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Saturday, December 6, 2014

Reading List: A Troublesome Inheritance

Wade, Nicholas. A Troublesome Inheritance. New York: Penguin Press, 2014. ISBN 978-1-59420-446-3.
Geographically isolated populations of a species (unable to interbreed with others of their kind) will be subject to natural selection based upon their environment. If that environment differs from that of other members of the species, the isolated population will begin to diverge genetically, as genetic endowments which favour survival and more offspring are selected for. If the isolated population is sufficiently small, the mechanism of genetic drift may cause a specific genetic variant to become almost universal or absent in that population. If this process is repeated for a sufficiently long time, isolated populations may diverge to such a degree they can no longer interbreed, and therefore become distinct species.

None of this is controversial when discussing other species, but in some circles to suggest that these mechanisms apply to humans is the deepest heresy. This well-researched book examines the evidence, much from molecular biology which has become available only in recent years, for the diversification of the human species into distinct populations, or “races” if you like, after its emergence from its birthplace in Africa. In this book the author argues that human evolution has been “recent, copious, and regional” and presents the genetic evidence to support this view.

A few basic facts should be noted at the outset. All humans are members of a single species, and all can interbreed. Humans, as a species, have an extremely low genetic diversity compared to most other animal species: this suggests that our ancestors went through a genetic “bottleneck” where the population was reduced to a very small number, causing the variation observed in other species to be lost through genetic drift. You might expect different human populations to carry different genes, but this is not the case—all humans have essentially the same set of genes. Variation among humans is mostly a result of individuals carrying different alleles (variants) of a gene. For example, eye colour in humans is entirely inherited: a baby's eye colour is determined completely by the alleles of various genes inherited from the mother and father. You might think that variation among human populations is then a question of their carrying different alleles of genes, but that too is an oversimplification. Human genetic variation is, in most cases, a matter of the frequency of alleles among the population.

This means that almost any generalisation about the characteristics of individual members of human populations with different evolutionary histories is ungrounded in fact. The variation among individuals within populations is generally much greater than that of populations as a whole. Discrimination based upon an individual's genetic heritage is not just abhorrent morally but scientifically unjustified.

Based upon these now well-established facts, some have argued that “race does not exist” or is a “social construct”. While this view may be motivated by a well-intentioned desire to eliminate discrimination, it is increasingly at variance with genetic evidence documenting the history of human populations.

Around 200,000 years ago, modern humans emerged in Africa. They spent more than three quarters of their history in that continent, spreading to different niches within it and developing a genetic diversity which today is greater than that of all humans in the rest of the world. Around 50,000 years before the present, by the genetic evidence, a small band of hunter-gatherers left Africa for the lands to the north. Then, some 30,000 years ago the descendants of these bands who migrated to the east and west largely ceased to interbreed and separated into what we now call the Caucasian and East Asian populations. These have remained the main three groups within the human species. Subsequent migrations and isolations have created other populations such as Australian and American aborigines, but their differentiation from the three main races is less distinct. Subsequent migrations, conquest, and intermarriage have blurred the distinctions between these groups, but the fact is that almost any child, shown a picture of a person of European, African, or East Asian ancestry can almost always effortlessly and correctly identify their area of origin. University professors, not so much: it takes an intellectual to deny the evidence of one's own eyes.

As these largely separated populations adapted to their new homes, selection operated upon their genomes. In the ancestral human population children lost the ability to digest lactose, the sugar in milk, after being weaned from their mothers' milk. But in populations which domesticated cattle and developed dairy farming, parents who passed on an allele which would allow their children to drink cow's milk their entire life would have more surviving offspring and, in a remarkably short time on the evolutionary scale, lifetime lactose tolerance became the norm in these areas. Among populations which never raised cattle or used them only for meat, lifetime lactose tolerance remains rare today.

