The tree of life has gained a new branch with the discovery of a vanishingly small deep-sea microbe. The minuscule organism, described in the 2 May issue of Nature, could shed light on the earliest stirrings of our single-celled ancestors.
The new organism was found by Karl Stetter and colleagues of Regensburg University, Germany, while studying rock samples taken from hot underwater vents north of Iceland. Although most of the ocean floor is cold and nearly lifeless, these vents--and the plumes of hydrogen and sulfur that escape from them--serve as oases for some of Earth's oddest life-forms.
A group that flourishes there is the Archaea, one of the three main divisions of life. (The other two divisions are the Eukaryota, which are made of cells like ours with organelles and DNA packed into a nucleus, and the prokaryotic Bacteria, which lack these features.) The prokaryotic Archaea branched off from bacteria long before our eukaryotic lineage, perhaps more than 2 billion years ago. Like the coelacanth, the Archaea are considered "living fossils" because they retain features of ancient life-forms--invaluable for piecing together the evolutionary history of today's creatures.
Stetter's group was cultivating a deep-sea archaeote in the lab called Ignicoccus, when they noticed some unusually tiny cells about 400 nanometers in diameter. Rarely floating free, the spherical cells were attached to the Ignicocci, sometimes in groups of two or three and sometimes bunched like strings of pearls. They absorbed a dye that binds to DNA, which suggested that they are alive. But their DNA sequence defied classification into known groups of microbes. Further analysis proved that these cells are not only alive, but form an entirely new phylum of the Archaea.
The so-called Nanoarchaea seem to be linked symbiotically with the Ignicocci, because they could not be cultured alone. Indeed, the tiny microbe has perhaps the world's most streamlined genome, around 500 million base pairs. In spite of its small size, says biologist Ford Doolittle of Dalhousie University in Canada, "it is as worthy of our notice as any coelacanth or other macroscopic living fossil." Insights into the Nanoarchaea, he says, could illuminate the murkiest depths of evolutionary history.