Protists are eukaryotes that first appeared approximately 2 billion years ago with the rise of atmospheric oxygen levels.
Eukaryotes, having probably evolved from prokaryotes, have more complex traits in both cell and DNA organization.
Eukaryotes may have been a product of one cell engulfing another and evolving over time until the separate cells became a single organism.
Mitochondria are energy-producing organelles that are thought to have once been a type of free-living alpha-proteobacterium.
Plastids may derive from cyanobacteria engulfed via endosymbiosis by early eukaryotes, giving cells the ability to conduct photosynthesis.