Analysing data from a space probe focused on the planet, researchers noted an unusually large amount of methane gas suggesting biological sources, which could include rice cultivation, biomass burning and 'flatulence from domesticated animals'. They conceded that the instruments trained on Australia could have been more sensitive, but said data from the rest of the planet proved more conclusive.
The team worked on the basis of life as 'the hypothesis of last resort'. It examined Earth to test techniques used to seek life on other planets. If they had found nothing, it would have raised suspicions that scientists using the same techniques had missed tell- tale signs of life elsewhere.
Robert Carlson, one of the team, and a scientist at the Jet Propulsion Laboratory in Pasadena, California, said the result did not mean the instruments were not to be trusted. 'When we say 'probably' we are just being extremely cautious. We see oxygen, methane and nitrous oxide which are all very indicative of biological processes.
'A spacecraft could never prove there is life on a planet. To do that you would have to land on the surface, shake a hand or see something growing. But you would sure bet a lot of money that life was there.'
It still seems Earth is unique in harbouring life. Space probes have so far flown past more than 60 planets, satellites, comets and asteroids. 'In none of these encounters has compelling or even strongly suggestive evidence for extra-terrestrial life been found,' the paper in today's issue of the science journal Nature says.
The team, led by Dr Carl Sagan, who became a household name through his programmes in the 1970s on the 'Cosmos', produced its analysis by examining data the Galileo space probe sent back after it passed Earth in 1990.
It is now bound for Jupiter.