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.

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.

View Images The yellow-footed rock wallaby, a bipedal marsupial. Photograph 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.

View Images A Verreaux’s sifaka, showing an upright posture. Photograph 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.

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