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.
“No Particular Reason”
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 favoured 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. This is a big deal, to put it mildly, because it means what virtually every “man on the street” believes about human brain evolution is simply not true.
Random and Constrained Evolution
Instead, the study finds that random evolution — not natural selection — is the reason our brains got bigger — and there were other forces that prevented continued growth in brain size. New Scientist summarizes the findings this way:
The changes in size and shape of the braincase were best explained by neutral evolution. This means random mutations affected the size and shape and changes accumulated over time, without the new braincase designs ever being advantageous compared with the old ones. There was also evidence of stasis: evolution pressuring the braincase to stay roughly the same shape and size.
The technical paper summarizes the findings as follows:
In conclusion, our results are consistent with previous work suggesting a limited role for gradual directional selection in human evolution and underscore the importance of stabilizing selection and constraints, which our analyses strongly supported relative to other evolutionary processes. The cranial morphology of the genus Homo was therefore likely shaped by factors constraining the accumulation of variation (i.e., that promoted evolutionary stasis), as well as by stochastic processes.
Think Bigger
The human brain is “the most complex object in the known universe,” according to famed neuroscientist Christof Koch. And now we’re being told that evolution claims this momentously important organ arose “for no particular reason.” Just wow.
In more technical terms, this study is fascinating because it shows that one of humanity’s defining traits is not well explained by the idea of incremental, selection-driven increases gradually making our ancestors smarter over time. To resort to “stochastic,” “neutral,” “random” evolutionary processes — or as New Scientist puts it, evolution “for no particular reason” — is to admit that they have no compelling Darwinian narrative for why our bigger and smarter brains arose. It is tantamount to saying the human brain, chief marvel in the cosmos as far as we can tell, evolved because, well, why the heck not?
Apart from that incredibly uncompelling conclusion, the study supports a core ID postulate: that change in living systems isn’t unlimited. As the technical paper finds, there are “evolutionary limits constraining their potential to evolve new phenotypes,” in this case referring to limits on our ability to evolve bigger, smarter brains.
All this goes to show that sometimes even the problems you think evolution has solved turn out to be anything but.