Saturday, August 22, 2015

A genetic marker for empathy?


 
The Starry Night, Vincent van Gogh (1853-1890). The more you empathize with the world, the more you feel its joy and pain, but too much can lead to overload.

 

One of my interests is affective empathy, the involuntary desire not only to understand another person's emotional state but also to make it one's own—in short, to feel the pain and joy of other people. This mental trait has a heritability of 68% and is normally distributed along a bell curve within any one population (Chakrabarti and Baron-Cohen, 2013). Does it also vary statistically among human populations? This is possible. Different cultures give varying importance to affective empathy, and humans have adapted much more to their cultural environments than to their natural environments. This is why human genetic evolution accelerated over 100-fold about 10,000 years ago when humans began to abandon hunting and gathering for farming, which in turn led to increasingly diverse forms of social organization (Hawks et al., 2007).

I have argued previously that Europeans to the north and west of the Hajnal Line (an imaginary line running from Trieste to Saint-Petersburg) have adapted to a cultural environment of weaker kinship and, conversely, greater individualism. In such an environment, the reciprocal obligations of kinship are insufficient to ensure compliance with social rules. This isn’t a new situation. Weak kinship is inherent to the Western European Marriage Pattern, which goes back to at least the 12th century, if not earlier.

This cultural environment has selected for a package of mental adaptations:

- capacity to internalize punishment for disobedience of social rules (guilt proneness)

- capacity to simulate and then transfer to oneself the emotional states of people who may be affected by rule-breaking (affective empathy)

- desire to seek out and expel rule-breakers from the moral community (ideological intolerance).

The above mental package has enabled Northwest Europeans to free themselves from the limitations of kinship and organize their societies along other lines, notably the market economy, the modern State, and political ideology. They have thus managed to meet the threefold challenge of creating larger societies, ensuring greater compliance with social rules, and making possible a higher level of personal autonomy.

So much for the theory. What direct evidence do we have that affective empathy is stronger on average in Northwest Europeans? We know that a higher capacity for affective empathy is associated with a larger amygdala, which seems to control our response to facial expressions of fear and other signs of emotional distress (Marsh et al., 2014). Two studies, one American and one English, have found that "conservatives" tend to have a larger right amygdala (Kanai et al., 2011; Schreiber et al., 2013). In both cases, my hunch is that "conservatives" are disproportionately drawn from populations that have, on average, a higher capacity for affective empathy.

But testing this kind of hunch would require a large-scale comparative study, which in turn would require cutting up a lot of cadavers or doing a lot of MRIs. It would be nicer to have a genetic marker that shows up on a simple test. It would also be cheaper.

We may now have that marker: a deletion variant of the ADRA2b gene. Carriers remember emotionally arousing images more vividly and for a longer time, and they also show more activation of the amygdala when viewing such images (Todd and Anderson, 2009; Todd et al., 2015). This is not to say that the ADRA2b deletion variant is the sole reason or even the major reason why some people have increased capacity for affective empathy. As with intelligence, an increase in capacity seems to have come about through changes of small effect at many genes.

Nor can we say that "emotional memory" is equivalent to affective empathy. Instead, it seems to be one component, albeit a critical one: the capacity to imagine an emotional state based on visual information (a picture of a person's face, a puppy dog, etc.) and then keep it as part of one's current emotional experience. Emotional memory may be upstream to affective empathy, being perhaps closer to cognitive empathy—the ability to imagine how another person feels without involuntarily making that feeling one's own.

Does its incidence differ among human populations?

This variant was first studied in the United States. Small et al. (2001) found a higher incidence in Caucasians (31%) than in African Americans (12%). Belfer et al. (2005) likewise found a higher incidence in Caucasians (37%) than in African Americans (21%).

In a press release, the authors of the latest study noted that this variant is not equally common in all humans:

The ADRA2b deletion variant appears in varying degrees across different ethnicities. Although roughly 50 per cent of the Caucasian population studied by these researchers in Canada carry the genetic variation, it has been found to be prevalent in other ethnicities. For example, one study found that just 10 per cent of Rwandans carried the ADRA2b gene variant. (UBC News, 2015)

Curiously, its incidence seems higher among “Canadian Caucasians” (50%) than among "American Caucasians” (31-37%). This may reflect differences in participant recruitment or in ethnic mix between the two countries. Indeed, the "Caucasian" category may prove to be problematic because it includes people from both sides of the Hajnal Line. If the average incidence is 31% to 50%, there may be populations that score much higher.

I have found only three studies on specific European ethnicities. The first study found an incidence of 50% in Swiss participants (de Quervain, 2007). The second one found an incidence of 56% in Dutch participants (Cousijn et al., 2010). The third one had two groups of participants: Israeli Holocaust survivors and a control group of European-born Israelis who had emigrated with their parents to the British Mandate of Palestine. The incidence was 48% in the Holocaust survivors and 63% in the controls (Fridman et al., 2012).

From East Asia, a study on Chinese participants reported an incidence of 68% (Zhang et al., 2005). This is surprising because Chinese seem less likely to distinguish between cognitive empathy and affective empathy (Siu and Shek,2005). Japanese participants had an incidence of 56% in one study (Suzuki et al., 2003) and 71% in another (Ishii et al., 2015). Among the Shors, a Turkic people of Siberia, the incidence was 73%. Curiously, the incidence was higher in men (79%) than in women (69%). It may be that male non-carriers had a higher death rate, since the incidence increased with age (Mulerova et al., 2015).

Conclusion

The picture is still incomplete but the incidence of the ADRA2b deletion variant seems to range from a low of 10% in some sub-Saharan African groups to a high of 50-65% in some European groups and 55-75% in some East Asian groups. Given the high values for East Asians, I suspect this variant is not a marker for affective empathy per se but rather for empathy in general (cognitive and affective).

It may be significant that a high incidence was found among the Shors, who were largely hunter-gatherers until recent times. This suggests that empathy reached high levels in Eurasia long before the advent of complex societies, or even farming. The example of the Shors also suggests that non-carriers of the deletion variant suffer from higher mortality—a somewhat surprising finding, given the evidence that carriers have a higher risk of heart disease.

More research is needed on how this variant interacts with variants at other genes. For instance, it has been found that people with at least one copy of the short allele of 5-HTTLPR tend to be too sensitive to negative emotional information. This effect seems to be attenuated by the deletion variant of ADRA2b, which either keeps one from dwelling too much on a bad emotional experience or helps one anticipate and prevent repeat experiences (Naudts et al., 2012). Nonetheless, too much affective empathy may lead to an overload where one ends up helping others to the detriment of oneself and one’s family and kin.

