions of years ago. We keep looking for a forward placed foramen magnum as a marker of  humanity because of what  our species is like today, but who knows what may yet be found among the little-known, early branches of the ape tree from which our ancestors split.

Reference:

Russo, G., Kirk, E. 2013. Foramen magnum position in bipedal mammals. Journal of Human Evolution. http://dx.doi.org/10.1016/j.jhevol.2013.07.007

There are 6 Comments. Add Yours.

  1. David Bump
    September 28, 2013

    Well done! A thoughtful article on a valuable study suggesting several interesting possibilities for further research.

  2. John McKay
    September 28, 2013

    A while back, I gave some thought to the placement of the foramen magnum in quadrupeds. I was considering how a semi-literate person, whose main familiarity with large animals was through game and farm animals would reconstruct a partially decayed carcass.

    I’m thinking of the Adams mammoth. Roman Boltunov, the first person to try to reconstruct it, drew a picture of a huge boar. He saw the mammoth after the tusks had been cut off and predators had started to eat the face (which almost certainly means the trunk was gone). The obvious observation is that the exposed nasal cavities might have resembled a boar’s snout, as would have the red fur and the idea of tusks.

    My thought was that, along with those superficial features, he also considered some deeper anatomical details, primarily the neck. Almost all animals he was familiar with–deer, horses, cattle, fur bearing mustelids, dogs–have long necks and can change the posture of their heads in relation to their torsos. The only animals he would know that have short muscle-bound necks, with very limited movement would have been boars and pigs. Boltunov didn’t entirely go with a boar. He struggled with the placement of the tusks and how they might have fit in the upper jaw instead of the lower, as in a boar. He realized that he was looking at an animal unlike anything known.

    The relevance of all this is that along with the placement of the foramen magnum in the skull, physical anthropologists and paleontologists need to consider the length and thickness of the neck when determining whether a specimen would have been exclusively quadrupedal, semi bipedal, or exclusively bipedal. Trousers and dress jackets might also shed some light on this topic, but such non-calcareous fibers rarely survive more than a few thousand years.

  3. Zachary Long
    October 1, 2013

    Really it seems that much more study needs to go into this before becoming fact, since it states that there are species with this foramen magnum in non-bipedal species, or at least species who aren’t fully bipedal. This would mean that it is more of how the head tends to be held rather than how the organism tends to walk that causes it. This means that more species are likely to be discovered having it that aren’t full bipedal considering how none survive, which almost completely discredits the theory that bipedalism is directly connected to the foramen magnum.

    While this might be true, it doesn’t seem that it will be entirely true. More details should be added and should be reviewed more for consistency in the statement that bipedalism is connected to the foramen magnum. I say this because even in the article, it provides examples of species that aren’t fully bipedal who have the foramen magnum, but just tend to hold their head in a certain position. Though the article was quite detailed and gave me new information that I had previously not known, it does seem that the study needs to be further explored.

  4. Sarah Longo
    October 2, 2013

    Although the article never says it specifically (except as a keyword) it is essentially about convergence across mammals in a trait traditionally linked to being specifically human. This paper is a great example of how a broader comparative context (beyond apes and primates) can help us learn about our own evolutionary history. Thanks for bringing it to my attention!

  5. Austin
    October 3, 2013

    @John McKay

    “[P]hysical anthropologists and paleontologists need to consider the length and thickness of the neck when determining whether a specimen would have been exclusively quadrupedal, semi bipedal, or exclusively bipedal.”

    They have. And do. The muscles of the neck leave marks on the occipital bone of the skull, called nuchal lines. Nuchal lines in hominids have been extensively studied.

  6. Monado
    November 5, 2013

    If Ardipithecus ramidus walked along tree branches in lemur-like fashion, it may well have been more upright than chimps and gorillas, which seem to have diverged in the direction of knuckle-walking and leaning forward.

Add Your Comments

All fields required.

Related Posts