Eukaryotic cells differ in many respects from the generally smaller cells of bacteria and archaea. Even the smallest single–celled eukaryote is far more complex in structure than any prokaryote. The most fundamental question is to know how these complex eukaryotic cells evolved from much simpler prokaryotic cells.
A process called endosymbiosis probably led to mitochondria and plastids (choloroplasts). The theory of endosymbiosis proposes that mitochondria and plastids were formerly small prokaryotes living within larger cells. The term endosymbiont refers to a cell that lives within another cell, which is called host cell.
After the first eukaryotes appeared, a great range of unicellular forms evolved, giving rise to the diversity of single–celled eukaryotes that continue to flourish today. But multicellular forms also evolved. Their descendants include a variety of algae, plants (mosses, ferns, gymnosperms and angiosperms), fungi, animals and human beings.