Protists are eukaryotes that first appeared approximately 2 billion years ago with the rise of atmospheric oxygen levels.
Characteristics of Eukaryotes
Eukaryotes, having probably evolved from prokaryotes, have more complex traits in both cell and DNA organization.
Endosymbiosis and the Evolution of Eukaryotes
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.