Horses may have been domesticated as long as 10,000 years ago. Evidence of wild horses being trained by humans dates back as far as 5000BC and the process of selective breeding and horse training was certainly widespread by 2000BC. Horses have been bred for specific purposes. Wild horses were not originally particularly well suited to riding or for agricultural tasks such as ploughing. It is not just their behaviour but their physique that has been changed as a result of selective breeding. Now some breeds are, as racing thoroughbreds more than adequately demonstrate, so finely tuned to their domesticated purpose that they only really have one use in life. Of course no amount of carefully selected breeding will breed out all of the characteristics an animal shows in the wild. The flightiness of horses, their tendency to shy away from unexpected sounds or sights is directly related to their behaviour in the wild where the fight or flight response was essential to their survival.