Eukaryotic cells are believed to have evolved from prokaryotic cells through a symbiosis between separate single-celled organisms.
According to the endosymbiotic theory,
and plastids such as
are organelles representing formerly free-living bacteria (prokaryotes) that were taken inside another cell as an endosymbiont around 1.5 billion years ago.
These organelles carry DNA that is
the remainder of the eukaryotic cell.