References 

Belfer, I., B. Buzas, H. Hipp, G. Phillips, J. Taubman, I. Lorincz, C. Evans, R.H. Lipsky, M.-A. Enoch, M.B. Max, and D. Goldman. (2005). Haplotype-based analysis of alpha 2A, 2B, and 2C adrenergic receptor genes captures information on common functional loci at each gene. Journal of Human Genetics, 50, 12-20.
http://www.researchgate.net/profile/Mary_Anne_Enoch/publication/8134892_Haplotype-based_analysis_of_alpha_2A_2B_and_2C_adrenergic_receptor_genes_captures_information_on_common_functional_loci_at_each_gene/links/02e7e53559c67a2c02000000.pdf 

Chakrabarti, B. and S. Baron-Cohen. (2013). Understanding the genetics of empathy and the autistic spectrum, in S. Baron-Cohen, H. Tager-Flusberg, M. Lombardo. (eds). Understanding Other Minds: Perspectives from Developmental Social Neuroscience. Oxford: Oxford University Press.
http://books.google.ca/books?hl=fr&lr=&id=eTdLAAAAQBAJ&oi=fnd&pg=PA326&ots=fHpygaxaMQ&sig=_sJsVgdoe0hc-fFbzaW3GMEslZU#v=onepage&q&f=false

Cousijn, H., M. Rijpkema, S. Qin, H.J.F. van Marle, B. Franke, E.J. Herman, G. van Wingen, and G. Fernández. (2010). Acute stress modulates genotype effects on amygdala processing in humans. Proceedings of the National Academy of Sciences U.S.A., 107, 9867-9872.
http://www.pnas.org/content/107/21/9867.full.pdf

de Quervain, D.J. F., I.-T. Kolassa, V. Ertl, L.P. Onyut, F. Neuner, T. Elbert, and A. Papassotiropoulos. (2007). A deletion variant of the alpha2b-adrenoceptor is related to emotional memory in Europeans and Africans. Nature Neuroscience, 10, 1137-1139.

Fridman, A., M.H. van IJzendoorn, A. Sagi-Schwartz, and M.J. Bakermans-Kranenburg. (2012). Genetic moderation of cortisol secretion in Holocaust survivors: A pilot study on the role of ADRA2B. International Journal of Behavioral Development. 36, 79
http://www.researchgate.net/profile/Abraham_Sagi-Schwartz/publication/230887396_Genetic_moderation_of_cortisol_secretion_in_Holocaust_survivors__A_pilot_study_on_the_role_of_ADRA2B/links/0912f505c75bb4a01d000000.pdf 

Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences (USA), 104, 20753-20758.
http://harpending.humanevo.utah.edu/Documents/accel_pnas_submit.pdf

Ishii, M., H. Katoh, T. Kurihara, and S. Shimizu. (2015). Catechol-O-methyl transferase gene polymorphisms in Japanese patients with medication overuse headaches. JSM Genetics and Genomics, 2(1), 1-4.
http://www.researchgate.net/profile/Masakazu_Ishii/publication/273587694_Catechol-O-methyl_transferase_gene_polymorphisms_in_Japanese_patients_with_medication_overuse_headaches/links/5507e87a0cf26ff55f7f719d.pdf

Kanai, R., T. Feilden, C. Firth, and G. Rees. (2011). Political orientations are correlated with brain structure in young adults. Current Biology, 21, 677 - 680.
http://www.cell.com/current-biology/abstract/S0960-9822(11)00289-2

Marsh, A.A., S.A. Stoycos, K.M. Brethel-Haurwitz, P. Robinson, J.W. VanMeter, and E.M. Cardinale. (2014). Neural and cognitive characteristics of extraordinary altruists. Proceedings of the National Academy of Sciences, 111, 15036-15041.
http://www.pnas.org/content/111/42/15036.short 

Mulerova, T.A., A.Y. Yankin, Y.V. Rubtsova, A.A. Kuzmina, P.S. Orlov, N.P. Tatarnikova, V.N. Maksimov, M.I. Voevoda, and M.Y. Ogarkov. (2015). Association of ADRA2B polymorphism with risk factors for cardiovascular diseases in native population of mountain Shoria. Bulletin of Siberian Medicine, 14, 29-34.

Naudts, K.H., R.T. Azevedo, A.S. David, K. van Heeringen, and A.A. Gibbs. (2012). Epistasis between 5-HTTLPR and ADRA2B polymorphisms influences attentional bias for emotional information in healthy volunteers. International Journal of Neuropsychopharmacology, 15, 1027-1036.

Schreiber, D., Fonzo, G., Simmons, A.N., Dawes, C.T., Flagan, T., et al. (2013). Red Brain, Blue Brain: Evaluative Processes Differ in Democrats and Republicans. PLoS ONE, 8(2): e52970.
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0052970

Siu, A.M.H. and D.T. L. Shek. (2005). Validation of the Interpersonal Reactivity Index in a Chinese Context. Research on Social Work Practice, 15, 118-126.
http://rsw.sagepub.com/content/15/2/118.short

Small, K.M., and S.B. Liggett. (2001) Identification and functional characterization of alpha(2)-adrenoceptor polymorphisms. Trends in Pharmacological Sciences, 22, 471-477.

Suzuki N, Matsunaga T, Nagasumi K, Yamamura T, Shihara N, Moritani T, et al. (2003). a2B adrenergic receptor deletion polymorphism associates with autonomic nervous system activity in young healthy Japanese. The Journal of Clinical Endocrinology & Metabolism, 88, 1184-1187.
http://www.researchgate.net/profile/Tetsuro_Matsunaga/publication/10863380_Alpha(2B)-adrenergic_receptor_deletion_polymorphism_associates_with_autonomic_nervous_system_activity_in_young_healthy_Japanese/links/00b7d530379a0f06bf000000.pdf

Todd, R.M. and A.K. Anderson. (2009). The neurogenetics of remembering emotions past. Proceedings of the National Academy of Sciences U.S.A., 106, 18881-18882
http://www.pnas.org/content/106/45/18881.short

Todd, R.M., M.R. Ehlers,  D. J. Muller, A. Robertson, D.J. Palombo, N. Freeman, B. Levine, and A.K. Anderson (2015). Neurogenetic Variations in norepinephrine availability enhance perceptual vividness. The Journal of Neuroscience, 35, 6506-6516.

UBC News. (2015). How your brain reacts to emotional information is influenced by your genes, May 6
http://news.ubc.ca/2015/05/06/how-your-brain-reacts-to-emotional-information-is-influenced-by-your-genes/ 

Zhang, H., X. Li, J. Huang, Y. Li, L. Thijs, Z. Wang, X. Lu, K. Cao, S. Xie, J.A. Staessen, J-G. Wang. (2005). Cardiovascular and metabolic phenotypes in relation to the ADRA2B insertion/deletion polymorphism in a Chinese population. Journal of Hypertension, 23, 2201-2207.
http://www.staessen.net/publications/2001-2005/05-30-P.pdf

Saturday, August 15, 2015

The past is another country


 
Male figurine, pottery, c. 7,000–5,000 years ago, Greece, Archaeological Museum of Heraklion, Wikicommons

 

A very important recent finding is the recovery of the entire genomes of three prehistoric farmers who lived in northern Greece 7500-5500 years BP. The data have been analyzed and are expected to shed light on the ancestral relationships of the first Europeans and provide a wealth of information about functional and morphological characteristics. Already it is known that some of our Neolithic ancestors could not digest milk, i.e., they were intolerant to lactose, and had brown eyes and dark skin. (Anon, 2015)

This is one of several findings with a common theme: the farther back in time we go, the less familiar people look. And we don't have to go very far.

