The Foramen Magnum: How Do We Know?

The location of the foramen magnum plays a crucial role in our understanding of human evolution. Raymond Dart’s original argument for the hominid status of Australopithecus africanus relied heavily on the position of the foramen magnum, as did the argument for the hominid status of Sahelanthropus tchadensis and Ardipithecus ramidus. Usually, the location of the foramen magnum is linked to bipedal behavior – or the lack thereof – so it would be nice if we could quantify this somehow.

So let’s start at the beginning. The foramen magnum is located on the occipital.

At birth, the occipital (which forms the part of the rear and bottom of the skull) is composed of four different pieces: the squama (circled in yellow), two lateral pieces (which contains the condyles for articulation with the first cervical vertebra, circled in red), and the basilar (circled in light blue). The two lateral pieces usually fuse to the squama by about age four. The basilar portion fuses to the laterals by about age seven and to the sphenoid sometime between puberty and age twenty one.
The position and orientation of the foramen magnum, as mentioned above, is usually considered to be an indicator of the type of locomotion since it provides clues about the orientation of the head relative to the rest of the body. For example the picture below is from a red fox skull found in the Van-Yoncatepe necropolis in eastern Anatolia.

The foramen magnum is circled in red. Note that rather than being under the skull it is at the back. Below is a drawing of an ape skull.

As you can see, the foramen magnum has moved more underneath the skull. Below is a drawing of a human skull.

As you can see it has moved even further. So just by visual inspection we can spot some differences. This raises three questions. First, can we quantify this somehow? Second, does it genuinely allow us to distinguish between apes and humans. Third, can we apply it to the fossil record? The answer to the first question is yes. Physical anthropologists love to measure things and have created a number of measures to address this issue. If you look at the two above pictures again you will note that the basilar part of the occipital is longer than in humans. There are two different ways to measure this.

Picture A is of a chimp skull, it shows the distance from basion (midpoint of the anterior margin of the foramen magnum to the biporion chord (porion is a paired landmark on the margin of the external auditory meatus – or ear opening). Picture B shows the distance from basion to the bicarotid chord (or line connecting the openings for the carotid arteries). The pictures come from a paper, published in the AJPA in 2005 (Ahern, AJPA 127:267-276) which examined the issue in some detail. The results of the paper were quite interesting and raised a few interesting issues. The paper examined the above in 69 chimps, 42 humans and eight fossils (five of which were research quality casts), the results were then applied to Sahelanthropus tchadensis and Ardipithecus ramidus. One interesting result is that in chimps the basion – bicarotid measurement (picture B) varied by sex (such was not the case in humans). Male chimps tended to have bicarotid chords which lie more anterior (towards the front – meaning the distance between the bicarotid chord and basion was greater) than female chimps. Do either help distinguish between apes and humans? Yes, it turns out that the basion-bicarotid does distinguish between apes and humans (basion-biporion did a poorer job, for example it gave ambiguous results for KNM-ER 3833, KNM WT-15000 to name a few). Basion-bicarotid also correctly classified the fossil skulls.

Which is, of course, interesting but the paper raises two points that bear pointing out as suggestions for future research (like any good piece of research should). First, the paper used eight fossils: three A. africanus, one A. aethiopecus, two A. boisei, and two erectus/ergaster (plus Toumai and Ardipithecus). Clearly we need a larger sample size to get a grip on the range of variation in each of the above species. Second, what is the condition in Miocene/early Pliocene apes? We don’t have much of a fossil record for apes so:

…we can not be sure that the chimpanzee condition would be the same as in late Miocene and early Pliocene apes, those we might mistake for hominids (Ahern 2005: 275 – see above for where this paper can be found)

Finally, in looking at the area of the foramen magnum, one other interesting feature comes to light.

This is picture B from above. If you draw a line from the carotid foramina to the most anterior point of the petrous portion of the temporal the angle it makes to the bicarotid line is 60 degrees in monkeys and apes (and possibly A. africanus). A picture of a human skull is below.

In humans (and specimens referred to Paranthropus) the angle (marked in red) is only 45 degrees (this example comes from Aiello and Dean’s An Introduction to Human Evolutionary Anatomy – which I would love to see updated since it was published 16 years ago).
Finally, two pieces of trivia about the foramen magnum. In A. boisei the foramen magnum is heartshaped. In Neanderthals it is oval and elongated…
Added Later: I forgot to mention that the next post in this series will be on the semicircular canal…

Update 03/30/2013: Edited to fix the pictures in the post, and to correct a few typos.