American Scientist

The earliest bridges are generally believed to have been found structures, in that they were fortuitous arrangements of stones, felled tree trunks or hanging vines that provided a more-or-less convenient way for people to cross a stream without getting their feet wet. These chance crossings naturally led to the deliberate placing of flattish stepping stones, the manipulation of straight logs, and the rearrangement and draping of vines to gain improved footing or reach at the most desirable locations. The evolution from such elementary improvements led in time to the placing of longer stones on narrower ones to produce what are known as clapper bridges; the division of logs into timbers to produce wooden bridges with flat and level decks; and the braiding and anchoring of vines to make the main drooping cables of primitive suspension bridges.