Bergmann’s rule is an observation that body mass of endotherms increases with altitude and colder climate. Neandertals fit this rule, their barrel chests and wide hips, indicate they had large bodies, and thus smaller surface area relative to their body mass. This feature made them comparatively inefficient at radiating their body heat off into the surrounding environment than smaller bodied hominins…. which is an advantageous trait to have in cold environments.
Another similar theorem, Allen’s rule, summarizes the observation that endotherms from colder climates usually have shorter limbs compared to counterparts from warmer climates. Similar to Bergmann’s rule, shorter limbs keep blood and heat closer to the core of the body, reducing heat loss.
All this may sound like adaptionist mumbo-jumbo, but researchers have observed these adaptations to cold climate in other animals. Compare the profiles of polar bears, walruses, and mammoths to cheetahs, antelopes, and giraffes, and you’ll see how some of this makes sense.
Homo neanderthalensis from la Chapelle aux saints
Anyways, I digress. There are two different camps currently hashing out whether the Neandertal facial morphology is due to random genetic drift or a mix of archaic traits and climate influenced adaptations. One of the more hotly debated facial traits, the Neandertal nose, doesn’t quite fit what we expect to see in cold climate adapted species. The Neandertal nose is broad and wide, a feature seen in tropical climates. Physiologists have shown that narrow noses better warm the air being inhaled and prevent evaporation of water in such dry environments by recapturing moisture during exhalation. Wide noses dissipate heat very efficiently.
Some researchers, like Milford Wolpoff, have suggested that there’s a growth and development reason to why we don’t see narrow Neandertal noses. For example, the effects of large teeth and broad palates could have affected the reduction of the nasal aperture, and were most likely inherited traits from Pleistocene ancestors. In a new Journal of Human Evolution paper that Dienekes pointed out this week, researchers from the University of Iowa have investigated the relationships between nasal breadth, intercanine breadth, and facial prognathism. The paper is titled, “The paradox of a wide nasal aperture in cold-adapted Neandertals: a causal assessment.” They tested variants of the following hypothesis: Does the distance between the two upper canines correlated with nasal breadth in modern and archaic Homo?
Intercanine Breadth Measurement
Their sample set of modern humans included 119 crania of Bantu people and 112 crania of Western Europeans. The sample of human ancestors included 11 from the early Upper Paleolithic, 9 from the late Upper Paleolithic all coming from Eurasia. They also included 15 samples from the late Stone Age in Africa, and 14 Pleistocene Homo. Like I mentioned earlier, they measured the distance between the two canines, known as the ICB. In anatomical terms that’s the distance between the lingual tubercles of the maxillary canines, or the pointy parts of your vampire teeth. The lower facial prognathism (BPL) is a measurement of basion to prosthion. Upper facial prognathism (BNL) is a measurement of basion to nasion. Of course some fossils didn’t have all the measurements so predictions were made by least squares regression.
The authors conclude that intercanine breadth cannot fully explain nasal breadth from their sample set, which goes against what anatomist Gustav Schwalbe said in the later 1800′s and what E.V. Glanville reconfirmed in the late ’60s. They also note that the development of the anterior palatine bone does not affect the growth trajectory of the breadth of the nose. While, they do suggest that nasal breadth is affected by the ICB, the lower facial prognathism impacts nasal breadth more than any other trait.
They finally suggest that the plesiomorphic retention of a prognathic lower face is due to the extended time the premaxillary sutures remain open during development. This is lower face prognathism is not a derived trait, but rather a retention of archaic traits, seen in early humans. But that’s not to say the Neadertal nose never adapted to cold climates while the rest of their body did. Neandertals from colder climate have been characterized with a more narrow superior nasal dimensions, which have ultimately been linked to aspects of airflow dynamics.
N HOLTON, R FRANCISCUS (2008). The paradox of a wide nasal aperture in cold-adapted Neandertals: a causal assessment Journal of Human Evolution DOI: 10.1016/j.jhevol.2008.07.001