We are told that our brains started to shrink about 3000 years ago:
Your ancestors had bigger brains than you. Several thousand years ago, humans reached a milestone in their history – the first known complex civilisations began to emerge. The people walking around and meeting in the world’s earliest cities would have been familiar in many ways to modern urbanites today. But since then, human brains have actually shrunk slightly.
The lost volume, on average, would be roughly equivalent to that of four ping pong balls, says Jeremy DeSilva, an anthropologist at Dartmouth College in the US. And according to an analysis of cranial fossils, which he and colleagues published last year, the shrinkage started just 3,000 years ago.
“This is much more recent than we anticipated,” says DeSilva. “We were expecting something closer to 30,000 years ago.”
Chris Baraniuk, “Why human brains were bigger 3,000 years ago” at BBC (May 8, 2022)
The researchers cite writing as one possibility for shrinking brains:
One other possibility is that the emergence of writing – which occurred roughly 2,000 years before the reduction in human brain size set in – also had an effect. Writing is one of relatively few things that separates us from all other species and DeSilva questions whether this could have influenced brain volume through “externalising information in writing and being able to communicate ideas by accessing information that’s outside your own brain”.
Chris Baraniuk, “Why human brains were bigger 3,000 years ago” at BBC (May 8, 2022)
But then we need to explain how we started writing.
You may also wish to read: Yes, the human brain is the most complex thing in the universe. But that’s not even the most remarkable thing about our brains. Our complex brains mirror the universe — 27 orders of magnitude bigger — yet some humans function with only half a brain or split brains.