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.

Decreases in Brain Size and Encephalization in Anatomically Modern Humans.

Growth in human brain size and encephalization is well documented throughout much of prehistory and believed to be responsible for increasing cognitive faculties. Over the past 50,000 years, however, both body size and brain mass have decreased but little is known about the scaling relationship between the two. Here, changes to the human brain are examined using matched body remains to determine encephalization levels across an evolutionary timespan. The results find decreases to encephalization levels in modern humans as compared to earlier Holocene H. sapiens and Late Pleistocene anatomically modern Homo. When controlled for lean body mass, encephalization changes are isometric, suggesting that much of the declines in encephalization are driven by recent increases in obesity. A meta-review of genome-wide association studies finds some evidence for selective pressures acting on human cognitive ability, which may be an evolutionary consequence of the more than 5% loss in brain mass over the past 50,000 years.