"Evolutionary biology has lots of problems. But one problem that most folks think the field doesn’t have is coming up with a reason for why humans evolved bigger brains. After all, we’ve heard countless times that bigger brains evolved to allow for higher intelligence in our hominid ancestors, because higher intelligence was selected to help our ancestors to better survive and reproduce. These kinds of arguments are everywhere — here are a few typical examples: A 2007 chapter titled “The Evolution of Brain Size and Intelligence” in an MIT Press book reads: “It is reasonable to hypothesize that bigger brains evolved based on natural selection for increased intelligence.” (Rushton and Ankney, 2007)
A 2025 paper in Philosophical Transactions of the Royal Society B states that “hominin brain expansion underpinned large increases in cognitive abilities.” (González-Forero and Gómez-Robles, 2025)
According to a 2011 paper in Frontiers in Human Neuroscience, “An increase in brain size could also result in an increase in the complexity of its functions. … the evolution of encephalization essentially involves an increase in the brain’s capacity to process information.” (Cairo, 2011)
A new article at Phys.org notes: “For decades, researchers generally assumed that these changes were the result of sustained, directed natural selection; larger brains were favored because they improved cognitive abilities.”
It’s so simple it’s a no brainer (pun intended!): Bigger brains evolved to make us smarter, a trait that was gradually favored by natural selection over time. And yet, even here evolutionary theory is apparently not supported by the evidence.
According to an article in New Scientist, “Human brains may have got bigger for no particular reason.” Says science writer Michael Marshall, “there is no evidence that evolution strongly favored big brains in the past 2 million years of human evolution, according to a new analysis of fossil skulls.”
The article cites a newly published study in Nature Communications, by anthropologist Mark Hubbe and paleoanthropologist Katerina Harvati, which notes that although brain size in Homo does get bigger over time, the changes appear to be “stochastic” — i.e., random — and “did not support gradual directional selection as a driving force.” In other words, the old idea that natural selection gradually favored bigger and bigger brains in our ancestors is not supported by the evidence." MindMatters
