This symbiosis was fantastically improbable. In 3.5 billion years of history and trillions of trillions of interactions between protozoa and cyanobacteria it seems to have happened exactly once. But this single incident had huge effects—it is responsible for the existence of plants. Over the eons the cyanobacterium shed many of its original characteristics, and became a chloroplast: the free-floating body in plant cells in which photosynthesis occurs. Plant cells today can have hundreds of chloroplasts, each a descendant of that long-ago cyanobacterium.
