Eukaryotic organisms encompass a range of organisms, from humans to single-celled microorganisms such as protozoa. Eukaryotes are fundamentally different from prokaryotic microorganisms, such as bacteria, in their size, structure and functional organization.

The oldest known eukaryote fossil is about 1.5 billion years old. Prokaryote fossils that are over 3 billion years old are known. Thus, prokaryotic cells appeared first on Earth. The appearance of eukaryotic cells some 1.5 billion years ago became possible when cellular function was organized into regions within the cell called organelles.

The eukaryotes are organized into a division of life that is designated as the Eukaryota. The Eukaryota are one of the three branches of living organisms. The other two branches are the Prokaryota and the Archae.

The evolutionary divergence of life into these three groups has been deduced in the pasts several decades. Techniques of molecular analysis have been used, in particular the analysis of the...