Study dismisses hypothesized reduction in modern human brain size

Scientific theories that the human brain shrank as we transitioned to modern urban societies during the Holocene have been dismissed by a new study.

Mark Grabowski, of Liverpool John Moores University and Brian Villmoare, of the University of Nevada, Las Vegas, concluded that any assumption that a reduction in skeleton size in human evolution implied a shrinking brain is unfounded in an analysis published in the journal Frontiers in Ecology and Evolution.

“This is quite an exciting result because it is generally accepted that there was a reduction in brain size within modern humans,” said Dr Grabowski  

A recent study by DeSilva et al (2021), suggested that about 3,000 years ago humans experienced a decrease in brain size spurred by the organisation of society which meant that information storage was ‘pooled in the group’.

Egypt's and China's rise

Grabowski and Villmoare considered this particularly unlikely at a time when important innovations and advances took place -  the appearance of Egypt's New Kingdom, the development of Chinese script, the emergence of the Olmec civilization, among many others.

The researchers looked again at DeSilva et al’s data set - a collection of 987 fossil and museum specimens ranging from Miocene hominid Rudapithecus (9.85 million years ago) to modern humans (300 ka to 100 years). They found that when adjusting for the fact that the vast majority of specimens were modern, any patterns suggesting significant changes disappeared.

“Not only have we not found any change in brain size in modern humans around 3,000 years ago, we could not identify a reduction in brain size over any time-period since the origins of our species.”

Right hypothesis

Grabowski and Villmoare argue that in questions of micro-evolutionary change, the analysis and data need to be appropriate for the specific scale of that hypothesis. Given that the adoption of agriculture and the transition to complex societies occurred in different times at different places, the samples need to be specific enough to test the hypothesis across different times and populations, which does not appear to be the case in previous studies.

-Did the transition to complex societies in the Holocene drive a reduction in brain size? A reassessment of the DeSilva et al. (2021) hypothesis, by Brian Villmoare, Mark Grabowski, published in Frontiers in Ecology and Evolution, section Social Evolution.