This fact came up in a column I wrote about the Americas. If we turn back the clock, Amerindians look more and more European, yet their genes say they're still Amerindian. We're just getting closer to the time when both groups were the same people. If we turn back the clock even farther, those "proto-Amerindians" give way to a very different sort of human, much like the inhabitants of Papua New Guinea (Frost, 2015).

What happened to those first Americans? They were "replaced." If you're looking for family entertainment, don't study history or prehistory.

Ironically, one of the comments on that column argued that European settlers had stolen this land from the Native Americans and had thus forfeited any moral right to complain about immigration. Well, one genocide doesn’t justify another. I would also venture to say that the universe cares little about our notions of morality. There is only survival or extinction. Everything else is sophistry.

Early and not-so-early Europeans

Ancient DNA is telling a similar story about early Europeans. As late as 8,000 years ago, only the hunting peoples of northern and eastern Europe had white skin and a diverse palette of hair and eye colors. Farther west and south, in Spain, Luxembourg, and Hungary, we find hunter-gatherers with a strange mix of brown skin and eyes of blue, green, or grey. Central Europe was also home to early farmers with white skin, dark hair, and brown eyes. If we go still farther south, beyond the Alps, we see faces and bodies that seem to evoke another continent (Gibbons, 2015; Olalde and Lalueza-Fox, 2015).

This is in line with earlier work on skeletal remains. Angel (1972) found that "one can identify Negroid (Ethiopic or Bushmanoid?) traits of nose and prognathism appearing in Natufian latest hunters [...] and in Anatolian and Macedonian first farmers." In the Middle East, the Natufians (15,000-12,000 BP) were anatomically more similar to present-day West Africans than to present-day Middle Easterners (Brace et al., 2006).

Many African-looking skulls and skeletons have been found in an arc of territory stretching from Brittany, through Switzerland and northern Italy, and into the Balkans. Most are from the Neolithic, but some are as recent as the Bronze Age and the early Iron Age (Boule and Vallois, 1957, pp. 291-292)

Does this mean that prehistoric Greek farmers were more closely related to sub-Saharan Africans than to present-day Greeks? The genome analysis isn't complete, but I think not. They may have looked un-European, but their genomes would probably place them a lot closer to present-day Europeans than to anyone else. We saw the same thing with Kennewick Man. His skull looked European, yet genetically he was closer to Amerindians.

Those prehistoric Greeks were descended from a wave of modern humans that entered Europe some 40,000 years ago. In the north and east, the new settlers encountered selection pressures that recolored and reshaped their most visible features, making them look very different from their African-like relatives to the south and west. Yet this new look came about through changes to just a tiny subset of the genome.

This is not to argue that "we're all pretty much the same under the skin." One could just as well say that humans and chimps are pretty much the same under the skin. They are, actually, if one looks only at flesh and blood. Nonetheless, a human is not a chimp with a body shave.

A second look at the spread of farming

This portrait of early Europeans is still incomplete, and some findings seem contradictory. For instance, why did those Greek farmers lack the alleles for white skin and lactose tolerance when the same alleles were present in Central European farmers from the same time period? In fact, it now seems that both traits evolved in Europe (Gibbons, 2015). A year ago, almost everyone pointed to those Central Europeans as proof that white skin and lactose tolerance must have come from the Middle East, along with farming itself.

It has become popular to argue that farming spread out of the Middle East and into Central Europe through a process of population replacement. The argument seems logical. Because farming supports a larger population per unit of land area, immigrants from the Middle East should have overwhelmed the native hunter-gatherers of Europe by force of numbers. Apparently, things weren’t so simple. Early European farmers were a mixed bunch, and their relationship to the Middle East looks just as problematic. Farming did spread out of the Middle East, but the extent to which this diffusion was genetic or cultural is far from clear. Even the hard evidence looks soft when given a second look.

For instance, we know that a sharp genetic boundary separates late hunter-gatherers from early farmers in Europe. That's good evidence for population replacement. But when a Danish team used a more complete time series of ancient DNA samples, they found that the genetic boundary actually separated early farmers from somewhat later farmers. Haplogroup U, the supposed genetic signature of Europe's ancient hunter-gatherers, reached its current low level after the Neolithic, according to that time series (Melchior et al., 2010). The genetic boundary must therefore be partly due to something else than population replacement, perhaps new selection pressures.

Another piece of hard evidence is the cultural conservatism of hunter-gatherers, who generally prefer to die out than embrace farming and who especially dislike having to plan their lives over a yearly cycle. But that finding is based on tropical hunter-gatherers. Northern hunter-gatherers plan ahead over the coming year and are better able to make the leap. If we take the Mississippian culture of the American Midwest and Southeast (c. 800 -1600), we find that small groups of hunter-gatherers had little trouble making the shift not only to large-scale intensive maize farming but also to life in large towns of up to 40,000 people—all this in half a millennium.

Indeed, if we look at pre-Columbian America, we see that farming first developed in Mesoamerica and then spread north through cultural diffusion. There were very few cases of farmers demographically replacing hunter-gatherers. Why would the situation have been so different in prehistoric Europe? As a general rule, it seems that population replacement occurs only when there is a profound difference in mental makeup that cannot be easily changed.

A final question

Southern Europe and the Middle East were initially home to dark African-like people, who were then replaced by European-like people, apparently from the north, beginning around 12,000 ago. The process of replacement was still incomplete, however, during the time of those northern Greek farmers 7,500 to 5,500 years ago. That last date is very close to the dawn of history. Only a millennium and a half later, the Minoans were building the palace of Knossos. Are those African-like people remembered in European myths, legends, and folk tales?

h/t to Dienekes

References

Angel, J.L. (1972). Biological relations of Egyptian and eastern Mediterranean populations during Pre-dynastic and Dynastic times, Journal of Human Evolution, 1, 307-313. 

Anon. (2015). The Archeological Museum of Thessaloniki. Report on ancient DNA -Learn what eye color your ancestor had and what he ate in the Neolithic! Iefimerida
http://www.iefimerida.gr/news/219751/ekthesi-gia-arhaio-dna-mathe-ti-hroma-matia-eihe-kai-ti-etroge-o-neolithikos-progonos

Boule, M. & Vallois, H.V. (1957). Fossil Men. New York: Dryden Press. 

