Genetic data shows natural selection on immune and metabolic traits intensified dramatically 5,000 to 2,000 years ago. This suggests that high-density living and close contact with animals during the Bronze Age created a more powerful evolutionary pressure than the initial shift to farming.
Contrary to expectations of increasing societal complexity, the strongest selection for genetic variants predicting modern IQ test scores and educational attainment occurred between 4,000 and 2,000 years ago. In the last 2,000 years, including the industrial revolution, there has been no detectable selection on these traits.
The TYK2 gene variant, a risk factor for tuberculosis, increased in frequency for thousands of years before plummeting in the last 3,000. This suggests it protected against an earlier threat but became a liability with the rise of endemic tuberculosis in denser populations, showing how selection can reverse direction.
Neanderthals share modern human Y-chromosomes, mitochondrial DNA, and key cultural technologies. This suggests an early modern human group expanded, mixed with local archaics, and became genetically swamped while retaining key cultural and matrilineal/patrilineal traits, challenging the idea they were a completely separate sister species.
While hunter-gatherer life seems cognitively demanding, their genetic profile predicts dramatically lower scores on modern intelligence tests. The subsequent rise in Europe's average score was driven primarily by the migration of farming populations with a different genetic setpoint, not gradual evolution within the hunter-gatherer lineage.
All populations that developed agriculture descend from ancestors who lived long before its invention, implying the necessary cognitive abilities were in place. The simultaneous, independent emergence of farming worldwide points to a global environmental trigger: the unprecedented climate stability of the last 12,000 years (the Holocene).
To test if the "years of schooling" genetic signal was an artifact, researchers applied it to a separate dataset of Chinese individuals. The fact it still predicted educational outcomes strongly suggests the genes are linked to a fundamental, cross-cultural biological trait, not just a quirk of European society.
Most changes in gene frequencies are due to population movements (migration) and random chance (genetic drift), which create statistical noise. The true signal of adaptation is a tiny fraction (2%) of this noise, explaining why it was so difficult to detect with smaller datasets before recent methodological breakthroughs.
Behavioral traits are genetically complex, shaped by thousands of genes with tiny effects (highly polygenic). Current methods can detect strong selection on simpler immune traits but lack the statistical power to pick up the weak, distributed signals acting on complex behaviors. Absence of evidence is not evidence of absence.
Despite the explosion of art and complex tools 50,000-100,000 years ago, there are no genetic "selective sweeps" from that period shared by all living humans. This rules out a single, powerful mutation for language or cognition, pointing instead to gradual, multi-gene adaptation or purely cultural developments.
The same genes predicting educational attainment also predict a woman's age at first birth, body mass index, and household wealth. This suggests selection acts not on "studiousness" but an underlying trait like executive function or propensity to defer gratification, which manifests differently across environments.
Once a population reaches millions, every possible mutation occurs regularly. Therefore, the rapid selection seen in the Bronze Age wasn't enabled by larger populations creating more variants. Rather, it reflects sufficient time (thousands of years) for strong selective pressures to act on existing genetic variation.