This is /possibly/ part of a long term trend of decreasing human reliance on brainpower.

Brain size doesn’t neatly correlate to intelligence, but it’s still kind of used as a proxy, over human evolution. It gets bigger for a long time, then starts getting smaller again, starting maybe 20k-3k years ago.

This has been attributed to group size, agriculture, writing or statistical error. So maybe it’s not a thing.