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

Author: David Reich | Published: 2018

Summary

Who We Are and How We Got Here is geneticist David Reich’s account of the revolution in ancient DNA analysis that has transformed our understanding of human prehistory since 2010. Reich’s laboratory at Harvard has been central to this revolution, developing techniques to extract and sequence DNA from ancient human remains and using that data to reconstruct population movements, admixtures, and genetic relationships across deep time. The book covers the emergence of modern humans in Africa, the dispersal out of Africa, the surprising discovery that modern Eurasians carry DNA from Neanderthals (who interbred with modern humans multiple times), the peopling of the Americas, the dramatic population turnovers in prehistoric Europe, and the origins of South Asian population structure.

The most startling findings Reich describes concern the dramatic population replacements that ancient DNA reveals: the farmers who spread agriculture across Europe did not simply teach techniques to the hunter-gatherers already there—they largely replaced them. Similarly, the steppe pastoralists who spread across Eurasia around 3000-2000 BCE (associated with the spread of Indo-European languages) substantially replaced the populations of Europe and South Asia. These findings—that the people living in a place today are often not primarily descended from the people who lived there 5,000 years ago—have profound implications for how we understand national and ethnic identities, and for the political claims made in their names.

Reich is careful throughout to acknowledge the sensitive political dimensions of genetic research on human populations: research on race, ancestry, and the genetics of human variation has a catastrophic history of misuse. He argues that geneticists have a responsibility to be honest about the complexity and uncertainty of their findings and to actively resist the distortions of both racist and politically overcautious readings of genetic data. The final chapters on the ethics of population genetics and the specific dangers of the “race is a social construct” vs. “race is a biological reality” debate are among the most nuanced in popular genetics writing.

Critical Takeaways

  • Ancient DNA revolution: The book documents a genuine scientific revolution—the ability to sequence ancient DNA has given prehistory a new empirical dimension that archaeology and historical linguistics alone could not provide.
  • Population replacement vs. continuity: The discovery that prehistoric Europe experienced massive population turnovers—that the farmers replaced the foragers, that the steppe pastoralists replaced the farmers—has fundamentally revised long-standing assumptions about in-situ cultural evolution.
  • South Asian origins: The finding that South Asian populations carry genetic contributions from three major source populations (the Indus Valley civilization, steppe pastoralists, and Southeast Asian hunter-gatherers) in varying proportions has direct implications for debates about Indo-European origins, the Aryan migration hypothesis, and Indian identity politics.
  • Neanderthal admixture: The discovery that modern non-African humans carry 1-4% Neanderthal DNA (through multiple admixture events) is one of the most remarkable findings of the ancient DNA era; Reich’s lab played a central role in establishing it.
  • Ethics of genetics: Reich’s engagement with the ethical dimensions of population genetics—the history of misuse, the political sensitivities, the responsibility to communicate uncertainty—is one of the most serious in the popular genetics literature.

My Takeaways

  1. The population replacement findings permanently changed my mental model of prehistory: I had imagined cultural change happening largely in place, through diffusion. The reality—waves of population movement and replacement—is more dynamic and more dramatic.
  2. The South Asian genetics sections were personally resonant: the three-way ancestry model (Ancestral North Indians, Ancestral South Indians, and steppe component) gives a genetic framework for understanding the diversity of the Indian population that had no clear form before.
  3. Reich’s treatment of the ethics of race and genetics—refusing both the denial of genetic population structure and the racist misuse of that structure—is the most balanced I have found. The problem is real; the responsible path is narrow.
  4. The Neanderthal admixture is philosophically interesting: we are hybrid beings, and the boundary between “modern human” and “archaic human” was permeable in ways that complicate any simple account of human uniqueness.

Footnotes