No, human brains didn’t shrink just 3,000 years ago: study

UNLV research team questions several of the hypotheses of popular 2021 study suggesting shrinkage

By Kiran N. Kumar

Anthropologists believe that brain size has remained dynamic in size. It nearly quadrupled in the six million years since Homo last shared a common ancestor with chimpanzees, but human brains are thought to have decreased in volume since the end of the last Ice Age.

Researchers long held that beginning with Australopithecus cranial capacities which were on average 20% larger than those of the late Miocene hominins Sahelanthropus and Ardipithecus or modern chimpanzees. These ∼450 cc brains remained roughly unchanged in size from 3.5 to 2.0 million years ago despite extensive diversification in homins.

Read: Scientists growing human brains in Petri dishes to study autism, schizophrenia (October 7, 2016)

After the fourfold increase, well-documented studies trace the decrease in brain size since the end of the Pleistocene era. However, the precise timing of this decrease remains unclear. Some have placed it in the late Pleistocene ∼35 kyr and others in recent Holocene ∼10 kyr.

Popular 2021 study
Setting aside all these beliefs, in 2021, a new study led by Jeremy M. DeSilva et al., raised the eyebrows of many anthropologists with their publication titled, “When and Why Did Human Brains Decrease in Size? A New Change-Point Analysis and Insights From Brain Evolution in Ants.”

It concluded that the human brain shrank during the transition to modern urban societies about 3,000 years ago basing their hypotheses on a comparison of evolutionary patterns seen in ants, among others.

The 2021 study provides a larger picture that brain differentiation can occur in association with division of labor in the absence of striking changes in body size, illustrating how selection may have operated in humans as social groups began to swell in size.

They said, “Our ancestors’ ability to store information externally in social groups decreased our need to maintain large brains.” Summarizing ten popular studies, they concluded that the human brain size might have decreased 3,000 years ago.

Read: Brain’s part that recognises facial expressions discovered (April 20, 2016)

UNLV team questions the hypotheses
The University of Nevada, Las Vegas (UNLV) research team questioned several of the hypotheses that DeSilva et. al gleaned from a dataset of nearly 1,000 early human fossil and museum specimens.

They cited the rise of agriculture and complex societies which occurred at different times around the globe, which means there should be variation in timing of skull changes in different populations.

In their new paper published last week in Frontiers in Ecology and Evolution, the UNLV-led team analyzed the dataset used in 2021 study and dismissed their findings.

They also demand multiple hypotheses on causes of brain reduction in humans be reassessed to see whether human brains haven’t actually changed at all in size since the arrival of our species.

Since DeSilva’s dataset sampled only 23 crania from the timeframe critical to the brain shrinkage hypothesis and lumped together specimens from locations including England, China, Mali, and Algeria, said the UNLV team

Hence, the conclusion is heavily skewed as more than half of the 987 skulls examined represent only the last 100 years of a 9.8-million-year span of time, they point out. The 2021 study doesn’t give scientists a good idea of how much cranial size has changed over time, argues the UNLV team.

Not so fast, said UNLV anthropologist Brian Villmoare. “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.”

Read: No, the human brain did not shrink 3,000 years ago (August 7, 2022)

Another UNLV researcher questioned ignoring the developments 3,000 years ago. “Did the 12th century BCE — a time when humans were forging great empires and developing new forms of written text — coincide with an evolutionary reduction in brain size? Think again,” he asks.

Denying the DeSilva et al. study, the UNLV team insists that human brain size has not changed in 30,000 years, and probably not even in 300,000 years.

“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,” Villmoare said.

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