Brace, C.L., N. Seguchi, C.B. Quintyn, S.C. Fox, A.R. Nelson, S.K. Manolis, and P. Qifeng. (2006). The questionable contribution of the Neolithic and the Bronze Age to European craniofacial form, Proceedings of the National Academy of Sciences U.S.A., 103, 242-247
http://www.pnas.org/content/103/1/242.full 

Dienekes. (2015). Prehistoric farmers from northern Greece had lactose intolerance, brown eyes, dark skin, Dienekes' Anthropology Blog, August 7
http://dienekes.blogspot.ca/2015/08/prehistoric-farmers-from-northern.html 

Frost, P. (2015). Guess who first came to America? The Unz Review
http://www.unz.com/pfrost/guess-who-first-came-to-america/ 

Gibbons, A. (2015). How Europeans evolved white skin, Science, Latest News, April 2
http://news.sciencemag.org/archaeology/2015/04/how-europeans-evolved-white-skin 

Melchior, L., N. Lynnerup, H.R. Siegismund, T. Kivisild, J. Dissing. (2010). Genetic diversity among ancient Nordic populations, PLoS ONE, 5(7): e11898
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0011898#pone-0011898-g002 

Olalde, I. and C. Lalueza-Fox. (2015). Modern humans' paleogenomics and the new evidences on the European prehistory, Science and Technology of Archaeology Research, 1
http://www.maneyonline.com/doi/pdfplus/10.1179/2054892315Y.0000000002

Saturday, August 8, 2015

In the eye of the ancient beholder


 
Egyptian painting of a Libyan, a Kushi, a Syrian, and an Egyptian.  In the Middle East, the Egyptians were seen as the Dark Other (Wikicommons)

 

Mention the term ‘skin color’ and people usually think of race or ethnicity. Yet this way of thinking became dominant only when Europeans began moving out and colonizing the rest of the world, beginning in the 16th century. Previously, physical features were less useful as ethnic markers. We knew about and quarrelled with those groups of people who lived within close range, and they tended to look a lot like us. People farther away looked more different, but we had less to do with them. Often, we didn’t even know they existed. So we separated "us" from "them” mainly on the basis of culture—language, religion, customs, and so on.

In those earlier times, skin color was used to distinguish among individuals of the same people and between the two sexes, women being paler and men ruddier and browner. A pale color also set infants apart, particularly in those societies where everyone else was much darker-skinned.

Skin color thus had meanings related to gender, age, or simply the identity of any one individual. This was true for all cultures. For example, in pre-Islamic writings from Arabia:

Human beings are frequently described by words which we might translate as black, white, red, olive, yellow, and two shades of brown, one lighter and one darker. These terms are usually used in a personal rather than an ethnic sense and would correspond to such words as "swarthy," "sallow," "blonde," or "ruddy" in our own modern usage more than to words like "black" and "white." (Lewis, 1990, p. 22)

Similarly, the Japanese would use the terms shiroi (white) and kuroi (black) to describe their gradations of skin color (Wagatsuma, 1967). The Igbo of precolonial Nigeria used ocha (white) and ojii (black) in the same way, so that nwoko ocha (white man) merely meant an African with a yellowish or reddish complexion (Ardener, 1954).

Jews of Antiquity

This older way of viewing skin color—personal, relativistic, and gender-oriented—has been studied by David Goldenberg with respect to the Jews of the ancient world.

The Jews considered their skin to be light brown. A second-century rabbi compared it to “the boxwood tree, neither black nor white, but in between" (Goldenberg, 2003, p. 95). In papyri from Ptolemaic Egypt, Jews are almost always described as "honey-colored" (Cohen, 1999, pp. 29-30).

Nonetheless, Jewish women were preferentially referred to as "white." This reflected the naturally lighter complexion of women, which was made lighter still by sun avoidance and various cosmetics. One rabbinic text advises, "He who wishes to whiten his daughter's complexion, let him give her milk and young fowl," while another recommends using olive oil as a body lotion for the same purpose. A Midrash recounts that after returning from exile in Babylon the men didn't wish to marry the women who came with them because the sun had darkened their faces on the long journey home (Goldenberg, 2003, p. 86). This preference is implicit in a rabbinic discussion of a vow "not to marry a particular woman who is ugly, but it turns out that the woman is beautiful; or black (dark; shehorah), but it turns out that she is white (fair; levanah); or short, but she is tall. Even if she was ugly, but became beautiful; or black, and became white" (Goldenberg, 2003, pp. 85-86).

"White" was also the preferred color of infants. According to a rabbinic tradition, if a woman was suspected of infidelity and found innocent, she would go through the following changes: "if she formerly bore ugly babies, she will now bear beautiful babies; if she formerly bore dark [shehorin] children, she will now bear fair [levanim] children" (Goldenberg, 2003, p. 96).

In the above cases, the terms "white" and "black" were projected onto individuals and onto the two sexes in a relative sense that is better translated by "light" and "dark." This relativism also held true when the same terms were projected onto ethnic groups. Hence, the Jews often called themselves "white" in relation to darker-skinned peoples, usually Egyptians or kushi (black Africans).

For example, in one parable a kushit maidservant claims she is the most beautiful of her household. Her matronah (a free woman of good family) replies: "Come the morning and we'll see who is black [shahor] and who is white [lavan]" (Goldenberg, 2003, p. 88). Interestingly, the Jews also considered themselves “white” in comparison to Arabs (Goldenberg,2003, pp. 120-124).

There was also the reverse semantic process: the description of an individual’s skin color by a word that originally applied to an ethnic group. A lighter-skinned Jew could for instance be called a germani, and a darker-skinned Jew a kushi. There are even cases of the word kushi being used for inanimate objects, like dark wine (Goldenberg, 2003, p. 116).

Whatever the case, use of color terms in an ethnic sense tended to carry over values from the non-ethnic sense, specifically the aesthetic ones associated with the lighter skin of women and infants. We see this in a commentary on Gen 12:11 where Abraham enters Egypt and, fearing that the Egyptians will covet his wife, says: "Now I know that you are a beautiful woman." This is explained in the commentary as meaning: "Now we are about to enter a place of ugly and dark [people]" (Goldenberg, 2003, p. 86).

The Egyptians were the Dark Other. Depreciation of their darker skin became associated with negative values, not only ugliness but also uncleanliness and servility. In rabbinic writings, Egypt is called "a house of slaves" and the Pharaoh himself is said to be a "slave." In one text, Jacob debates whether to go to Egypt: "Shall I go to an unclean land, among slaves, the children of Ham?" (Goldenberg, 2003, pp. 160-161). This view is preserved in a homily by the third-century Christian writer Origen: 

But Pharao easily reduced the Egyptian people to bondage to himself, nor is it written that he did this by force. For the Egyptians are prone to a degenerate life and quickly sink to every slavery of the vices. Look at the origin of the race and you will discover that their father Cham, who had laughed at his father's nakedness, deserved a judgment of this kind, that his son Chanaan should be a servant to his brothers, in which case the condition of bondage would prove the wickedness of his conduct. Not without merit, therefore, does the discolored posterity imitate the ignobility of the race.
Homily on Genesis XVI

Most academics argue that dark skin became mentally associated with slavery through the Atlantic slave trade of the 16th to 19th centuries. Others, like Bernard Lewis, believe this mental association goes back to the expansion of the Muslim world into Africa in the seventh century (Lewis, 1990). Actually, it seems to go even farther back, at least to the third century and perhaps even to the establishment of Roman rule over the region (Goldenberg, 2003, pp. 155-156, 168-174). From that time onward, a pigmentocracy took shape in Egypt with Greeks, Jews, and Romans forming the dominant class. Meanwhile, a trade in slaves grew and developed between sub-Saharan Africa and the Middle East. Once the Roman Empire had stopped growing, and stopped taking large numbers of prisoners of war, trade became the main source of domestic servants. It is perhaps significant that the kushit maidservant appears as a recurring motif in rabbinic literature, since that period—Late Antiquity—would correspond to the time when the black slave trade was slowly but steadily growing (Goldenberg, 2003, pp. 126-128). 

