Eukaryotes (from the Greek eu "good" and karyon "nucleus") are the only domain that possesses a true nucleus in their cells, which surrounds and protects the DNA. Of the three existing domains, eukaryotes by far have the most species, being the only one that has truly macroscopic organisms, with an extremely high diversity of forms but with a very low metabolic diversity.

Life appeared on Earth approximately 3.8 billion years ago, but it was not until about 900 million years ago that eukaryotes appeared, making them the last domain to form, with a difference of about 3 billion years. Prior to the appearance of eukaryotes, for almost 3 billion years, life was composed only by the Bacteria and Archaea domains. After this time, there was a period when some species from these two domains began to live in symbiosis. Specifically, a bacterium from the group Proteobacteria managed to survive inside the cell of an archaeon from the group Asgardarchaeota, originating a new organism by the fusion of these two completely different organisms. This gave rise to the Eukaryota domain. The fusion of two completely different metabolisms that complemented each other in a single organism provided it with endless possibilities, greatly increasing its biological efficiency. Subsequently, this lineage followed a completely different evolutionary path, giving rise to a large number of structures that are only present in eukaryotes, such as the nucleus, the cytoskeleton, the internal membranous structures, certain genetic patterns, ... leading to a lineage that would eventually form all the macroscopic species that can be found on Earth.

Although the true diversity of eukaryotes lies in the numerous microscopic life forms it possess, their kingdoms Animalia (animals), Plantae (plants), and Fungi (fungi) are well known. Despite the high morphological diversity of this domain, the molecular diversity of both their metabolic pathways and their DNA is very low compared to the other two domains, and comparable, for example, to a single kingdom of the Bacteria domain.