The Salisbury Cathedral in England is said to be home to the world's oldest clock. The mechanical device's wrought iron hands are believed to have been tracking the passage of time since at least 1386. The clock survived war, fire and inattention before being rediscovered in the early 20th century and restored [source: Salisbury Cathedral].
The Salisbury Cathedral clock is but a wee whippersnapper when compared to the natural clocks that track our circadian rhythms. Scientists believe internal clocks evolved more than 3 billion years ago in cyanobacteria (what we also call blue-green algae), but they don't know exactly why it happened. Some say this was nature's way of leveling the playing field for organisms all competing for the same sources of energy. Circadian rhythms developed so that some creatures feed during the day and others do it at night. Others say the body clock evolved in algae to stagger the sludge's processes for photosynthesis—converting light into energy to be stored for later—and nitrogen fixation—in which plants convert nitrogen from the air into energy — so as not to counteract one another [source: Newitz].