We’ve lost the equivalent of a tennis ball over the last 20,000 years – globally.
So it’s has to be a global factor – climate change?
More likely, as the human population grows larger we don’t need to have as much brain power because we are not reliant solely on ourselves.
Moreover, group think may free up a large portion of our computing power
the more complex societies, the more diversity the more equality, the greater our capacity has to be for understanding and cooperation
less cooperation more herd/group thoughts and behaviour, the less brain power we need
or the brain could become more efficient, but the dino to brain ratio, suggests there is no hope in that idea