What would life on other worlds be like? On worlds in our solar system, extraterrestrial life, if it exists at all, may not be as diverse as it is on Earth. Our planet is especially hospitable to life due to its "just-right" distance from the sun, but this is not the case for the rest of the solar system.
One notable feature of life on Earth is that different lifeforms interact. Living things here exist as parts of ecosystems and depend on one another for survival. Until recently, in fact, biologists assumed that life could not exist outside
an ecosystem composed of many species. This assumption was based on the observation that the species they observed were always, without exception, dependent on other species around them.
For astrobiologists, scientists who investigate the possibility of life on other worlds, this assumption carries significant implications. Many potential sites where extraterrestrial life may exist are so inhospitable that they may not be able to support more than one species, even of microbes. The energy source may not be the sun, but geothermal energy, heat from decaying radioactive elements, or even tidal stress (in the case of Jupiter's moon Europa). Some astrobiologists speculate that these paltry resources would be insufficient to support a diverse ecosystem.
The assumption that life must be part of an ecosystem was based on empirical observation, and only one observation would be necessary to show that it is false. Enter Desulforudis audaxviator, a bacterial species found in the depths of the Earth. D. audaxviator was discovered in the groundwater of a South African gold mine, where it is the lone lifeform. Since it lives alone, it must be self-sufficient, and scientists have confirmed that it has all the genes it needs to extract the energy and nutrients it needs to grow from the water and minerals of its surroundings.
D. audaxviator's energy source is the radioactive decay of uranium. It acquires carbon from carbon dioxide dissolved in the water and converts nitrogen from surrounding rocks to a form it can use to generate all the amino acids it needs (a process called nitrogen fixing).
Unlike a true alien species, this bacterium is not unique in origin. It is distantly related to Clostridium bacteria, which are notorious for causing deadly infections in humans. It shares the same DNA code and the same set of amino acids as all other life on earth. Still, it has value to astrobiologists as a concrete demonstration that a species can exist without depending on any other species, and that it can obtain all its energy requirements from a source other than the sun.
"Audax viator" is Latin for "bold traveller," and comes from a quotation in the Jules Verne novel, Journey to the Center of the Earth: "Descend, bold traveller, and attain the center of the Earth." Its discovery tells us that the bold travellers who may one day travel away from the Earth into the solar system may find that our world is not alone in accommodating life.