Professor Davies will argue that demonstrating that life has appeared more than once on Earth would be the best evidence yet that it must exist elsewhere in the Universe."Royal Society meet to discuss if extra-terrestrials are here on Earth"
He told The Times: "We need to give up the notion that ET is sending us some sort of customised message and take a new approach."
According to Professor Davies, "weird" microbes that belong to a completely separate tree of life, dubbed the "shadow biosphere", could be present in isolated ecological niches in which ordinary life struggles to survive. Likely hiding places include deserts, scalding volcanic vents, the dry valleys of Antartica or salt-saturated lakes.
Not all are convinced by the "shadow biosphere" concept. Colin Pillinger, who led the Beagle 2 Mars landing mission, said: "I prefer to deal in scientific fact -- this is wildly science fiction. You'd be off your trolley to go searching for arsenic-based life."
Professor Pillinger, who is due to speak at the Royal Society today, argues that Mars remains the best bet for finding alien organisms.