Nobody knows the origin of viruses—how they came into existence or when they appeared in the history of life on earth. Viruses may be examples or relics of life forms that operated at the dawn of life. Viruses may have come into existence with the first stirrings of life on the planet, roughly four billion years ago. Or they may have arisen after life started, during the time when single-celled bacteria had already come into existence—nobody knows.
