Did Cold Make Human Brains Bigger?
The idea that cold environments might favor larger brains did not come out of nowhere.
Across animals, there is evidence that harsher and more variable environments can favor larger brains. In birds, for example, species exposed to greater environmental variation tend to have larger brains relative to body size, and the proposed mechanism is straightforward: when food, weather, and survival conditions change sharply across the year, behavioral flexibility becomes more valuable (Sayol et al. 2016).
Humans are not birds, but the logic is relevant. Cold environments are unusually demanding. Food is seasonal, winters are dangerous, and survival can depend on planning, storage, mobility, clothing, shelter, technology, and cooperation. A large brain is costly tissue, but in the right ecological setting those costs may be offset by better problem-solving and behavioral flexibility.
Ancient DNA now lets us ask a more precise version of this old question.
Instead of comparing modern countries or modern skull measurements, we can ask whether ancient individuals sampled from colder environments carried higher polygenic scores for brain volume.
What was tested
I used a clumped brain-volume polygenic score and asked whether it varies with paleoclimate at each ancient sample’s location and date. The climate variables come from CHELSA-TraCE21k-style estimates matched to ancient coordinates and dates (Karger et al. 2023). The ancient samples come from the Allen Ancient DNA Resource, or AADR, a curated compendium of ancient human genomes (Mallick et al. 2024).
There are two different hypotheses here.
The first is geographic:
Do ancient individuals from colder places have higher brain-volume PGS?
The second is dynamic:
Within the same location, did cooling over the previous millennium predict higher brain-volume PGS?
Those questions sound similar, but they are not the same.
A cold place can have high brain-volume PGS because of long-term adaptation, migration, ancestry structure, body-size biology, or sampling. A within-location cooling model asks something more specific: when a location becomes colder over time, does the score rise?
Brain volume: cross-sectional climate
The first analysis is the broad geographic test. It asks whether ancient individuals from colder locations have higher brain-volume PGS, after accounting for sample age.
Here the climate variables are coded as climate values themselves. Annual mean temperature is annual mean temperature. Coldest-quarter temperature is coldest-quarter temperature. Warmest-quarter temperature is warmest-quarter temperature.
So the sign is straightforward:
Negative temperature beta = warmer places have lower brain-volume PGS = colder locations have higher brain-volume PGS.
I also include temperature seasonality because the cold-brain hypothesis is not only about annual averages. Winter severity and seasonal amplitude may matter too. But seasonality is not the same thing as cold, so it has to be interpreted separately.
The first figure shows the standardized climate coefficients for brain-volume PGS in AADR. This is the simplest way to see whether the cold-location pattern is actually present.
