Who We Are and How We Got Here. Ancient DNA and the New Science of the Human Past

Topic: 
Poverty - Inequality - Aid Effectiveness
Review by: 
Eduardo Lora
Year: 
2018
Author(s): 
David Reich
Publisher: 
Random House
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Economics is a fairly isolationist discipline, which until recently looked with contempt at other social disciplines and blatantly ignored psychology or biology. But this is changing. It is already recognized that the human being is "predictably irrational" because their mechanisms of perception, reaction and decision making did not evolve for the urban environment, financial planning and interaction with strangers, but for the wildlife of gathering and hunting and for sharing with others in small bands. Thanks to Daniel Kahneman and the modern psychologists who have followed in his footsteps, economics has been forced to better understand how the human mind actually works.

A revolution similar to the one initiated by Kahneman in psychology is happening with David Reich in the field of genetic studies. It is now possible to obtain the sequence of the genome of our remote ancestors and to explore with rigorous statistical methods the ancestral relations of the current populations. This will open a window to observe how some populations have dominated and displaced others and how this has left its marks in the social structures that we observe today. Soon we economists will be able to explore the ancestral roots of inequality and institutions.

Since all this may sound very remote to the mind of almost any economist, I have organized this review around the questions that the previous paragraph may raise.

Who is that Mr. Reich? 

David Reich is a professor in the Department of Genetics at the Harvard Medical School and a pioneer in the analysis of the DNA of fossil remains of humans –from the Neanderthal to the first inhabitants of the Americas–  and its use to explore the relationships between different populations, both past and current. In 2015, the magazine Nature highlighted him as one of "the 10 people that matter" in the world of science. His academic articles have about 124,000 citations, according to Google Scholar. (Those of Daniel Kahneman have about 18,500, according to the same source).

What is the genome?  

The human genome is the code of operating instructions for each and every cell of the human body. It is written in the two interlaced chains that make up the double helix of the DNA that we have in each cell. In each chain there are about three billion characters of an alphabet that only has four letters: A (adenine), C (cytosine), G (guanine) and T (thiamine). "What we call a gene is a tiny piece of those chains, typically a thousand letters long, which the organism uses as a template to assemble the proteins that do most of the cell work." Between one gene and another there is a lot of "junk" code that does not do any known function, but that is very useful to establish the similarity between the genomes of two individuals (or of the two chains of the same individual). 

How can the genome sequence be established? 

Establishing the sequence of the genome consists in reading in order and putting in a computer file the six billion letters of the DNA of an individual. This is now done with machines that generate chemical reactions that release light flashes of different colors according to the letter, which allows scanning and archiving the results at high speed.

How do the genomes of two people differ? 

Typically the genomes of two people with no family relationship differ by about three million letters. Two identical twins have the same genomes. Two brothers who are not twins have long segments of their genomes equal, because they are different mixtures of pieces of the genomes of their parents. In addition, they can differ in a few letters due to copy errors of the original of their father or mother (those errors are mutations). The shorter the common segments of the genomes of two people, the farther in time their common ancestors will be. As the mutations occur at a completely regular rate, it can be deduced how many generations apart are two individuals of their common ancestors in each segment of their genomes. 

What is the Reich revolution?  

The reading of the genome of the ancestors is something very recent. Before 2010, since the genome of ancient remains could not be read, morphological similarities or objects found near the remains were used to establish the possible genealogical relationship between ancestral populations. In turn, the relationship between these ancestral populations and the current populations was inferred basically from the place where the remains had been found. It was presumed that the ancestral migratory currents had always been divergent and that the current populations were always descendants of the ancestral populations that had arrived at those same places. They were very crude methods, which left a lot of room for imagination and assumptions (like in economics until recently). 

In 2010 the first three archaic genomes were published, one of them a Neanderthal. In 2015, a real explosion began. Three academic articles published the genomes of 249 ancestral individuals. In August 2017 David Reich's lab had already generated more than 3,000 observations. The speed at which more observations accumulate in the near future will be such that lack of data will quickly cease to be a limitation to explore the relationships between all current and ancestral population groups (although some indigenous communities and some countries prohibit the sequencing of the genome of archaic remains found in their territories).

