 The first model known as the " <unk> model " proposes that a symbiotic relationship between the archaea and bacteria created the nucleus @-@ containing eukaryotic cell . ( <unk> of the <unk> and <unk> domain have no cell nucleus . ) It is hypothesized that the symbiosis originated when ancient archaea , similar to modern <unk> archaea , invaded and lived within bacteria similar to modern <unk> , eventually forming the early nucleus . This theory is analogous to the accepted theory for the origin of eukaryotic mitochondria and <unk> , which are thought to have developed from a similar <unk> relationship between proto @-@ eukaryotes and <unk> bacteria . The <unk> origin of the nucleus is supported by observations that archaea and <unk> have similar genes for certain proteins , including histones . Observations that <unk> are <unk> , can form multicellular complexes , and possess <unk> and G proteins similar to <unk> , support a bacterial origin for the eukaryotic cell . 
