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Of Brains and Faces
May 16, 2010Posted by on
A new paper in the Journal of Human Evolution discusses the effect of both brain size and facial size on the basicranium. I am excited to see it because it talks about an old hypothesis by one of my favorite Great Anatomists, Josef Biegert.
The basicranium is basically the bottom of your skull. When you look at it in humans and other primates, there is one feature which is particularly notable: The foramen magnum. The foramen magnum is literally the “Big Hole” that transmits your medulla oblongata from your skull and connects it with your spinal column. In humans, the foramen magnum is situated so that it’s really far forward on the bottom of the skull- toward the nose. In most quadrupeds, it comes straight out the back of the skull, and you have to look at the skull from a different angle to see it. The traditional explanation for this is that humans, who are upright, needed to balance the head on top of the spinal column. To accomplish this, the foramen magnum moved forward so that we wouldn’t have to have so many muscles holding our heads up.
If you look at other primates, they also have foramina magna that are situated more toward the front of the skull, and the “balancing hypothesis” would attribute this to the fact that many of those primates use an orthograde posture when moving around.
Biegert didn’t buy the balancing hypothesis. He was a brilliant comparative anatomist, and noticed that in squirrel monkeys, the foramen magnum was almost as far forward as it is in humans. However, when you look at Howlers, the foramen magnum is way at the back of the skull. We’re talking a really huge, dramatic difference here. There are many differences between squirrel monkeys and howler monkeys- the least of which is their posture and locomotion. Squirrel monkeys are teeny tiny, while Howlers are relatively large for New World Monkeys. Howler monkeys have an extremely derived lower face which acts as a resonating chamber for their loud, noisy howls, while squirrel monkeys have a pretty typical monkeyface. Squirrel monkeys have a pretty large brain- which may be the effect of their petite body size- while Howler monkeys don’t have all that much brain to brag about.
Biegert thought that the face and the brain were particularly intriguing. Not only in Howlers, but across the primate family, he found that animals with bigger faces had a flatter basicranium, which positioned the foramen magnum at the back of the skull. In contrast, animals with bigger brains had a more “flexed” basicranium, with sort of a little kink in it at the joint between the sphenoid and the occipital. In these guys, the foramen magnum was situated more toward the front of the skull.
Biegert’s observations are particularly interesting in light of humans, who have a famously large brain and a notably small face. Since the australopithecines, brains have been getting larger, faces have been getting smaller, and the foramen magnum has been migrating ever-anteriorly.
This new paper, by Markus Bastir and his colleagues, tests Biegert’s hypothesis in anthropoid primates, humans, and fossil hominins. Their results support Biegert’s work and show that around 60% of the variation in basicranium shape is explained by the interplay of face size and brain size. Facial size can be large either because of a “tall” face, where the jaw is enlarged from top to bottom like in the robust australopithecines, or it can be because of a “long” face, where the face is longer from front to back, as in baboons and macaques.
Why would this happen? The authors suggest that it might be a respiratory requirement, since the space between the foramen magnum and the back of the nose is occupied by all of the soft tissue structures that we need in order to breathe. Biegert suggested that rotating the face forward relative to the foramen magnum might provide a biomechanical advantage by making the face taller.
The authors draw a few interesting conclusions about some fossils, as well. Neandertals are (obviously) just as bipedal as we are, and have bigger brains. Yet, their basicranium is a little flatter than ours. This is probably the result of their marked midfacial prognathism. If it were me, I’d go all the way back to the australopithecines. They were also obligate bipeds, but have a flatter basicranium than later humans. Doesn’t seem like very good support for the balancing hypothesis to me.
There are a few fossils which don’t seem to fit the pattern, though. Sangiran 17 (the Homo erectus from Java) and OH-5 (Australopithecus boisei, aka “Zinj” or “Nutcracker Man”) both show more flexion that you’d expect for their brain size and large faces. Of course, that is assuming that the skulls have been reconstructed properly, which may not always be a safe assumption when you’re looking at fine details like these. But, the model only explains 60% of the variation, so there could be something else that is influencing the anatomy of these guys.
Bastir, M., Rosas, A., Stringer, C., Manuel Cuétara, J., Kruszynski, R., Weber, G., Ross, C., & Ravosa, M. (2010). Effects of brain and facial size on basicranial form in human and primate evolution Journal of Human Evolution, 58 (5), 424-431 DOI: 10.1016/j.jhevol.2010.03.001
Biegert, J. 1963. The evaluation of characteristics of the skull, hands, and feet for primate taxonomy. In: Washburn, SL, Classification and Human Evolution. Aldine Publishing Co, Chicago. pp.116-145. (The original work from 1957 is in German).