Meanwhile, 140k years ago, our 3rd civilization approached our current level of tech. Fashion designers, specifically a group lead by Argot the Great, decided the tail made pants difficult, and broke the lay of suit jackets.

So they hired radicals to genetically engineer a retrovirus, to remove the tail. This worked, but due to unconsidered balance issues, all tail devoid children lost the capability for bipedal balance and therefore walking.

Most of these children, during their formative years spent their time on all fours, and thud their hand dexterity was pitiful, and society crumbled when they came of age.

It was another 46k years before a brain mutation emerged, re-enabling bipedal balance, but by that time all traces of the old civilization was lost, and thus, progress began anew.