A eukaryote is an organism composed of eukaryotic cells, which have membrane-bounded nuclei and other membranous organelles, as well as cytoskeletons for support. Eukaryotes include animals, plants, and fungi, which are mostly multicellular, and protists, which are unicellular. In contrast, prokaryotes, such as bacteria, consist of simpler cells that lack true nuclei.

 

The first eukaryotes are believed to have evolved from prokaryote ancestors some 1.6 to 2.1 billion years ago, although the earliest fossil traces of modern eukaryotic groups date back to only about 800 million years ago.