Over 1 billion years ago, a relationship began between the ancestor of all living plants and a type of bacterium that paved the way for the evolution of life as we know it. The single-celled algal ancestor engulfed, but crucially, did not destroy, a cyanobacterium-like organism with which it established a mutually beneficial bond. This symbiotic relationship provided energy in the form of sugars derived from photosynthesis (whereby sunlight is converted into chemical energy) from the cyanobacterium to its host.