Archaea, ahr KEE uh, sometimes called archaebacteria, are a group of single-celled organisms that make up one of three basic divisions of life. Scientists often call these basic divisions domains. The other two domains of living organisms are Eukaryota, which includes animals and plants, and Bacteria, which includes all true bacteria and the algaelike cyanobacteria. Eukaryotes have eukaryotic cells, or cells with a nucleus, while bacteria and archaea have prokaryotic cells, or cells that lack a nucleus. Scientists had traditionally classified archaea with bacteria because of their similar cell structures. But beginning in the 1970's, close analyses of their genes revealed that archaea and bacteria are too different to be grouped together. In many respects, archaea more closely resemble eukaryotes than they do bacteria.