Wednesday, 26 October 2011

The Neanderthal Face: All the Better to Smell You With!

Ambitious anthropologists take note: 
I'm in no position to flesh out [cough] the points that I make in this thesis. I'll gladly grant anyone leave to investigate this further as long as the energetic (and fearless) anthropologist who does so is careful to acknowledge from whom they acquired the skeleton of the argument—moi.
The Answer Was Right Under Our Noses All Along...
In all the time I've spent in and around archaeology and palaeoanthropology I've never heard a satisfying explanation for three features of the Neanderthal skull that make them distinct from ours: mid-facial prognathism, an enlarged nasal aperture, and a large infraorbital foramen. I want to suggest that the answer lies in the sense of smell needed to succeed as a carnivore, which arose in the European Homininan lineage, of which the Neanderthals were almost certainly the final expression. These Ice-Age Europeans were in all likelihood growing a snout.

Biological anthropologists have trouble deciding whether or not the large nasal aperture and mid-facial prognathism is the result of anterior dental loading, or something biomechanically and behaviorally independant. For years it was argued that they needed more nasal real estate to warm the cold air that they would have encountered in Ice-Age Europe and western Asia, where they lived. 
     To say the least, the jury is still out on these matters of the Neanderthal face. In this post I want to posit a new explanation, one that I imagine hasn't occurred to any Very Serious Anthropologists because they imagine the Neanderthals to be a lot like us. As you'll see, this has probably blinkered them from recognizing the possibility that Neanderthals had evolved specialized structures for enhancing their sense of smell. In other words, they grew a snout like others in the carnivore guild, because the olfactory sense is of prime importance at that trophic level. 
     As you'll recall, Herve Bocherens and his colleagues have determined isotopically that the Neanderthals were probably obligate carnivores—their stable isotope signature is on a par with the big cats and a far cry from that of, say, cows and horses. My understanding of evolution means that I can't ignore the trophic level occupied by a species in trying to understand its skeletal evolution. What Neanderthals ate and how they acquired their food would have been reflected in their skeleton, no less and no more than that of any other animal species, including our own. 
     It's also likely that the Neanderthal's trophic habits didn't evolve overnight. Given that we think earlier Hominina were seed eaters or omnivores, a shift to carnivory in a homininan lineage would have taken a very long time, perhaps stretching back to the early Pleistocene, at a time when our ancestors emerged from Africa for the first time.
     So, we must hypothesize a long, mosaic evolutionary process from omnivory to carnivory in the Neanderthal lineage. And, from what we know of other species' evolution, such a huge behavioral transformation would have been accompanied by biomechanical and concomitant skeletal changes, which I would argue worked together to produce the distinctive face of Homo neanderthalensis.
     The swing from omnivory to carnivory, it seems to me, would have necessitated a major realignment of the priority of the senses, with the sense of smell asserting itself. Because of this, the soft- and hard-tissue structures supporting the olfactory sense could not help but have changed, too. Change would have been necessary because homininans had inherited a much-reduced olfactory sense compared with their Primate forbears. Remember that one of the major trends in Primate evolution has been the progressive reduction in dependence on the olfactory sense, with concommitant changes in the facial skeleton—including reduced prognathism. The Primate nose took up less and less facial real estate as time passed in the Cenozoic. So, what's a homininan carnivore wannabe with a small nasal aperture and terminally impaired sense of smell to do? Grow a snout!
     Look a little closer at the Neanderthal face and you'll see that the nasal aperture is one of the most prominent structures of the Neanderthal face when viewed in norma frontalis, as in the La Ferrassie specimen shown above. It's significantly larger than that of Homo sapiens. The same is true of the infraorbital foramen, another outsized landmark when compared with that of modern humans. 
     The infraorbital foramen transmits the infraorbital nerve (ION), which only recently has revealed its anatomy to interested facial surgeons Kyung-Seok Hu et al. The ION is a ramified structure that supplies the skin and mucous membranes of the middle portion of the face, including the inside of the nose, and is therefore critical to the sense of smell and therefore of great importance in this discussion of Neanderthal facial skeletal evolution. According to Kyung-Seok Hu et al. the ION has four main branches, of which one, the internal nasal branch, innervates the skin of the philtrum and has a terminal branch that supplies the nasal septum and vestibule. These structures help to support the olfactory sense.
     For those of you who, like me, don't have a clue what the philtrum is, a picture is worth a thousand words. The philtrum is vestigial in humans and higher primates. However it and its related structures are of the utmost importance in most mammals. It is
a medial cleft common to many mammals, extending from the nose to the upper lip, and, together with a glandular rhinarium and slit-like nostrils, is believed to constitute the primitive condition for mammals in general
The Philtrum  (Mikael Häggström photo [Creative Commons])
     I wouldn't suggest that the Neanderthals were re-expressing the rhinarium of the lesser primates. Nonetheless, all of this means that the ION is intimately connected with olfaction. Moreover, in carnivores the ION's size usually tracks that of the overall muzzle size. Dogs, for example, have long snouts and relatively very large IONs, and depend to a high degree on their sense of smell. Same with cats and other meat eaters, but to a lesser degree.
     So, what are we looking at here? Neanderthals exhibit obligate carnivory. They have an oversized nasal aperture, which implies greater surface area inside the nose, along with the potential for enhanced olfactory capacity. A larger infraorbital foramen would have transmitted a larger ION, necessary to support an augmented olfactory sense. The mid face was become overall larger and more prognathic. All of these factors mean that Neanderthals must have had a much better olfactory sense than you or I, and would thus have been more successful as members of the carnivore guild. Nothing says they needed to be top carnivores. They could have done very well as scavengers, or as occasional hunters, like, for example, hyenas.
     Problem solved! Any questions?

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