Eukaryotes are multicelled organisms having cells that, from species to species, are arranged into various complex structures.
The evolutionary origin of the eukaryotic cell was a milestone in the enabling of interesting species. Before eukaryotes emerged, prokaryotes were the only sort of life that existed -- single-celled bacteria that were not capable of much more than just floating around in water. The evolutionary emergence of eukaryotes is hard to determine, although there is evidence to show they existed at least 2 billion years ago.
Eukaryotic cells are differentiated from prokaryotes by a membrane-bound nucleus. The duplication of nuclei among all cells of a eukaryote carries the necessary information from cell to cell as to what each cell's role is in the overall organism.
