https://www.sciencedirect.com/science/article/pii/S0278262625000764
Highlights
• Human brain size expanded rapidly, then
plateaued over the past 300,000 years, with
significant glacial–interglacial differences
emerging in the last 100,000 years.
•Large brains imposed costs that may have
heightened extinction risk, especially
during warming climates.
•Cognitive tools and symbolic culture likely
reduced pressure for larger brains by
enabling cognitive offloading.
Abstract
Increasing brain size is a hallmark of human
evolution. While a larger brain offers
evolutionary advantages driven by social and
cognitive adaptations, it also imposes
considerable energetic, metabolic, and
thermoregulatory costs. As a result, brain
size may have biological limits that impose
survival pressures during periods of extreme
environmental change. Here, temporal trends
in absolute brain size across the genus Homo
are analyzed, with a focus on a marked
slowdown in growth beginning around 300,000
years ago. The results suggest that strong
directional selection for brain expansion in
early Homo was followed by a shift toward
stabilizing selection in later populations.
Comparisons across glacial and interglacial
periods indicate that the physiological costs
of large brains may have become especially
disadvantageous during warming interglacial
periods in the last 100,000 years,
potentially increasing extinction risk. This
evolutionary shift coincides with the
emergence of cognitive and cultural
innovations — such as symbolic tools and
language — that may have enabled cognitive
offloading, reducing selective pressure for
continued encephalization. Together, these
findings support the hypothesis that
stabilizing selection, mediated in part by
behavioral and technological adaptations,
buffered later Homo populations against the
ecological and physiological costs associated
with large brains.
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