Recent work in classification of life on Earth has shown that what were once called prokaryotes are much more diverse than ever suspected. The prokaryotae are now divided into two domains, the Eubacteria and the Archaeabacteria, as different from each other as either is from the Eukaryota. One organism, the blue-green algae or cyanobacteria, has left a fossil record that dates back nearly 3.5 billion years and is among the oldest fossils known. These blue-green algae had cell walls and were photosynthetic. A few hundred years ago, this organism was classified as a plant, then as a protist. Today it is classified in the Eubacteria because it