The "miserable" existence ascribed by modern day humans to past human life is colored by their modern day psychological profile. If they were born and raised at that environment their psychological profile would be very different. A modern day human can be easily traumatized by something that past humans would consider trivial. Sure death was more common, possibly even violence, but that would not mean people were less happy. Satisfaction in human psychology has a certain profile, and that profile mainly follows the things i talked about. Close human relationships in small cohort groups, perception of agency, among a few other important factors. things that are missing among many citizens of modern day societies world wide. My point is that on average if you performed a statistical analysis of how happy people were, the claim is that they were happier back then then now.