Algae gain chloroplasts

About 1.6 billion years ago, give or take a few hundred million, a single-celled eukaryote swallowed a cyanobacterium and, instead of digesting it, kept it alive. Over generations the captive became a permanent organ called a chloroplast, which carries out photosynthesis inside the host cell. This partnership is called primary endosymbiosis, and it happened once. Its descendants include red algae, green algae and glaucophytes, and from the green algae came every land plant.

The ingredients were already in place. Eukaryotes, cells with a nucleus and internal compartments, had evolved earlier and could engulf other cells. Cyanobacteria had been making oxygen and sugar from sunlight for a long time. The idea that chloroplasts began as bacteria was first suggested by the Russian botanist Konstantin Mereschkowski in 1905, and Lynn Margulis defended it in 1967 against heavy skepticism. The evidence is now overwhelming: chloroplasts have a double membrane, their own circular DNA, bacteria-like ribosomes, and they divide by pinching in two, like bacteria.

The merger became permanent as most of the cyanobacterium's genes moved into the host's nucleus, so neither partner could live alone. Fossils from India that are about 1.6 billion years old may be the earliest red algae, though dating and identity are still discussed. Later, other eukaryotes engulfed algae in turn, a process called secondary endosymbiosis, producing brown algae, diatoms and dinoflagellates. Algae soon became a major part of ocean life, supplying food and oxygen, and their green relatives eventually made the move onto land.