Leonardo Da Vinci is credited with developing the idea of the chain and cog in the 15th century. However, it took nearly 400 years for the idea to become a practical aspect of bicycle design. For a chain drive to be effective it needs to transmit power efficiently from the rider's legs to the back wheel. It also must be designed so that pedalling resistance is within a comfortable range for the cyclist. The development of stronger materials and other technological and engineering advances made this possible. By the 1880s, the chain drive was commonplace.

The Benefits of Gears

A chain drive alone (without gears) is effective on flat surfaces and going downhill. However, when it comes to headwinds, hill climbing, and even starting on a bicycle without gears--the cyclist has to stand on his pedals and strain while pedalling at a very low rate. Gears allow the cyclist to pedal at a comfortable and efficient rate while travelling either uphill or downhill or with a headwind or a tailwind.
On the old high-wheelers, the pedals were attached directly to the wheel. One turn of the pedals equalled one turn of the wheel. Gears allows the cyclist to change that ratio. For steep hills, we choose a gear that lets us turn the pedals many times to turn the wheel just once; on flats or downhills, we might choose a gear that turns the wheel many times for each turn of the pedals.