India, June 8 -- Humans have been using tracks, of one kind or another, for over 20,000 years. Travois, for instance, were used to drag heavy loads. These were two wooden poles used to form a sort of sledge, that could be loaded with heavy materials, and dragged by humans, or by dogs or horses. The earliest travois tracks, found in New Mexico, date to 22,000 years ago, predating agriculture and settled living. For a largely migratory species, such transport would have been vital. By the time of Ancient Greece, track-based transport was getting rapid upgrades. But first. Buried in the bogs of Somerset, England, is a network of wooden logs that was once used by Neolithic humans, to cut across this marshy ground. Now called the Sweet Track, af...