Arguably the most useful domesticated animal in mankind's history, horses have existed for some 50 million years. While significant evolution has taken place (early ancestors were under a foot tall), the basic appearance of horses has remained relatively unchanged. The earliest interaction between horses and men was of game, with early man hunting them for food. Around 2500 B.C. the first signs of domesticated breeds began appearing in Asia, but the idea took hold and spread rapidly. Their contributions to man's conquest of his fellow man were immense, enabling the vast campaigns by Genghis Khan and Alexander the Great. The Spaniards introduced the modern breed to North America in the 16th century, often turning their imported herds loose before returning to Europe (making valuable room for gold and other treasures). While the importance of horses has dwindled since the advent of the internal combustion engine, they remain a boon in the forms of entertainment and farming.