Did increasing brain size place early humans at risk of extinction?

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.