bio

Serial endosymbiotic theory

The idea — argued into the mainstream by Lynn Margulis in the late 1960s against much resistance, now among biology's best-supported theories — that complex (eukaryotic) cells arose through a series of symbiotic mergers, not by gradual mutation alone. An ancient host cell engulfed an aerobic bacterium and kept it as the mitochondrion; later a lineage acquired a cyanobacterium and made it the chloroplast. Major evolution sometimes proceeds by merger, not only by branching.