David Reich – Bronze Age shock, the Neanderthal puzzle, & the sudden spread of farming — Summary & Key Points
TL;DR
David Reich argues that natural selection was not quiescent over the last 10,000 years but was actually rampant, particularly during the Bronze Age. Using a dataset of 10,000 ancient genomes and a new method based on relatedness, his team found 3,800 genetic positions under selection, showing a 'Bronze Age shock' that intensified immune and metabolic traits. This challenges the view that the Neolithic farming transition was the main biological shift, suggesting the Bronze Age was a more profound environmental wrenching process.
Key Quotes
"Maybe the degree of that wrenching process moving into the Bronze Age was qualitatively greater than the degree of the wrenching process that happened from the initial transition to growing plants."
The argument
The quiescent selection hypothesis
The mainstream view in human evolution holds that natural selection has been quiescent over the last several hundred thousand years because Europeans and East Asians show almost no genetic variants that are 100% different in frequency. Reich argues this view failed because sample sizes were too small to detect the weak but pervasive signals of selection.
The methodological breakthrough
Ali Akbari developed a method to predict genetic type based on relatedness across 22,000 genomes, allowing the team to detect frequency changes over time that were previously invisible. By analyzing 10 million positions, they identified 3,800 locations with 50% confidence of being under selection, a massive increase from previous studies that found only a dozen.
The Bronze Age shock
The data reveals a 'Bronze Age shock' where selection intensified between 5,000 and 2,000 years ago, particularly for immune and metabolic traits. This period saw the TYK2 tuberculosis risk variant rocket up and then down, and a massive selection for depigmentation, suggesting the Bronze Age was a more profound biological wrenching than the initial Neolithic farming transition.
Selection on intelligence and behavior
Contrary to the intuition that hunter-gatherers needed more intelligence, the data shows strong selection for cognitive performance between 5,000 and 2,000 years ago, peaking at two standard deviations. This selection has since vanished, and the genetic predictor of intelligence correlates with years of schooling and wealth, suggesting a general trait of executive function or delayed gratification.
A new Neanderthal theory
Reich proposes a new theory that Neanderthals are culturally modern humans who mixed with an archaic population 300,000 years ago. This model explains the shared Middle Stone Age tools and mitochondrial DNA by suggesting a matrilineal expansion that was culturally dominant but genetically swamped by local archaic DNA.
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