Climate Change Influences Brain Size in Humans.

Brain size evolution in hominins constitutes a crucial evolutionary trend, yet the underlying mechanisms behind those changes are not well understood. Here, climate change is considered as an environmental factor using multiple paleoclimate records testing temperature, humidity, and precipitation against changes to brain size in 298 Homo specimens over the past fifty thousand years. Across regional and global paleoclimate records, brain size in Homo averaged significantly lower during periods of climate warming as compared to cooler periods. Geological epochs displayed similar patterns, with Holocene warming periods comprising significantly smaller brained individuals as compared to those living during glacial periods at the end of the Late Pleistocene. Testing spatiotemporal patterns, the adaptive response appears to have started roughly fifteen thousand years ago and may persist into modern times. To a smaller degree, humidity and precipitation levels were also predictive of brain size, with arid periods associated with greater brain size in Homo. The findings suggest an adaptive response to climate change in human brain size that is driven by natural selection in response to environmental stress.