Long-distance running, or endurance running, is a form of continuous running over distances of at least five kilometres (3.1 miles). Physiologically, it is largely aerobic in nature and requires stamina. The body structure of a skeleton of a 12 years old Nariokatome boy is suggested to prove that early humans from 1.5 million years ago were eating more meat and less plants, and hunted by running down animals. The most prominent long-distance running sports are grouped within the sport of athletics, where running competitions are held on strictly defined courses and the fastest runner to complete the distance wins. The one hour run is an endurance race that is rarely contested, except in pursuit of world records. The 20,000 metres is also rarely contested, and all world records in the 20,000 metres have been set while in a one hour run race. In contrast to the relatively flat courses in track and road races, cross country usually incorporates obstacles such as muddy sections, logs and mounds of earth.