The wheel is probably history's most important technological innovation. Yet, as obvious as the device seems today, in ancient times it was anything but that. Archaeological evidence indicates that although human beings first settled in villages about 10,000 years ago, nearly 5,000 years would pass before anyone invented a wheel or, at least, before the idea would become popular enough to leave evidence of its existence. Through that time, people hauled heavy loads of lumber, stone, farm produce and other things without wheeled vehicles. The oldest evidence of a wheel, dating from 5,500 years ago, is a Sumerian picture, carved in clay, of a sledge equipped with wheels, making it a cart. Sumer, in what is now southern Iraq, also produced some of the world's first cities and, more important, invented writing. Historians of technology suspect that the idea for the wheel came from an older technology that used logs as rollers. This is probably how ancient Egyptians moved blocks of stone, weighing many tons and used to build pyramids. As the rolled cargo overshot the frontmost roller, workers would carry one left behind and place it in front of the block or sledge. What distinguishes a wheel from a log roller is its axle, the rod attached to the cart's platform that keeps the wheel in place under the cargo,eliminating the need to lay more rollers ahead of the moving load. The earliest wheels consisted of three carved wooden planks clamped together by wooden cross pieces, or battens. A hole in the center was fitted with a carved, wooden, cylindrical axle. To keep the axle from wearing away, bearings were added. The simplest was a sleeve of leather fitted between axle and wheel. Some historians think that the idea of the wheel then spread slowly. The oldest evidence of wheels in India, for example, dates from 4,500 years ago. The wheel did not reach Europe until 3,000 years ago. In the Old World, one of the last peoples to adopt the wheel were the Britons just 2,500 years ago. In the New World, the only evidence that anyone thought of wheels consists of small toy carts made of clay that were dug up in ancient Mexican sites. Curiously, the concept appears never to have been developed to make a practical device. Many variations of the wheel have emerged. Cutting holes through the wooden planks, for example, lightened the wheel. Further attempts to cut weight led to invention of the spoked wheel whose wooden rods transmitted the load's weight to a wooden rim made in carved segments. The ultimate in spoked wheels is the bicycle wheel, in which spokes do not transmit the load to the bottom of the rim. Instead, the load hangs from the top of the rim, which the rest of the spokes keep from collapsing. To keep the wooden rim from destruction, early inventors developed the tire. The earliest appear to have been made of iron.

CAPTION: One of the earliest wheels consisted of three planks held together with smaller cross pieces.

An early chassis had the wheel fixed to the axle, both turning together. The axle passed through a hole in the chassis, which absorbed the wear. A bearing was added to reduce wear. It consisted of a leather sleeve placed between the chassis and the axle. For a fixed axle (the axle is attached to the chassis, the wheel turning on the axle), the leather bearing was placed between the wheel and the axle.

Cut-outs lighten the load.

The bicycle wheel does not distribute the load to the bottom of the rim like the older wheels. It instead hangs from the top of the rim, the spokes keeping it from collapsing.