This trade may have undermined the status of Egyptians as the Dark Other. Initially, the kushi were often seen as an especially dark sort of Egyptian, perhaps because they were usually encountered in the Middle East as subjects of the Pharaoh (Goldenberg, 2003, pp. 17, 109, 301 n111). In Late Antiquity, they emerged more and more as a distinct category, probably because they were becoming more and more numerous as slaves, particularly in the eastern provinces of the Empire. It was during this time that their dark skin came to be explained as a curse on their forefather Kush, whose father Ham had sinned either by seeing Noah naked or by copulating in the Ark. In one text, Noah curses Ham with the words: "May your progeny be dark and ugly" (Goldenberg, 2003, p. 97). This is not a specifically Jewish tradition, being also attested in early Christian and early Islamic writings (Goldenberg, 2003, pp. 150-177).

Conclusion

We perceive human skin color by means of mental algorithms that originally processed non-ethnic differences in pigmentation: 1) the minor variability that exists among individuals; 2) the difference between infants (who are born with little pigmentation) and older humans; and 3) the sex difference, female skin being paler than male skin because it has less melanisation and less blood flowing through its outer layers. This is a universal sex difference, although it is most visible in humans of medium color (Frost, 2007).

Initially, these algorithms focused on the second source of variability. At some point in evolution, human skin acquired a new meaning when the adult female body began to mimic the relative lightness of infant skin, as well as other visible, audible, and tangible aspects of infants—smoother, more pliable skin, a higher-pitched voice, and a more childlike face. This mimicry arose apparently as a means to provide the adult female with the psychological effects that these traits induce in other adults, particularly males, i.e., a lower level of aggressiveness and a greater desire to provide care and nurturance (Frost, 2011).

After being a sign of age difference and then gender difference, skin color took on a third meaning within historic times—to varying degrees in Antiquity and then overwhelmingly with the expansion of the European world from the sixteenth century onward. Today, this new meaning has eclipsed the older ones, at least at the level of conscious thought.

References 

Ardener, E.W. (1954). Some Ibo attitudes to skin pigmentation, Man, 54, 71-73.
http://www.jstor.org/stable/2793760?seq=1#page_scan_tab_contents

Cohen, S.J.D. (1999). The Beginnings of Jewishness, Berkeley. 

Frost, P. (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 133, 779-781.
http://www.researchgate.net/publication/6480611_Human_skin-color_sexual_dimorphism_A_test_of_the_sexual_selection_hypothesis  

Frost, P. (2011). Hue and luminosity of human skin: a visual cue for gender recognition and other mental tasks, Human Ethology Bulletin, 26(2), 25-34.
http://www.researchgate.net/publication/256296588_Hue_and_luminosity_of_human_skin_a_visual_cue_for_gender_recognition_and_other_mental_tasks/file/72e7e5223eb2c3eb3b.pdf

Goldenberg, D.M. (2003). The Curse of Ham. Race and Slavery in Early Judaism, Christianity, and Islam, Princeton: Princeton University Press.

Goldenberg, D.M. (2009). Racism, Color Symbolism, and Color Prejudice, in M. Eliav-Feldon, B. Isaac, and J. Ziegler (eds.) The Origins of Racism in the West, Cambridge.
http://www.researchgate.net/profile/David_Goldenberg2/publication/263161501_Racism_Color_Symbolism_and_Color_Prejudice/links/00b7d53a09ef919429000000.pdf 

Lewis, B. (1990). Race and Slavery in the Middle East, Oxford: Oxford University Press.

Origen (2010). Homilies on Genesis and Exodus, transl. by R.E. Heine., Washington D.C.: Catholic University of America Press
https://books.google.ca/books?id=X_mSBavPcq4C&pg=PA214&source=gbs_toc_r&cad=2#v=onepage&q&f=false  

Wagatsuma, H. (1967). The social perception of skin color in Japan, Daedalus, 96, 407-443.
http://www.jstor.org/stable/20027045?seq=1#page_scan_tab_contents  

Saturday, August 1, 2015

Guess who first came to America?


Semang from the Malayan Peninsula, Wikicommons

 

Before the Europeans came, the Americas were settled by three waves of people from northeast Asia: the oldest wave beginning some 12,000 to 15,000 years ago, which gave rise to most Amerindians, and two later waves, which gave rise respectively to the Athapaskan and Inuit peoples of northern Canada and Alaska. That's the conventional view.

Kennewick Man. An earlier form of Northeast Asian?

There is growing evidence, however, for earlier waves of settlement. There's Kennewick Man, who lived nine thousand years ago in the American northwest and who looked more European than Amerindian, the closest match being the Ainu of northern Japan. He also looked a lot like Patrick Stewart.

Nonetheless, a DNA study has found him to be closer to Amerindians than to any other existing population in the world (Rasmussen et al., 2015). He was apparently descended from the same Northeast Asians who would later become today's Native Americans. Those earlier Northeast Asians looked more European because they lived closer to the time when these two groups were one and the same people. It may be that the Ainu best preserve the appearance of this ancestral population that would later develop into present-day Europeans, East Asians, and Amerindians.

But why would Kennewick Man be closer anatomically to an Ainu while being closer genetically to an Amerindian? The answer is that the genes that shape our anatomy are a tiny subset of the entire genome. Most genes are of low selective value, often being junk DNA, so they change at a steady rate through random processes. Taken as a whole, the genome thus provides a "clock" that can measure how long two populations have been moving apart since their common ancestors. Genealogically speaking, Kennewick Man is closer to present-day Native Americans than he is to the Ainu. Anatomically speaking, the reverse is true ... probably because his ancestors had escaped the extreme Ice Age conditions that affected northeast Asia 20,000 - 15,000 years ago by retreating to an ice age refugium on the Northwest Pacific Coast. The Ainu may have similarly sat out the Ice Age in another refugium on the other side of the Pacific.

[...]  ancient plant and animal remains found on several offshore islands provide evidence that some areas of land on the outer coast remained unglaciated and habitable during the Ice Age. These ice-free areas are called refugia, and evidence for their existence has been found off the Pacific coast from Alaska to southern British Columbia.

Although there is no direct evidence for human occupation of these refugia during the mid-glacial period, it is clear that a chain of habitable environments existed along the Pacific Northwest Coast, and that these environments could have supported people as they made their way down the coast.

If people moved down the West Coast, and then into the interior from there, where and when did this inward movement occur? Is there any archaeology suggesting that populations on the coast began moving inland? 

