Genetic determinants of nose shape in response to climate adaptation

This publication [attached] reminds me of a book I received (around 1996) as a gift from Anil Menon (at Univ Cincinnati), because he was aware of my interest in “evolution and migration of human tribes.” I studied every page of the book carefully and found the entire concept intriguing. The 1995 book’s title is “The Great Human Diasporas. History of Diversity and Evolution,” by Luigi Luca Cavalli-Sforza [95-year-old professor emeritus at Stanford] and his son Francesco Cavalli-Sforza. This topic is an excellent example of “gene-environment interactions.”

For those not famliar with the word –– “diaspora” means “scattering about,” and Luigi Luca (as a genetics physician-scientist) initiated “the science of human origins.” Early in the book, I recall the authors writing about facial characteristics of the various human subbgroups influenced by climate. Although sociologists use the term “races,” human geneticists/geographers describe “people that have lived and interbred in relative isolation for 10,000-15,000 years”, as “geographically-isolated subbgroups“.

There are five fundamental geographically-isolated subbgroups: African, East Asian, Oceanian, Caucasian, and Amerindian –– from which all other specific groups have diverged. Today, 22 years after publication of this book, due to human sequencing of fossilized bones, we now know more of the complexities of Human Diasporas, including “back-migrations” and admixture of Neanderthals and Denisovians with modern Homo sapiens.

Evolutionary reasons for variation in nose shape across human populations have been subject to debate for decades. One important function of the nasal cavity is to “modulate” inspired air before it reaches the lower respiratory tract. For this reason, it is thought the observed differences in nose shape among populations are not simply the result of genetic drift, but rather adaptations to climate. The two (continuous climate) extremes that come to mind are chronically cold air vs chronically hot humid air. [a] Inuits and Laplanders would benefit from thin nares (to filter and prevent excessive amounts of cold air from rushing directly into the lungs), whereas [b] Pygmies in sub-Saharan jungle heat and humidity are better suited to have broad nares, i.e. no need for protection of lungs from cold air.

To address the question of whether local adaptation to climate is responsible for nose shape divergence across populations, authors [attached article] used QSTFST comparisons to show that nares width and alar base width are more differentiated across populations than expected under genetic drift alone. [Under genetic neutrality, QST = FST; for further details, one can study the text of this article.]

To test whether this differentiation in nose shape is due to climate adaptation, authors compared the spatial distribution of these variables with the global distribution of temperature, absolute humidity, and relative humidity. They found that width of the nares is correlated with temperature and absolute humidity –– but not with relative humidity. Authors thus conclude that some aspects of nose shape may indeed have been driven by local adaptation to climate. However, this is likely to be a highly simplified explanation of a very complex evolutionary history, which possibly might also involve other non-neutral forces such as sexual selection/attraction.

PloS Genet   Mar 2o17; 13: e1006616

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