Protoctists have co-evolved with photosynthetic bacteria for some time. Swallowed up but not digested, some bacteria provide the protoctist with a constant supply of food, which makes them self-sufficient algae. The engulfed bacteria receive a safe, comfortable home and rapid transport to sunlight. With their endosymbiotic (living inside) bacteria, the protoctists, now algae, are virtual living greenhouses.
These cross-species associations celebrate success with permanent mergers. The bacteria transform into new organelles called plastids. Eventually, splashes of green and red seaweed will decorate Earth's coasts.