Prior work has linked decreases in the brain size of modern humans to increases in obesity. A new paper published in Brain, Behaviour, and Evolution considers climate change as a potential factor affecting brain size evolution, finding that the observed decreases in brain size might be driven by natural selection in response to environmental stress, beginning roughly 15,000 years ago.
“As a cognitive scientist, understanding how the brain has changed over time in hominins is critical but very little work has been done on this subject,” said Jeff Morgan Stibel, who serves as a board member and trustee of the Natural History Museum. “We know the brain has grown across species over the past few million years but we know very little about other macroevolutionary trends. I wrote about decreases in brain size in my last book Breakpoint, so this research was a natural extension of exploring the reasons for these changes.”
This work utilized a sample of 298 remains; estimates of body size were derived from latitude and sex, which were used to control for encephalization differences. Fossils in this research were limited to the past 50,000 years, allowing for the analysis of two dramatic temperature periods (before and after the last interglacial). Fossils were categorized into 100-year, 5000-year, 10,000-year and 15,000-year groups.
Cranial data was obtained from a variety of published sources (e.g., meta-analyses) for a total of 373 independent cranial capacity measurements for 298 skulls, from which brain size was estimated. Brain size data was compared to four climate records. Data was obtained from a variety of sources, such as EPICA Dome C, which provides surface temperature data dating back roughly 810,000 years. Further details of the data set are available at the following link.
“The most important thing to understand is that the human brain continues to evolve. Here, we found macro-evolutionary trends in brain size that happened in as few as 5-17 thousand years,” Stibel told PsyPost. “The Holocene warming period has led to more than a 10% reduction in brain size in modern humans. If global temperatures continue to warm, this could place increased evolutionary pressure on the human brain.”
The adaptive response of shrinking brain size started roughly 15,000 years ago, and may continue into present day. The researcher noted that humidity and precipitation levels were also influential variables on brain size, such that periods of little or no rain were associated with larger brain size. However, these factors were predictive of brain size to a smaller degree.
“It is surprising how little we know about the human brain, despite it being a pretty important organ,” the researcher added.
Are there questions that ought to be addressed in the future? Stibel responded, “of particular importance to this research, there is evidence that both brain and body size are under natural selection in response to climate change. One particularly relevant question is whether one phenotype is under direct selection whereas the other is simply responding to an evolutionary correlation.”
The researcher concluded, “Even a slight reduction in brain size across extant humans could materially impact our physiology in a manner that is not fully understood.”
The research, “Climate Change Influences Brain Size in Humans”, was authored by Jeff Morgan Stibel.