A eukaryote is a single-celled or multicellular organism whose cells contain a distinct membrane-bound nucleus or nuclei. Eukaryotes make up plants, fungi, animals, and certain types of protest. Eukaryotes have a number of membrane-bound bodies called organelles. These include the endoplasmic reticulum and ribosomes to name a few. Mostly, eukaryotes are multicellular. A eukaryote is much more complex than a prokaryote. The oldest eukaryotic fossil is about 1.5 billion years old. Many biologists believe that eukaryotes evolved from prokaryotes years ago.