Human brain not shrinking but 'remarkably stable' since last 300K years

A University of Nevada, Las Vegas-led team of scientists counter the idea that the human brain began to shrink during the transition to modern urban societies.

The scientists of the UNLV-led study, published in the Frontiers in Ecology and Evolution write that in 2021, DeSilva et al. suggested that “modern human brain size has decreased, starting at roughly 3,000 years ago. They offer a model in which directional selection for decreased brain size, and/or stabilizing selection for maintaining large brains, was relaxed due to the ability to store information externally in social groups.”

DeSilva and his colleagues offer an analogy with ants, noting that “following the development of complex societies, the cumulative intelligence and knowledge of the social group acted to relax the strong forces of selection that had been present in earlier human populations. They propose that ‘group-level cognition may select for reduced brain size and/or adaptive brain size variation’.”

The scientists carrying out the new study, namely Brian Villmoare of UNLV and Mark Grabowski of Liverpool John Moores University, say that the human brain did not change size as suggested by DeSilva et al.

“We were struck by the implications of a substantial reduction in modern human brain size at roughly 3,000 years ago, during an era of many important innovations and historical events — the appearance of Egypt's New Kingdom, the development of Chinese script, the Trojan War, and the emergence of the Olmec civilization, among many others,” Villmoare said.

“We re-examined the dataset from DeSilva et al. and found that human brain size has not changed in 30,000 years, and probably not in 300,000 years,” Villmoare said. “In fact, based on this dataset, we can identify no reduction in brain size in modern humans over any time-period since the origins of our species.” 

Main findings

Brian Villmoare and Mark Grabowski went over the dataset from DeSilva et al. and used the information gleaned from nearly 1,000 early human fossil and museum specimens.

They say that “Our analysis of these data fails to find a decrease in human brain size over the last few thousands of years. When the large sample sizes of the most recent human samples are adjusted for, the pattern disappears, and the arguments no longer need to be invoked.”

Among other things, they found that:

• Villmoare and Grabowski say that the rise of agriculture and complex societies did not happen simultaneously around the world. This would mean that there should be variation in timing of skull changes seen in different populations. Yet DeSilva’s dataset, they say, “sampled only 23 crania” and “lumped together specimens from locations including England, China, Mali and Algeria.”
• Villmoare and Grabowski also suggest that the dataset is heavily skewed: “the temporal intervals between samples ranges from 2.85 million years to 100 years. Of the total of 987 total specimens in this 9.85 Ma [million years ago] analysis, 578 are in the final 100-year interval.” Thus the skulls examined don’t allow scientists to extrapolate a hypothesis on how much cranial size has changed over time.

The researchers end their paper by saying “With regards to testing the hypothesis in question [that suggests that human brains have shrunk over the past 3,000 years], our analyses showed no changes in brain size associated with the transition to agriculture during the Holocene. Overall, our conclusion is that, given a dataset more appropriate to the research question, human brain size has been remarkably stable over the last 300 ka [thousand years ago].”