A few sites from the interior areas of Washington State, Oregon and Idaho may demonstrate this. Stemmed projectile points are found in a site along the Snake River in Washington State, with dates ranging from 8,800 to 10,800 years ago. Another site in south-central Oregon, Fort Rock Cave, contained a layer of gravel that had two obsidian points within it. Dates from this layer are as old as 13,000 years BP. Wilson Butte Cave from Idaho also contains human made artifacts dating to between 14,500 and 13,000 years ago. Perhaps these sites are examples of early people moving in-land; however the small number of sites uncovered so far makes it hard to determine definitively whether the early settlers came from the coast, or from the east. (VMC, 2005)

Kennewick Man may thus have been part of an earlier wave of people moving into the Americas, which was long confined to the coastal Northwest. With the end of the ice age, circa 12,000 years ago, another wave of settlement opened up via an ice-free corridor running from Alaska to Montana along the eastern side of the Rockies. This second wave, associated with the Clovis culture, brought more people than the first one and ultimately contributed the most to present-day Amerindians.

There were humans even earlier

But the story doesn't end here. There seems to have been another people before the Amerindians and even before the older and more European-like Kennewick humans. A recent study has looked at the gene pool of Native Americans from the Amazon. Not surprisingly, most of it closely matches that of Northeast Asians. But a tiny portion is like what we see in the natives of Australia, Papua New Guinea, and Melanesia (Skoglund et al., 2015)

It would be easy to dismiss this finding as a fluke, were it not for other evidence of a very different people who once lived in the Amazon basin 16,000 to 9,000 years ago (Roosevelt et al., 1996). While overlapping in time with the Clovis culture, they show none of its emphasis on big game hunting, as seen in the well-known Clovis projectile point and other hunting tools. In fact, they were much like tropical foragers of central Africa or Papua-New Guinea. And their earliest remains precede the Clovis culture by at least three thousand years, even though the Amazon rain forest should have been one of the last areas to be penetrated by former denizens of the Arctic.

There's more. A site in central Brazil has yielded several skulls dated to between 8,200 and 9,500 years ago. They don't look at all Amerindian: 

[...] they exhibit strong morphological affinities with present day Australians and Africans, showing no resemblance to recent Northern Asians and Native Americans. These findings confirm our long held opinion that the settlement of the Americas was more complicated in terms of biological input than has been widely assumed. The working hypothesis is that two very distinct populations entered the New World by the end of the Pleistocene, and that the transition between the cranial morphology of the Paleoindians and the morphology of later Native Americans, which occurred around 8-9 ka, was abrupt. [...] The similarities of the first South Americans with sub-Saharan Africans may result from the fact that the non-Mongoloid Southeast Asian ancestral population came, ultimately, from Africa, with no major modification in the original cranial bau plan of the first modern humans. (Neves et al., 2003).

Similar findings have emerged from analysis of skulls from Mexico dated to between 9,000 and 11,000 years ago and skulls from Colombia dated to between 7,500 and 8,300:

[...] only 6 out of 25 comparisons displayed in Table 3 tend to tie an early Mexican specimen to an Amerindian sample. Conversely, 19 of the 25 comparisons reflect the greatest similarity to Africans (6/25), Paleoindians (5/25), Australians (3/25), Polynesians (3/25), South Asians (1/25), or the Ainu (1/25). When first-place positions are explored, all five are circum-Pacific, either recent or early. Among second-place positions, 4 out of 5 are circum-Pacific, and the remaining one is African.

[...] To summarize, analyses of individual skulls against reference samples suggest that the early Mexican fossils studied do not share a common craniofacial morphology with Amerindians or East Asians, as reported elsewhere for South Paleoindians, some North Paleoindian specimens […] and some modern groups like Fuegian-Patagonians and the Pericúes from Baja California.

[...] This study does not support continuity between Early and Late Holocene groups in the Americas: Archaic remains from Colombia are not an intermediate point between Paleoamericans and modern groups. Moreover, the data presented here support the idea that the first settlers of the New World preceded the origin of the more specialized morphology observed in modern populations from Northeast Asia. (Gonzalez-Jose et al., 2005)

This shouldn't be too surprising. Here and there in Southeast Asia we find relic groups of small, dark-skinned, and woolly-haired hunter-gatherers: the Andamanese of India, the Semang of Malaysia, and the Aeta of the Philippines. They used to predominate throughout that region as late as four thousand years ago. Farther back in time, in prehistory, they may have also lived farther north, perhaps at one point the entire East Asian littoral ... and from there into the Americas. This would be before the last ice age, and probably before another wave of modern humans moved into northern Eurasia.

The past is another country, just as the future is another country. We unthinkingly assume that a place has always been home to a people who look a certain way, behave a certain way, and organize their lives a certain way. This is as untrue for the Americas as it is for anywhere else. Going back in time, we see people who look more and more ancestral not only to Amerindians but also to Europeans and East Asians. Eventually, those ancestral Eurasians disappear and we meet a very different sort of human.

What happened to those first inhabitants of the Americas? Did they go peacefully into the night when the newcomers arrived, retreating farther and farther into more remote areas? Or did the two groups fight it out? There was probably a range of scenarios—perhaps small numbers of newcomers initially worked out a modus vivendi with the natives, which later broke down as they became more and more numerous. In any case, the process matters less than the result. Those first Americans went into the night, peacefully or not.

References

Gonzalez-Jose, R., W. Neves, M. Mirazon Lahr, S. Gonzalez, H. Pucciarelli, M. Hernandez Martinez, and G. Correal. (2005). Late Pleistocene/Holocene Craniofacial Morphology in Mesoamerican Paleoindians: Implications for the Peopling of the New World, American Journal of Physical Anthropology, 128, 772-780
http://www.hectorpucciarelli.com.ar/pdf/112.AJPA-Gonzalez-Jose%20et%20al.%202005(b).pdf

Neves, W.A., A. Prous, R. Gonzalez-Jose, R. Kipnis, and J. Powell. (2003). Early Holocene human skeletal remains from Santana do Riacho, Brazil: implications for the settlement of the New World, Journal of Human Evolution, 45, 19-42.
http://www.museunacional.ufrj.br/arqueologia/docs/papers/Prous/nevesprous2003skeletalremains.pdf 

Rasmussen, M., M. Sikora, A. Albrechtsen, T. Sand Korneliussen, J.Victor Moreno-Mayar, G. David Poznik, C.P.E. Zollikofer, M.S. Ponce de Leon, M.E. Allentoft, I. Moltke, H. Jonsson, C. Valdiosera, R.S. Malhi, L. Orlando, C.D. Bustamante, T.W. Stafford Jr. D.J. Meltzer, R. Nielsen, and E. Willerslev. (2015). The ancestry and affiliations of Kennewick Man. Nature, early view
http://www.nature.com/nature/journal/vnfv/ncurrent/full/nature14625.html 

