2. Two of the three hypotheses proposed to explain the evolution of Neandertal cranial features are adaptive (natural selection), and one is “neutral” (genetic drift).  Explain these competing hypotheses and why the authors support the latter, non-ultra-Darwinian view.

This article presents three hypotheses for the evolutionary explanation for Neandertal cranial features: adaptation to cold climates, adaptation to anterior dental loading, and genetic drift.  The first hypothesis, claiming adaptation to cold climates, is based on the observation that the geographic range of the Neandertals was centered  in northern Europe, which was considerably cooler at the time of the Neandertals.  However, just because they experienced colder temperatures does not require that their cranial features are adaptations to these colder conditions.  Since most of these type of hypotheses focused on the nasal region. Studies have been done on the internal nasal dimensions.  The narrow superior internal nasal dimensions, tall nasal apertures, and progecting nasal bridges are typically found in high latitude recent humans and could be adaptations to cold climates.  However, the Neandertal nasal region does not appear to be an adaptation to cold climates and if they were adapting to cold similarly to presen-day humans, then climatic adaptation is not a likely explanation for their cranial form.

Another characteristic of Neandertals is that they tend to have more worn anterior teeth than posterior ones.  In addition to that, these anterior teeth showed a high incidence of enamel chipping, microfractures, and microstriations on the labial surfaces which suggest that Neandertals used their mouths like a vise.  The second hypothesis, the anterior dental loading hypothesis, states that the facial form of the Neandertal are adaptations to dissipate the high mechanical loads produced by such behavior.  However, the article states that since the facial features of Neandertals appear early in development they cannot be direct mechanical responses to anterior dental loading.  The article goes on to say that instead, these would have to adaptations produced by natural selection after the species performed the behavior for several generations.  Another issue the author has with this hypothesis is that biomechanical modeling suggests that Neandertals were not able to produce high bite forces, at least not high enough to promote any resistance, and thus their cranial form cannot be adapted to resisting high bite forces if they were incapable of producing them.

The author favors the genetic drift hypothesis for many reasons.  First is that Neandertals and modern human populations became isolated from one another around 350,00 years ago.  This would have caused the two to diverge from each other, even in the absence of natural selection through events such as changing allele frequencies.  They tested this hypothesis and estimated that both groups diverged 311,000 to 435,00 years ago, which closely matches dates derived from ancient Neandertal and extant human DNA sequences, which they say is expected if genetic drift is responsible for the cranial divergence.  Another strong piece of supportive information the author brings up comes from fossil records.  He says that Neandertal features, like modern human features, did not appear all at once, rather, they gradually accumulated over hundreds of thousands of years.  This is the expected pattern if genetic drift were responsible.