Chronic shoulder pain in humans might be a literal evolutionary hangover from when our ancestors walked on four legs.

The human shoulder transitioned from a weight-bearing quadrupedal tool to a highly mobile bipedal joint over millions of years. A specific muscle called the pectoralis minor often pulls the shoulder blade back into an ancient, four-legged position. This neurological glitch creates a conflict between our modern skeleton and our ancestral muscle patterns. The resulting friction is a primary driver of chronic shoulder pain and instability in many people. Understanding this evolutionary mismatch allows physical therapists to target the specific muscles that are stuck in the past.