Roosevelt, A.C., M. Lima da Costa, C. Lopes Machado, M. Michab, N. Mercier, H. Valladas, J. Feathers, W. Barnett, M. Imazio da Silveira, A. Henderson, J. Silva, B. Chernoff, D.S. Reese, J.A. Holman, N. Toth, and K. Schick. (1996). Paleoindian cave dwellers in the Amazon: The peopling of the Americas, Science, 272, 373-384.
http://www.researchgate.net/profile/William_Barnett3/publication/235237012_Paleoindian_Cave_Dwellers_in_the_Amazon_The_Peopling_of_the_Americas/links/00b7d524c6599de57c000000.pdf 

Skoglund, P., S. Mallick, M.C. Bortolini, N. Chennagiri, T. Hunemeier, M.L. Petzl-Erler, F. Mauro Salzano, N. Patterson, and D. Reich. (2015). Genetic evidence for two founding populations of the Americas, Nature, early view.
http://www.nature.com/nature/journal/vnfv/ncurrent/full/nature14895.html 

VMC (2005). A journey to a new land. Coastal Refugia
http://www.sfu.museum/journey/an-en/postsecondaire-postsecondary/refuges_cotiers-coastal_refugia

Saturday, July 25, 2015

Survival of the nicest-smelling?


 
The Perfume Maker, Rudolf Ernst (1854-1932) (Wikicommons)

 

It has long been known that we vary not only in our sensitivity to different smells but also in our preferences for them—the degree to which they seem pleasant or unpleasant. This variability often contains a large genetic component (Gross-Isseroff et al., 1992; Karstensen and Tommerup, 2012; Keller et al., 2007; Keller et al., 2012; Weiss et al., 2011). In the case of one odor, a single gene explains over 96% of the variability in smell sensitivity (Jaegar et al., 2013). A twin study has similarly found two odorants to be 78% and 73% heritable (Gross-Isseroff, 1992). This hardwiring is selective, however, because sensitivity to other odors can show little or no heritable variation (Hubert et al., 1980). There is also selective hardwiring in smell preferences. Different individuals will perceive androstenone, for instance, as offensive ('sweaty, urinous'), pleasant ('sweet, floral'), or odorless (Keller et al., 2007). It seems that selection has produced specific algorithms in the human brain for specific smells and that these algorithms can differ from one individual to the next (Keller et al., 2007; Knaapila et al., 2012).

This genetic variability exists between men and women, and also between age groups (Keller et al., 2012). Does it also exist between different human populations? The sense of smell does seem to matter more in some than in others, particularly hunter-gatherers:

People pay attention to smells when they are important to their daily lives and are not just part of the sensory and emotional background. This is certainly the case with the Umeda of New Guinea: in a tropical rainforest scent plays as important a role as sight in terms of spatial orientation. The Waanzi in southeast Gabon use odors daily in fishing, hunting, and gathering, thanks to a kind of 'olfactory apprenticeship' in family life and rituals of initiation. In Senegal, the Ndut are even more skillful: they are able to distinguish the odors of the different parts of plants and they are able to give a name to these odors. In our society, of course, most of us are incapable of this. (Candau, 2004)

For hunter-gatherers farther away from the tropics, the sense of smell matters less because the land supports a lower diversity of plant species and has less plant life altogether per unit of land area. Parallel to this north-south trend, more food comes from hunting of game animals and less from gathering of plant items. The end point is Arctic tundra, where opportunities for gathering are limited even in summer and where most food takes the form of meat. There, the senses of sight and sound matter more, being of greater value for long-range detection and tracking of game animals.

Candau (2004) sees these differences between human groups as evidence for "cultural influences" rather than "genetic inheritance." The two are not mutually exclusive: culture itself can select for some heritable abilities over others. On this point, it may be significant that different human groups continue to show differences in smell sensitivity and preference long after their ancestors had moved to very different environments. Thus, in a study from New York City, Euro-American and African American participants were exposed to a wide range of odors. It was found that the two groups differed in their pleasantness rating of 18 of the 134 stimuli, generally floral or vegetative odors. Moreover in 14 of the 18, the African Americans were the ones who responded more positively (Keller et al., 2012).

In addition, the African Americans responded more readily to aromatic metabolites of the male hormone testosterone, i.e., androstadienone and androstenone. The authors note: "This is consistent with the finding from the National Geographic Smell Survey, which found that African respondents were more sensitive to androstenone than American respondents. This difference is undoubtedly at least partially caused by the fact that the functional RT variant of OR7D4 is more common in African-Americans than in Caucasians" (Keller et al., 2012).

Gene-culture coevolution during historic times

This coevolution did not stop with hunter-gatherers. Beginning 10,000 years ago, some of them became farmers and that change set off a lot of other changes: population growth, land ownership, creation of a class system, social inequality on a much greater scale, year-round settlement in villages and then in towns and cities ... And on and on. We entered new environments—not natural ones of climate and vegetation, but rather human-made ones.

These environments offered us new olfactory stimuli: salves, perfumes, incense, scented oils, aromatic baths ... Havlicek and Roberts (2013) argue that our sense of smell coevolved with human-made fragrances and that this coevolution went on for the longest in the Middle East and South Asia, where the use of perfumes is attested as early as the fourth millennium B.C. (Wikipedia, 2015). A sort of positive feedback then developed between use of these fragrances and praise of them in prose, song, and poetry, the two reinforcing each other and thereby strengthening the pressure of selection. This may be seen in the Bible:

The Hebrew Song of Songs furnishes a typical example of a very beautiful Eastern love-poem in which the importance of the appeal to the sense of smell is throughout emphasized. There are in this short poem as many as twenty-four fairly definite references to odors,—personal odors, perfumes, and flowers,—while numerous other references to flowers, etc., seem to point to olfactory associations. Both the lover and his sweetheart express pleasure in each other's personal odor. 

"My beloved is unto me," she sings, "as a bag of myrrh
That lieth between my breasts;
My beloved is unto me as a cluster of henna flowers
In the vineyard of En-gedi."

And again: "His cheeks are as a bed of spices [or balsam], as banks of sweet herbs." While of her he says: "The smell of thy breath [or nose] is like apples." (Ellis, 1897-1928)

In the 9th century the Arab chemist Al-Kindi wrote the Book of the Chemistry of Perfume and Distillations, which contained more than a hundred recipes for fragrant oils, salves, and aromatic waters (Wikipedia, 2015). Today, the names of our chief perfumes are often of Arabic or Persian origin: civet, musk, ambergris, attar, camphor …

Finally, the use of perfumes, like kissing and cosmetics in general, moved the center of sexual interest away from the genitals and toward the face, thereby creating a second channel of arousal:

[...] the focus of olfactory attractiveness has been displaced. The centre of olfactory attractiveness is not, as usually among animals, in the sexual region, but is transferred to the upper part of the body. In this respect the sexual olfactory allurement in man resembles what we find in the sphere of vision, for neither the sexual organs of man nor of woman are usually beautiful in the eyes of the opposite sex, and their exhibition is not among us regarded as a necessary stage in courtship. The odor of the body, like its beauty, in so far as it can be regarded as a possible sexual allurement, has in the course of development been transferred to the upper parts. The careful concealment of the sexual region has doubtless favored this transfer. (Ellis, 1897-1928)

Differences between human populations?

