Thousands of years ago doesn’t seem a good reference point. With advances in science, technology, better shelters and more accessible food - we absolutely should have way less stress today.
> anthropologists have found that hunter-gatherers tend to have significantly more leisure time than people in more complex societies.
Perhaps being able to think doesn't help much when it comes to happiness. Thinking is useful practically but it's as if you have to suppress it in order to experience happiness.
You should read a bit about their sex lives...
The less snarky remark would be that Robert Sapolsky has done plenty of research on stress responses in primates and found that the single largest predictor of high stress responses is inequality, which our modern times do seem to generate in abundance.