Eukarya, Archaea, and Eubacteria constitute the three major branches of living organisms (excluding viruses). Eukaryotes are organisms consisting of one or more eukaryotic cells, the most distinctive feature of which is the presence of a membrane-bound nucleus containing the cell's genetic information. This diverse group includes plants, animals, fungi, and protists (unicellular organisms). Scientists believe that eukaryotes probably evolved from a prokaryotic (cells without a nucleus) ancestry between 1.6 and 2.1 billion years ago. Thus, they are also distinguished by other cell structure complexities, including a protein-rich cytoskeleton and separate membrane-bound compartments (organelles), such as ribosomes and mitochondria, that carry out specialized cellular functions. About 60 lineages of eukaryotes have been identified thus far, but the relationships among many of them remain unclear.