Humans in Africa originally lived close to the equator and had dark skin to protect them from the ultraviolet radiation of the Sun. As human bands occupied northern latitudes in Europe and Asia, dark skin would prevent them from being able to synthesise sufficient Vitamin D from the wan, oblique sunlight of northern winters. These populations were under selection pressure for alleles of genes which gave them lighter skin, but interestingly Europeans and East Asians developed completely different genetic means to lighten their skin. The selection pressure was the same, but evolution blundered into two distinct pathways to meet the need.

Can genetic heritage affect behaviour? There's evidence it can. Humans carry a gene called MAO-A, which breaks down neurotransmitters that affect the transmission of signals within the brain. Experiments in animals have provided evidence that under-production of MAO-A increases aggression and humans with lower levels of MAO-A are found to be more likely to commit violent crime. MAO-A production is regulated by a short sequence of DNA adjacent to the gene: humans may have anywhere from two to five copies of the promoter; the more you have, the more the MAO-A, and hence the mellower you're likely to be. Well, actually, people with three to five copies are indistinguishable, but those with only two (2R) show higher rates of delinquency. Among men of African ancestry, 5.5% carry the 2R variant, while 0.1% of Caucasian males and 0.00067% of East Asian men do. Make of this what you will.

The author argues that just as the introduction of dairy farming tilted the evolutionary landscape in favour of those bearing the allele which allowed them to digest milk into adulthood, the transition of tribal societies to cities, states, and empires in Asia and Europe exerted a selection pressure upon the population which favoured behavioural traits suited to living in such societies. While a tribal society might benefit from producing a substantial population of aggressive warriors, an empire has little need of them: its armies are composed of soldiers, courageous to be sure, who follow orders rather than charging independently into battle. In such a society, the genetic traits which are advantageous in a hunter-gatherer or tribal society will be selected out, as those carrying them will, if not expelled or put to death for misbehaviour, be unable to raise as large a family in these settled societies.

Perhaps, what has been happening over the last five millennia or so is a domestication of the human species. Precisely as humans have bred animals to live with them in close proximity, human societies have selected for humans who are adapted to prosper within them. Those who conform to the social hierarchy, work hard, come up with new ideas but don't disrupt the social structure will have more children and, over time, whatever genetic predispositions there may be for these characteristics (which we don't know today) will become increasingly common in the population. It is intriguing that as humans settled into fixed communities, their skeletons became less robust. This same process of gracilisation is seen in domesticated animals compared to their wild congeners. Certainly there have been as many human generations since the emergence of these complex societies as have sufficed to produce major adaptation in animal species under selective breeding.

Far more speculative and controversial is whether this selection process has been influenced by the nature of the cultures and societies which create the selection pressure. East Asian societies tend to be hierarchical, obedient to authority, and organised on a large scale. European societies, by contrast, are fractious, fissiparous, and prone to bottom-up insurgencies. Is this in part the result of genetic predispositions which have been selected for over millennnia in societies which work that way?

It is assumed by many right-thinking people that all that is needed to bring liberty and prosperity to those regions of the world which haven't yet benefited from them is to create the proper institutions, educate the people, and bootstrap the infrastructure, then stand back and watch them take off. Well, maybe—but the history of colonialism, the mission civilisatrice, and various democracy projects and attempts at nation building over the last two centuries may suggest it isn't that simple. The population of the colonial, conquering, or development-aid-giving power has the benefit of millennia of domestication and adaptation to living in a settled society with division of labour. Its adaptations for tribalism have been largely bred out. Not so in many cases for the people they're there to “help”. Withdraw the colonial administration or occupation troops and before long tribalism will re-assert itself because that's the society for which the people are adapted.

Suggesting things like this is anathema in academia or political discourse. But look at the plain evidence of post-colonial Africa and more recent attempts of nation-building, and couple that with the emerging genetic evidence of variation in human populations and connections to behaviour and you may find yourself thinking forbidden thoughts. This book is an excellent starting point to explore these difficult issues, with numerous citations of recent scientific publications.

Posted at December 6, 2014 15:23