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For decades, scientists have heralded the idea that human brains have increased in size over the course of history, evolving in modern humans be much larger than that of our Neanderthal cousins. In October 2021, DeSilva et al., seemingly added more evidence to this hypothesis with a paper that concluded the human brain shrank during the transition to modern urban societies about 3,000 years ago. And while this supported previous literature, the research did establish a new timeline—marking brain decrease as late as the last Ice Age.
But, in a brand-new article published in the same journal, scientists Brian Villmoare and Mark Grabowsk dispute the conclusions of not only the DeSilva et al., paper, but the entire theory of an evolutionary decrease in human brain size.
“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,” said Villmoare, professor of anthropology at University of Nevada, Las Vegas (UNLV). “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.”
The 2021 paper
DeSilva et al., studied a dataset of 985 fossil and modern human crania. Applying a change-point analysis, according to their results, they found human brains increased in size 2.1 million years ago and 1.5 million years ago during the Pleistocene; but, decreased around 3,000 years ago.
Three-thousand-years-ago also happens to be the beginning of technical advancements, agriculture and complex society for Homo sapiens, leading the research team to turn to ants as a model for group-level cognition and division of labor.
Studying computational models and patterns of worker ant brain size, structure and energy use, the team observed that group-level cognition and division of labor could allow for adaptive brain size variation. In practice, this means that within a social group where knowledge is shared or individuals are specialists at certain tasks, brains may adapt to become more efficient, such as decreasing in size.
This find led the researchers to hypothesize that the suspected decrease in human brain size was due to “increased reliance on collective intelligence—the idea that a group of people is smarter than the smartest person in the group.”
The 2022 article
However, Villmoare and Grabowsk question DeSilva et al.’s dataset in multiple ways. First, as explained in their opinion paper published in Frontiers in Ecology and Evolution, they challenge if the dataset is appropriate to even test the hypothesis that brain size reduced due to technological changes.
Since the rise of agriculture and complex societies occurred at different times around the globe, Villmoare and Grabowsk argue there should be variation in timing of skull changes seen in different populations. However, DeSilva’s dataset sampled only 23 crania from the timeframe critical to the brain shrinkage hypothesis and lumped together specimens from various locations, including England, China, Mali and Algeria.
Additionally, the UNLV team says the dataset is heavily skewed age-wise. Of the 985 total specimens in the dataset, 578 specimens are from just two sources—the Morton Collection in Philadelphia and Peter Brown's Analysis of Australian and Asian recent modern humans. However, both these sources are from the last 100 years—which means more than half of the skulls examined in the initial study represent only the last 100 years of a 9.8-million-year span of time.
Given that timeframe, Villmoare and Grabowsk amended the analysis to only modern humans, focusing only on the last 300,000 years. However, they found even that data was skewed as 60% of the data fall within the last 10% of the modern human timeline. Thus, they created a subset of the data focusing on the most well-populated interval—the last 30,000 years.
“We suggest that, given the data available from DeSilva et al., this consolidated and narrowed data set is needed to produce an accurate test of the hypothesis of a shift in mean brain size in modern humans in the pre- and post-agricultural period,” write Villmoare and Grabowsk in their article.
Even so, using two different methods for identifying changepoints in the data, the UNLV team found no significant changepoint at or near 3,000 years ago.
“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. Thus, hypotheses of recent change are not supported by the evidence,” write Villmoare and Grabowsk.