To recapitulate, we humans vary a lot in the degree to which we have been exposed to perfumes and to a perfume-friendly culture, a possible analogy being the degree to which we have been exposed to alcoholic beverages. There may thus have been selection against individuals whose own smell preferences or body chemistry failed to match the perfumes available, this selection being not only stronger in some populations but also qualitatively different:

[...] individual communities vary considerably in the substances they employ for perfume production (in most of the speculations below we deliberately ignore recent trends such as technological advancement in global transfer of goods and production of synthetic chemicals used in perfumery: these phenomena appeared only very recently and one might not expect their immediate effect on biological evolution which operates on a much longer time scale). The absence of a specific ingredient in the perfumes of a particular community could be due to the following reasons: (1) the source of the odour is unavailable in the area and is not traded from neighbours. For example, we know that aromatic plants were an important commodity in trading networks in Ancient Egypt or Greece, but some of the scents routinely employed in that era in India were rare or absent in Mediterranean cultures. (2) The community is constrained by a technology. Some of the aromas can be extracted only using a specific technology which might not be available for or discovered by the particular community. In ancient Greece, for instance, ethanol distillation was not used and perfumers instead used mechanic extraction or enfleurage (Brun 2000). (3) Particular scents or their source (e.g. a particular plant) are believed to be inappropriate for body adornment. Such beliefs might stem from religious considerations.

[...] considering that only some scent ingredients will complement particular body odours (i.e. particular genotypes) and that a particular community employs only a restricted variety of scents for perfuming, it is plausible that some individuals may not be able to select a perfume which complements their body odour and may therefore suffer a social disadvantage. In the long run, the frequency of genotypes of such individuals would decrease in the particular community. (Havlicek and Roberts, 2013)

It is disappointing that Havlicek and Roberts do not develop this argument further with plausible evidence for such gene-culture coevolution. For instance, Hall et al. (1968) discussed how smell and touch hold greater importance for Arabs than for Americans. This point has likewise been remarked upon with respect to the Gulf countries:

The importance of good smell in Qatari homes is inherent in the requirement of cleanness and purity (taharah) in Islam, both physical and spiritual (Sobh and Belk, 2010). Purity, cleanness, and good smell are central to Muslims everywhere in the world, but the obsession with perfuming bodies and homes is something of a fetish in Gulf countries and is very prominent in Qatar. (Sobh and Belk, 2011)

This heightened smell sensitivity is all the more striking because plant life is less abundant and less diverse in the Middle East, certainly in comparison with the tropics. It seems unlikely, then, that it had been acquired during the hunter-gatherer stage of cultural evolution, being instead a later development, possibly through coevolution with the development of perfumes in historic times.

References 

Candau, J. (2004). The olfactory experience: constants and cultural variables, Water Science & Technology, 49, 11-17.
https://halshs.archives-ouvertes.fr/halshs-00130924/  

Ellis, H. (1897-1928). Studies in the Psychology of Sex, vol. IV, Appendix A. The origins of the kiss.
https://www.gutenberg.org/files/13613/13613-h/13613-h.htm 

Gross-Isseroff, R., D. Ophir, A. Bartana, H. Voet, and D. Lancet. (1992). Evidence for genetic determination in human twins of olfactory thresholds for a standard odorant, Neuroscience Letters, 141, 115-118.
http://www.sciencedirect.com/science/article/pii/030439409290347A 

Hall, E.T., R.L. Birdwhistell, B. Bock, P. Bohannan, A.R. Diebold, Jr., M. Durbin, M.S. Edmonson, J.L. Fischer, D. Hymes, S.T. Kimball, W. La Barre, F. Lynch, J.E. McClellan, D.S. Marshall, G.B. Milner, H.B. Sarles, G. L Trager, and A.P. Vayda  (1968). Proxemics, Current Anthropology, 9, 83-108
http://www.jstor.org/stable/2740724?seq=1#page_scan_tab_contents 

Havlícek, J., and S.C. Roberts (2013). The Perfume-Body Odour Complex: An Insightful Model for Culture-Gene Coevolution? Chemical Signals in Vertebrates, 12, 185-195.
http://www.scraigroberts.com/uploads/1/5/0/4/15042548/2013_csiv_perfume.pdf 

Hubert, H.B., R.R. Fabsitz, M. Feinleib, and K.S. Brown. (1980). Olfactory sensitivity in humans: genetic versus environmental control, Science, 208, 607-609.
http://www.sciencemag.org/content/208/4444/607.short 

Jaeger, S.R., J.F. McRae, C.M. Bava, M.K. Beresford, D. Hunter, Y. Jia et al. (2013). A Mendelian Trait for Olfactory Sensitivity Affects Odor Experience and Food Selection, Current Biology, 23, 1601 - 1605
http://www.cell.com/current-biology/abstract/S0960-9822(13)00853-1?_returnURL=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982213008531%3Fshowall%3Dtrue 

Knaapila, A., G. Zhu, S.E. Medland, C.J. Wysocki, G.W. Montgomery, N.G. Martin, M.J. Wright, D.R. Reed. (2012). A genome-wide study on the perception of the odorants androstenone and galaxolide, Chemical Senses, 37, 541-552.
http://www.researchgate.net/profile/Danielle_Reed2/publication/221859158_A_genome-wide_study_on_the_perception_of_the_odorants_androstenone_and_galaxolide/links/02e7e52b367564a573000000.pdf

Karstensen, H.G. and N. Tommerup. (2012). Isolated and syndromic forms of congenital anosmia, Clinical Genetics, 81, 210-215. 

Keller, A., M. Hempstead, I.A. Gomez, A.N. Gilbert and L.B Vosshall. (2012). An olfactory demography of a diverse metropolitan population, BMC Neuroscience, 13, 122
http://www.biomedcentral.com/1471-2202/13/122/ 

Keller, A., H. Zhuang, Q. Chi, L.B. Vosshall, and H. Matsunami. (2007). Genetic variation in a human odorant receptor alters odour perception, Nature, 449, 468-472
http://vosshall.rockefeller.edu/reprints/KellerMatsunamiNature07.pdf

Sobh, R. and R. Belk. (2011). Domains of privacy and hospitality in Arab Gulf homes, Journal of Islamic Marketing, 2, 125-137
http://www.researchgate.net/profile/Rana_Sobh/publication/241699027_Domains_of_privacy_and_hospitality_in_Arab_Gulf_homes/links/54a1374b0cf257a63602614b.pdf

Weiss, J., M. Pyrski, E. Jacobi, B. Bufe, V. Willnecker, B. Schick, P. Zizzari, S.J. Gossage, C.A. Greer, T. Leinders-Zufall, et al. (2011). Loss-of-function mutations in sodium channel Nav1.7 cause anosmia, Nature, 472, 186-190. 

Wikipedia (2015). Perfume
https://en.wikipedia.org/wiki/Perfume