The collapse of sewers had led to the discovery of bacteria able to survive in hostile conditions, places that were hot, pressurised and acidic. Scientists looked elsewhere for signs of life and, no matter how nasty the environment, from the salty Dead Sea to Antarctica, they found thriving colonies of bacteria. A camera housing on the Surveyor III transported bacteria to the moon, and the bugs were able to resurrect themselves upon returning to Earth two years later, having endured intense cosmic radiation and a vacuum.
In The Fifth Miracle, Paul Davies discusses the existence of bacteria several kilometres below the Earth's surface. For every kilometre downward, the temperature rises 20C/68F, and the pressure undergoes a similarly dramatic increase. In 1992, Tommy Gold, an American physicist, presented evidence of bacteria in Swedish granite originating from a depth of 7km. A couple of years later, drilling projects brought up living specimens, which confirmed Gold's belief in a bacterial underworld. In fact, a rough calculation suggests that one tenth of our planet's biomass may be living below the surface.
As far as Davies is concerned, the most interesting aspect of subterranean bacteria is that they may represent the earliest forms of life on Earth. Initially, scientists believed that subterranean bacteria were surface bacteria that had infiltrated deep rocks by gradually working their way downwards over billions of years. But a more recent theory suggests that life may have begun below the Earth's surface, and that these primitive bacteria then made their way upwards to seed the surface world.
Supporters of this theory point out that the surface environment was incredibly harsh during Earth's early history, largely because of asteroids and meteorites continually striking the planet. Also, the newborn Earth did not have an atmosphere, and so the surface was illuminated with lethal doses of ultra-violet radiation. Therefore, conditions for life were probably better below the Earth's surface.
Charles Darwin once wrote that life began in "some warm little pond". In the 1920s, biologists developed this idea and began to talk about the "primordial soup" model for the origin of life. Now it seems that Darwin might have misled us. Our most ancient ancestors may actually have lived deep below the Earth's surface in rocks.