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Many things can shrink your brain: Living in Antarctica, exposure to pollution, alcohol, and COVID-19, for example. However, living in cities isn’t one of them, at least not 3,000 years ago – that’s the conclusion of a new reassessment of data that was previously used to claim human brain size took a dive about the time large urban areas appeared.
Anthropologists have tracked the interior size of human ancestors’ skulls over millions of years as a proxy for intelligence. They have concluded brain size expanded slowly since our last common ancestor with chimpanzees five million years ago, and then accelerated around 2 million years ago.
Last year, however, a paper gained considerable attention for saying this increase reversed in the 12th century BCE, around the time that urban living became widespread. Plenty of people have argued over the implications of this, but a paper in Frontiers in Ecology and Evolution challenges the idea it happened at all.
The idea of a substantial reduction in brain size has been proposed in several papers recently, but the timing and cause were considered uncertain. Dartmouth College’s Professor Jeremy DeSilva and co-authors used a sample of 987 skulls from museum collections to date the decline to 3,000 years ago. They attributed the shrinkage to the development of technologies like writing, which meant that people didn’t need to remember as much. Likewise, the development of cities meant people could specialize more, possibly expanding their brains in the areas responsible for specific skills they used frequently, while sections involved with capacities they outsourced to others could atrophy.
"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," Dr Brian Villmoare of the University of Nevada said in a statement. "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."
Despite claims for declines in human cranial capacity dating back over forty years, there were some obvious problems with the evidence provided. Writing and large metropolises appeared at different times around the world – if they were the cause of brain shrinkage, that should have shown up a lot earlier in Egypt, for example, than places where even agriculture wasn’t widespread until much later.
Villmoare and co-author, Liverpool John Moore University’s Dr Mark Grabowski, point out that DaSilva’s skulls were from widely varied locations, with no allowance made for the state of civilization there at the time.
They also note that while 987 looks like a good sample size at first, half the skulls come from the last century. That leaves 500 specimens to span a range from 9.8 million years ago, before Australopithecines had evolved, let alone humans, to the 19th century. The period from 5,000-1,000 years ago was represented by just 23 skulls – certainly inadequate for sweeping conclusions given how variable any population can be.
The average size of the recent skulls in DeSilva’s database is noticeably below most other estimates for the modern average.
The question of whether living in complex societies shrunk our brains may appear interesting, but not particularly pertinent. However, measuring brain capacity has a long and ugly history, with dubious statistics being used to justify horrors such as slavery.
Recent years have seen an uptick in attempts to rehabilitate racism based on measures such as skull shape. DeSilva and co-authors’ intentions may have been entirely innocent, but drawing big conclusions based on insufficient samples can open the doors to those with more malevolent objections.
