Journeys Written in DNA

Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past by David Reich. Pantheon. 2018.

Image: pixabay.com and pmgimage.com

When I spent a summer on the campus of Saint John’s College in Santa Fe, New Mexico, in 1987, one of the groups meeting there was comprised of researchers working on planning the Human Genome Project. The project, which ran for about fifteen years, starting in 1990, cost several billion dollars and produced a single composite DNA sequence for Homo sapiens. Hard to believe that today, thousands of sequences are run routinely at a cost so low that you can get your own sequence in one to two days for $1000.

David Reich provides an account of the recent developments in studying ancient DNA, which is beginning to provide a picture of the evolution of our genus, Homo, over the last 50-150 thousand years, roughly the time that enough DNA remains in old bones or teeth to allow sequencing. The field is rapidly expanding, as more labs open across the world to explore the accumulated human remains in museums, as well as newly excavated material.

Among the findings he reviews are the discovery that early modern humans did indeed hybridize with the archaic populations of Homo that were already in Europe and Asia when the first Homo sapiens moved out of Africa. These earlier people included the Neanderthals and the newly recognized Denisovans. Another finding is that the spread of Indo-European language and culture was indeed accompanied by a spread of people with steppe genotypes into Europe as far west as the British Isles ( see my post on The Horse, the Wheel and Language) The modern human population of Europe turns out to have been the result of multiple waves of migration, bringing not only cultural innovations like farming, but also new human lineages that displaced or blended with the earlier people.

In fact, everywhere that geneticists examine ancient genomes, they find that multiple migrations have shaped human destiny. In North and South America, the most recent areas of human occupation, at least three different migrations can be seen in the genes, and there is still much more work to be done. Likewise, the Indian subcontinent holds a story of migration of Indo-European speakers from the steppes of Central Asia, displacing and blending with the earlier Dravidian language speakers. East Asia has similar patterns, spilling out into the Pacific. These are truly epic journeys of the human species.

Reich discusses the implications of these findings at length. The chapter on genetics and inequality was particularly interesting. The subject is the differences between and within sexes in numbers of offspring produced, as shown by the frequency of distinct gene sequences from a single ancestor among descendent populations. Because men can produce offspring with very little direct effort compared to women, it is possible for men to have many more children in a lifetime. I recall my world history teacher in high school saying that Augustus the Strong of Saxony, “only had about four hundred children.” Circumstantial evidence suggests that Genghis Kahn, thirteenth century Mongol conqueror is the male ancestor of millions, though this is disputed. Certainly, powerful rulers, if fertile and with access to a succession of willing or unwilling consorts, can father many, many sons and daughters.

Reich cites data that indicate that a number of individuals were the fathers of similarly large numbers of descendants during the period between five thousand and three thousand years ago as Neolithic farming peoples began to feel the effects of new technologies: pastoralism, the horse, the wheel and metalworking. In The Horse the Wheel and Language, David Anthony discusses the idea that the new technologies made it possible for tribal chiefs to accumulate power, wealth and prestige. Such men may have led aggressive expansions into new territory, like Genghis, fathering enough descendants for their distinctive Y-chromosomes to show up in genetic analyses. When peoples mix by this mode of male driven conquest, the result is that Y- chromosomes are from the conquering group, while mitochondrial DNA, which is maternally inherited, is from the original inhabitants.

I heard a similar idea many years ago in a seminar on the shift from mother goddess based religion to male sky god religions. Anthony suggests this too in his account of the steppe peoples whose culture and Indo-European language spread widely in this period. The long ago speaker attributed it to mining for metals de-sanctifying the earth, but I suspect it more likely related to the technologies listed above allowing a subset of males to accumulate power. There are now many other cases, from ancient China to medieval Ireland documented by genetic researchers.

Another point Reich makes is that genomics can become a very touchy issue for contemporary descendants of our diversified ancestors. Ethnocentrism is alive and well, from scientists from India who maintain that there were no migrants from the Asian steppes to Navajo elders who refuse to countenance genetic testing arguing that they already know how the Dine were created. Origins are disputed territory: how many Americans still believe in Adam and Eve? Reich points out the falsity of the politically motivated myths of Aryan origins promulgated by German nationalists and still alive today. These crumble in the face of incontrovertible evidence that the modern inhabitants of Northern Europe originated from an amalgam of previous populations with invaders from Central Asia. Migration and mixing of populations and cultures, as we know only too well, is often seen as a threat.

Reich is sensitive to the ethical issues raised by these powerful technologies. He finally consulted a rabbi on the question of whether it was morally right to disturb the dead to obtain genetic material from bones. The answer: only if the knowledge gained will contribute to human  understanding. On the even more fraught question of what population wide genetic studies may reveal about average differences between identifiable groups of people, Reich says two things: First, the question must be faced with accurate data, lest it become the province of pseudoscientific or politically motivated interpretations. Second, whatever the facts are, we know that all groups contain a wide range of potentialities, all of which deserve a chance to be fully realized. Even if a person is not in the upper percentiles of learning ability or athletic ability, the human capacity for hard work makes it possible to succeed. I think Reich means that while  admire the extreme standouts, the Einsteins and Usain Bolts, the bulk of the useful work in the world gets done by those of us closer to the average.

Finally, Reich discusses individual genetic testing. He is in favor of the study of DNA at the population level for medical reasons and also at the individual level, if this helps reduce the incidence of illness caused by recessive mutations. He doesn’t object to individual testing to discover ancestry, but he is not interested in learning about his own genetic background. He seems to feel that focusing on our own unique genomes distracts us from the heritage we share with everyone else, of which the most important part is non-genetic. The simple fact that we are alive tells us that we come from an unbroken genetic line of survivors. Being able to claim descent from particular populations really doesn’t prove much of anything about your own worthiness. Moreover, making such claims can lead to embarrassment, as Elizabeth Warren has discovered.

Humans have been evolving culturally for much longer than the period for which we can get DNA data. Given how much of our behavior is learned, it is likely that our cultural milieu has been a major part of our environment for a long time. Cultures evolve. In doing so, they change the selective environment for humans and the things that live with them. Cultural change drives natural selection. That is, culture shapes our genes indirectly through natural selection as much as genes shape our culture.

As a social species, cultural traditions matter as much or more than our particular DNA in shaping how we live. Many of us, however, know very little of that tradition, or only slivers of it, dependent on our nationality, ethnicity, religion or profession. Too many people grow up with almost no knowledge of any tradition. Even though he professes no religion now, Reich was raised in a deep cultural tradition, Ashkenazi Judaism. He recognizes that all human populations have come out of similarly rich traditions. Together, they represent the most important heritage of the human species; as much as DNA, they are who we are and how we got here.

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