Julian Jaynes, even if you probably disagree with his conclusions, amassed a lot of evidence that a few thousand years ago there was a significant change in how humans think. Much of this could have been due to cultural advances in childrearing, but apparently it was also genetic evolution.
According to this, human brain size shrank 10% in the last 5000 years. Unfortunately, that statistic just tells us that our brains changed, but not what the functional changes were. In that article, John Hawks speculates
As to why is it shrinking, perhaps in big societies, as opposed to hunter-gatherer lifestyles, we can rely on other people for more things, can specialize our behavior to a greater extent, and maybe not need our brains as much.
Maybe. Memory prosthetics, such as writing, could also be a factor. Or a change in diet. And the childrearing improvements mentioned above.
Whatever the explanation, it’s important to keep in mind that brain size doesn’t correlate with intelligence. Emily Sohn points out
overall size is irrelevant when it comes to brain power. Among humans, individuals with larger noggins don’t have higher IQs. Whales, with brains that weigh up to 20 pounds and have more than 200 billion neurons, are no smarter than people, with our measly 3-pound brains that have just 85 billion neurons.
Instead of contributing intelligence, big brains might just help support bigger bodies, which have larger muscles to coordinate and more sensory information coming in. Like computers, Chittka said, size might add storage capacity but not necessarily speed or usefulness. At the same time, it takes a lot of energy to support a big brain.
The main point is that something about our brains today really is significantly, biologically, genetically different than the brains of our ancestors 5000 years ago, and there’s probably a connection between that change and the evidence amassed by Jaynes.