What new things have been learned about our past? 

In the few years of this revolution, there have been extraordinary discoveries. It has been discovered that modern humans crossed not only with Neanderthals, but also with other human species, such as the Denisovans. The current populations of East Asia have the highest percentage of Neanderthal origin (2.1%), and not Europeans, as was believed by the fact that the Neanderthals lived in Europe. The reason is that the bulk of the current populations of Europeans are not descendants of the first waves of modern humans who came to Europe, but of Yamnaya populations from the Asian steppes that invaded Europe 4,500 years ago and displaced the previous populations thanks to their mastery of the horse and the technology of carts with wheels. The Yamnayas also arrived in India, which explains why the current languages ​​of Europe and India belong to the common family of "Indo-European" languages, something that nobody could explain before.

Going to the Americas, the first settlers did not arrive in Alaska but much farther south along the Pacific coast. Their descendants, who are currently in remote areas of the Amazon, have their closest ancestral relatives in Australia (in turn, common descendants of a "ghost population" whose existence is genetically proven although no remains have yet been found). And there are many, many more discoveries that will change our view of the past. 

What does this have to do with inequality? 

The invaders leave numerous traces in the genome of their descendants. It can be established, for example, if those traces are more marked in the genes that come from the paternal side or in those that come from the maternal side. A very dramatic evidence is found in Antioquia, Colombia: 94% of the Y chromosomes, which represent the paternal side of the current population, are of European origin, while 90% of the mitochondrial DNA that represents the maternal side, are of indigenous origin. [1] European settlers, mostly men, displaced or killed indigenous men and sexually subjugated indigenous women. Is it possible to find greater gender inequality ("sex bias" is the term used by Reich)?

The rigid caste system of India, through which inequality and segregation are perpetuated, is living evidence of the domination strategies used by the populations that arrived in Northeast India to prevail over those that arrived from the Southwest (displacing between both the original populations, whose only remnants can be found on a small island). The higher the caste to which an individual belongs today, the higher the genetic percentage of the ancestral populations of the Northeast (and the lower the percentage from the Southwest). India remains a patchwork quilt of groups that have been genetically separated for millennia as a result of ancestral domination strategies. In contrast, China is basically a single population where everyone shares the same mix of origins.  Such different genetic patterns may have influenced the ability to form more centralized systems of government in some countries than in others, and in the willingness of more homogeneous populations to share economic results and to collectively protect themselves from certain risks. 

In what kind of research can this information be used? 

The genetic revolution will help us to understand the cultural origins of gender inequalities, the institutional bases of economic inequalities, the practices of spatial and productive segregation, and perhaps also the roots of various forms of political organization. Studies as influential as Acemoglu and Robinson [2] (with more than 11,000 citations to date), which used settlers’ mortality rates as an instrumental variable to analyze the institutional determinants of development in colonized countries, could be done using ancestral DNA data as instruments to study many other aspects of the economy and the institutions that have ancient historical roots.

Surely some will dismiss this type of research as racist or deterministic, arguing that using genome information implies believing that the genetic differences or similarities between human groups determine the history or functioning of societies. No, the genetic revolution is just the opposite: it is the scientific reading of the marks left by the history of populations in their genome (more specifically in the "junk" code of their genome, which does not have a biological function), marks that have nothing to do with the characteristics, capabilities or behavior of individuals (which may at least partly be influenced by genes, ie the potentially active genome code). 

It took about two decades for the revolutionary ideas of Kahneman and his followers to have some acceptance in the academic circles of the economy. Hopefully the process will be faster with the genetic revolution.


1. L. G. Carvajal -Carmona et al. 2000. “Strong Amerind/White Sex Bias and a Possible Sephardic Contribution Among the Founders of a Population in Northwest Colombia,” American Journal of Human Genetics 67: 1287–95. 

2. Acemoglu, Daron, Simon Johnson, and James A. Robinson. 2001. "The Colonial Origins of Comparative Development: An Empirical Investigation." American Economic Review, 91 (5): 1369-1401.