How do you know that your ancestors didn't have numerous orthopedic problems caused by constantly running down and killing animals, which resulted in drastically lower life expectancies than we enjoy now?
However, I did have someone with a Ph.D. in kinesiology (I think?) tell me that there's essentially no evidence of knee, hip, or back dysfunction in developing countries where people don't sit all day.
Here's a somewhat nutty article on it called "third world squat": http://www.t-nation.com/free_online_article/sports_body_trai... but it is an actual thing that people and it makes for a different muscle development than chair-dwellers have.